Search

We present the results of a two-year long optical monitoring program of the narrow-line Seyfert 1 galaxy Akn 564. The majority of this monitoring project was also covered by X-ray observations (RXTE) and for a period of ~50 days, we observed the galaxy in UV (HST) and X-rays (RXTE & ASCA) simultaneously with the ground-based observations. Rapid and large-amplitude variations seen in the X-ray band, on a daily and hourly time-scale, were not detected at optical and UV wavelengths, which in turn exhibited much lower variability either on short (one day) or long (several months) time-scales. The only significant optical variations can be described as two 2--4 day events with ~10% flux variations. We detect no significant optical line variations and thus cannot infer a reverberation size for the broad-line region. Similarly, the large X-ray variations seem to vanish when the light curve is smoothed over a period of 30 days. The UV continuum follows the X-rays with a lag of ~0.4 days, and the optical band lags the UV band by ~2 days. No significant correlation was found between the entire X-ray dataset and the optical band. Focusing on a 20-day interval around the strongest optical event we detect a significant X-ray--optical correlation with similar events seen in the UV and X-rays. Our data are consistent with reprocessing models on the grounds of the energy emitted in this single event. However, several large X-ray flares produced no corresponding optical emission., Comment: 10 pages, 6 figures included, LaTeX emulateapj5.sty, accepted for publication in The Astrophysical Journal

We present the results of a recent reverberation-mapping campaign undertaken to improve measurements of the radius of the broad line region and the central black hole mass of the quasar PG 2130+099. Cross correlation of the 5100 angstrom continuum and H-beta emission-line light curves yields a time lag of 22.9 (+4.4 - 4.3) days, corresponding to a central black hole mass MBH= 3.8 (+/- 1.5) x 10^7 Msun. This value supports the notion that previous measurements yielded an incorrect lag. We re-analyzed previous datasets to investigate the possible sources of the discrepancy and conclude that previous measurement errors were apparently caused by a combination of undersampling of the light curves and long-term secular changes in the H-beta emission-line equivalent width. With our new measurements, PG 2130+099 is no longer an outlier in either the R-L or the MBH-Sigma relationships., Comment: 21 pages, 7 figures; Accepted for publication in ApJ

We present the optical to near-infrared (IR) spectrum of the galaxy TSPS J1329-0957, a red and bright member of the class of extremely red objects (EROs) at z = 1.26. This galaxy was found in the course of the Tokyo-Stromlo Photometry Survey (TSPS) which we are conducting in the southern sky. The spectroscopic observations were carried out with the Gemini Multi-Object Spectrograph (GMOS) and the Gemini Near Infra-Red Spectrograph (GNIRS) mounted on the Gemini-South telescope. The wide wavelength coverage of 0.6 - 2.3 um provides useful clues as to the nature of EROs while most published spectra are limited to a narrower spectral range which is dictated by the need for efficient redshift determination in a large survey. We compare our spectrum with several optical composite spectra obtained in recent large surveys, and with stellar population synthesis models. The effectiveness of using near-IR broad-band data, instead of the spectral data, in deriving the galaxy properties are also investigated. We find that TSPS J1329-0957 formed when the universe was 2 - 3 Gyr old, and subsequently evolved passively to become one of the most massive galaxies found in the z = 1 - 2 universe. Its early type and estimated stellar mass of M* = 10^{11.5} Msun clearly point to this galaxy being a direct ancestor of the brightest elliptical and spheroidal galaxies in the local universe., Comment: 18 pages, 4 figures. Accepted for publication in ApJ

We present relations of the black hole mass and the optical luminosity with the velocity dispersion and the luminosity of the [Ne V] and the [O IV] high-ionization lines in the mid-infrared (MIR) for 28 reverberation-mapped active galactic nuclei. We used high-resolution Spitzer Infrared Spectrograph and Infrared Space Observatory Short Wavelength Spectrometer data to fit the profiles of these MIR emission lines that originate from the narrow-line region of the nucleus. We find that the lines are often resolved and that the velocity dispersion of [Ne V] and [O IV] follows a relation similar to that between the black hole mass and the bulge stellar velocity dispersion found for local galaxies. The luminosity of the [Ne V] and the [O IV] lines in these sources is correlated with that of the optical 5100A continuum and with the black hole mass. Our results provide a means to derive black hole properties in various types of active galactic nuclei, including highly obscured systems., Comment: accepted for publication in ApJL

On 2006 May 5, a four second duration, low energy, ~10^49 erg, Gamma-Ray Burst (GRB) was observed, spatially associated with a z=0.0894 galaxy. Here, we report the discovery of the GRB optical afterglow and observations of its environment using Gemini-south, Hubble Space Telescope (HST), Chandra, Swift and the Very Large Array. The optical afterglow of this GRB is spatially associated with a prominent star forming region in the Sc-type galaxy 2dFGRS S173Z112. Its proximity to a star forming region suggests that the progenitor delay time, from birth to explosion, is smaller than about 10 Myr. Our HST deep imaging rules out the presence of a supernova brighter than an absolute magnitude of about -11 (or -12.6 in case of ``maximal'' extinction) at about two weeks after the burst, and limits the ejected mass of radioactive Nickel 56 to be less than about 2x10^-4 solar mass (assuming no extinction). Although it was suggested that GRB 060505 may belong to a new class of long-duration GRBs with no supernova, we argue that the simplest interpretation is that the physical mechanism responsible for this burst is the same as for short-duration GRBs., Comment: 7 pages, 3 figures, ApJ submitted

We present results from very high signal-to-noise spectropolarimetric observations of the Seyfert 1 galaxy NGC 3783. Position Angle (PA) changes across the Balmer lines show that the scatterer is resolving the Broad-Emission Line Region (BLR). A broad component seen in polarized light and located bluewards from the H$\beta$ line very likely corresponds to HeII$\lambda4686$. The lack of PA changes across this line suggests that the region responsible for this emission appears to the scatterer as unresolved as the continuum source, in agreement with the stratified BLR structure determined from reverberation mapping., Comment: Conference proceedings to appear in "The Central Engine of Active Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP)

The redshift distribution of the short-duration GRBs is a crucial, but currently fragmentary, clue to the nature of their progenitors. Here we present optical observations of nine short GRBs obtained with Gemini, Magellan, and the Hubble Space Telescope. We detect the afterglows and host galaxies of two short bursts, and host galaxies for two additional bursts with known optical afterglow positions, and five with X-ray positions (<6'' radius). In eight of the nine cases we find that the most probable host galaxies are faint, R~23-26.5 mag, and are therefore starkly different from the first few short GRB hosts with R~17-22 mag and z<0.5. Indeed, we measure spectroscopic redshifts of z~0.4-1.1 for the four brightest hosts. A comparison to large field galaxy samples, as well as the hosts of long GRBs and previous short GRBs, indicates that the fainter hosts likely reside at z>1. Our most conservative limit is that at least half of the five hosts without a known redshift reside at z>0.7 (97% confidence level), suggesting that about 1/3-2/3 of all short GRBs originate at higher redshifts than previously determined. This has two important implications: (i) We constrain the acceptable age distributions to a wide lognormal (sigma>1) with tau~4-8 Gyr, or to a power law, P(tau)~tau^n, with -1

We present host-galaxy velocity dispersions of 12 local (mainly Palomar-Green) quasi-stellar objects (QSOs) measured directly from the stellar CO absorption features in the H band. The mean bulge velocity dispersion of the QSOs in our sample is 186 km/s with a standard deviation of 24 km/s. The measurement of the stellar velocity dispersion in QSOs enables us to place them on observational diagrams such as the local black-hole mass to bulge-velocity-dispersion relation and the fundamental plane of early-type galaxies. Concerning the former relation, these QSOs have higher black hole masses than most Seyfert 1 AGNs with similar velocity dispersions. On the fundamental plane, PG QSOs are located between the regions occupied by moderate-mass and giant ellipticals. The QSO bulge and black hole masses, computed from the stellar velocity dispersions, are of order 10^11 M_sun and 10^8 M_sun respectively. The Eddington efficiency of their black holes is on average 0.25, assuming that all of the bolometric luminosity originates from the active nucleus. Our data are consistent with other lines of evidence that Palomar-Green QSOs are related to galaxy mergers with gas-rich components and that they are formed in a manner similar to the most massive Ultraluminous Infrared Galaxies, regardless of their far-infrared emission. However, PG QSOs seem to have smaller host dispersions and different formation mechanisms than QSOs with supermassive black holes of 5x10^8-10^9 M_sun that accrete at low rates and reside in massive spheroids., Comment: accepted for publication in the Astrophysical Journal

We present a detailed analysis of the intrinsic absorption in the Seyfert 1 galaxy NGC 4151 using UV spectra from the HST/STIS and FUSE, obtained 2002 May as part of a set of contemporaneous observations that included Chandra/HETGS spectra. In our analysis of the Chandra spectra, we determined that the soft X-ray absorber was the source of the saturated UV lines of O VI, C IV, and N V associated with the absorption feature at a radial velocity of ~ -500 km/sec, which we referred to as component D+E. In the present work, we have derived tighter constrains on the the line-of-sight covering factors, densities, and radial distances of the absorbers. We find that the Equivalent Widths (EWs) of the low-ionization lines associated with D+E varied over the period from 1999 July to 2002 May. The drop in the EWs of these lines between 2001 April and 2002 May are suggestive of bulk motion of gas out of our line-of-sight. If these lines from these two epochs arose in the same sub-component, the transverse velocity of the gas is ~ 2100 km/sec. Transverse velocities of this order are consistent with an origin in a rotating disk, at the roughly radial distance we derived for D+E., Comment: 51 pages, including 12 figures. Accepted for publication in ApJ Supplements

Over the past decade our physical understanding of gamma-ray bursts (GRBs) has progressed rapidly thanks to the discovery and observation of their long-lived afterglow emission. Long-duration (T < 2 s) GRBs are associated with the explosive deaths of massive stars (``collapsars''), which produce accompanying supernovae, while the short-duration (T > 2 s) GRBs arise from a different origin, which has been argued to be the merger of two compact objects, either neutron stars or black holes. Here we present observations of GRB060614, a 100-s long burst discovered by the Swift satellite, which require the invocation of a new explosive process: either a massive ``collapsar'' that powers a GRB without any associated supernova, or a new type of engine, as long-lived as the collapsar but without any such massive stellar host. We also discuss the properties of this burst's redshift z=0.125 host galaxy, which distinguish it from other long-duration GRBs and suggest that an entirely new type of GRB progenitor may be required.

We present detailed optical, near-infrared, and radio observations of the X-ray flash 050416a obtained with Palomar and Siding Springs Observatories as well as HST and the VLA, placing this event among the best-studied X-ray flashes to date. In addition, we present an optical spectrum from Keck LRIS from which we measure the redshift of the burst, z=0.6528. At this redshift the isotropic-equivalent prompt energy release was about 10^51 erg, and using a standard afterglow synchrotron model we find that the blastwave kinetic energy is a factor of 10 larger, E_K,iso ~ 10^52 erg. The lack of an observed jet break to t ~ 20 days indicates that the opening angle is larger than 7 deg and the total beaming-corrected relativistic energy is larger than 10^50 erg. We further show that the burst produced a strong radio flare at t ~ 40 days accompanied by an observed flattening in the X-ray band which we attribute to an abrupt circumburst density jump or an episode of energy injection (either from a refreshed shock or off-axis ejecta). Late-time observations with HST show evidence for an associated supernova with peak optical luminosity roughly comparable to that of SN 1998bw. Next, we show that the host galaxy of XRF 050416a is actively forming stars at a rate of at least 2 M_solar per year with a luminosity of L_B ~ 0.5L* and metallicity of Z ~ 0.2-0.8 Z_solar. Finally, we discuss the nature of XRF 050416a in the context of short-hard gamma-ray bursts and under the framework of off-axis and dirty fireball models for X-ray flashes., Comment: ApJ in press (accepted version; 40 pages, 7 figures, 4 tables)

We report on the photometric, spectroscopic and polarimetric, monitoring of the optical afterglow of Gamma-Ray Burst (GRB) 030328 detected by HETE-2. Photometry, collected at 7 different telescopes, shows that a smoothly broken powerlaw decay, with indices alpha_1 = 0.76 +/- 0.03, alpha_2 = 1.50 +/- 0.07 and a break at t_b = 0.48 +/- 0.03 days after the GRB, provides the best fit of the optical afterglow decline. This shape is interpreted as due to collimated emission, for which we determine a jet opening angle theta_{jet} of about 3.2 degrees. An achromatic bump starting around 0.2 d after the GRB is possibly marginally detected in the optical light curves. Optical spectroscopy shows the presence of two rest-frame ultraviolet metal absorption systems at z = 1.5216 +/- 0.0006 and at z = 1.295 +/- 0.001, the former likely associated with the GRB host galaxy. Analysis of the absorption lines at z = 1.5216 suggests that the host of this GRB may be a Damped Lyman-alpha Absorber. The optical V-band afterglow appears polarized, with P= (2.4 +/- 0.6) % and theta = (170 +/- 7) degrees, suggesting an asymmetric blastwave expansion. An X-ray-to-optical spectral flux distribution of the GRB 030328 afterglow was obtained at 0.78 days after the GRB and fitted using a broken powerlaw, with an optical spectral slope beta_{opt} = 0.47 +/- 0.15, and an X-ray slope beta_{X} = 1.0 +/- 0.2. The discussion of these results in the context of the "fireball model" shows that the preferred scenario for this afterglow is collimated structured jet with fixed opening angle in a homogeneous medium., Comment: 14 pages, 6 figures, 3 tables, accepted for publication on A&A, main journal. Abstract changed to comply with the new format of A&A abstracts

We present deep Hubble Space Telescope and Spitzer Space Telescope observations of the host galaxy of GRB 050904 at z=6.295. The host is detected in the H-band and marginally at 3.6 micron. From these detections, and limits in the z'-band and 4.5 micron, we infer an extinction-corrected absolute magnitude, M(UV)=-20.7 mag, or ~L*, a substantial star formation rate of 15 solar masses per year, and a stellar mass of a few 10^9 solar masses. A comparison to the published sample of spectroscopically-confirmed galaxies at z>5.5 reveals that the host of GRB 050904 would evade detection and/or confirmation in any of the current surveys due to the lack of detectable Ly-alpha emission, which is likely the result of dust extinction (A[1200]~1.5 mag). This suggests that not all luminous starburst galaxies at z~6 are currently being accounted for. Most importantly, using the metallicity of Z~0.05 solar inferred from the afterglow absorption spectrum, our observations indicate for the first time that the observed evolution in the mass- and luminosity-metallicity relations from z=0 to z~2 continues on to z>6. The ease of measuring redshifts and metallicities from the afterglow emission suggests that in tandem with the next generation ground- and space-based telescopes, a GRB mission with dedicated near-IR follow-up can provide unique information on the evolution of stars and galaxies through the epoch of re-ionization., Comment: Submitted to ApJ; 11 pages, 5 figures; A high-resolution version of figure 1 can be found at http://www.ociw.edu/~eberger/fig1.050904.berger.eps

We present four improved empirical relationships useful for estimating the central black hole mass in nearby AGNs and distant luminous quasars alike using either optical or UV single-epoch spectroscopy. These mass-scaling relationships between line widths and luminosity are based on recently improved empirical relationships between the broad-line region size and luminosities in various energy bands and are calibrated to the improved mass measurements of nearby AGNs based on emission-line reverberation mapping. The mass-scaling relationship based on the Hbeta line luminosity allows mass estimates for low-redshift sources with strong contamination of the optical continuum luminosity by stellar or non-thermal emission, while that based on the C IV lambda 1549 line dispersion allows mass estimates in cases where only the line dispersion (as opposed to the FWHM) can be reliably determined. We estimate that the absolute uncertainties in masses given by these mass-scaling relationships are typically around a factor of 4. We include in an Appendix mass estimates for all the Bright Quasar Survey (PG) quasars for which direct reverberation-based mass measurements are not available., Comment: 48 pages including 12 figures and 7 tables. Accepted by ApJ

We here report on the photometric, spectroscopic and polarimetric monitoring of the optical afterglow of the Gamma-Ray Burst (GRB) 030328 detected by HETE-2. We found that a smoothly broken power-law decay provides the best fit of the optical light curves, with indices alpha_1 = 0.76 +/- 0.03, alpha_2 = 1.50 +/- 0.07, and a break at t_b = 0.48 +/- 0.03 d after the GRB. Polarization is detected in the optical V-band, with P = (2.4 +/- 0.6)% and theta = (170 +/- 7) deg. Optical spectroscopy shows the presence of two absorption systems at z = 1.5216 +/- 0.0006 and at z = 1.295 +/- 0.001, the former likely associated with the GRB host galaxy. The X-ray-to-optical spectral flux distribution obtained 0.78 days after the GRB was best fitted using a broken power-law, with spectral slopes beta_opt = 0.47 +/- 0.15 and beta_X = 1.0 +/- 0.2. The discussion of these results in the context of the "fireball model" shows that the preferred scenario is a fixed opening angle collimated expansion in a homogeneous medium., Comment: To appear in the proceedings of the 16th Annual October Astrophysics Conference in Maryland, "Gamma Ray Bursts in the Swift Era", Washington DC., November 29-December 2, 2005, 4 pages, 3 figures

The most intense monitoring observations yet made in the optical (UBV) and near-infrared (JHK) wave bands were carried out for nearby Seyfert1 galaxies of NGC 5548, NGC 4051, NGC 3227, and NGC 7469. Over three years of observations with MAGNUM telescope since early 2001, clear time-delayed response of the K-band flux variations to the V-band flux variations was detected for all of these galaxies. Their H-K color temperature was estimated to be 1500-1800 K from the observed flux variation gradients, which supports a view that the bulk of the K flux should originate in the thermal radiation of hot dust that surrounds the central engine. Cross-correlation analysis was performed to quantify the lag time corresponding to the light-travel distance of the hot dust region from the central engine. The measured lag time is 47-53 days for NGC 5548, 11-18 days for NGC 4051, about 20 days for NGC 3227, and 65-87 days for NGC 7469. We found that the lag time is tightly correlated with the optical luminosity as expected from dust reverberation ($\Delta t \propto L^{0.5}$), while only weakly with the central virial mass, which suggests that an inner radius of the dust torus around the active nucleus has a one-to-one correspondence to central luminosity. In the lag time versus central luminosity diagram, the K-band lag times place an upper boundary on the similar lag times of broad-emission lines in the literature. This not only supports the unified scheme of AGNs, but also implies a physical transition from the BLR out to the dust torus that encircles the BLR. Furthermore, our V-band flux variations of NGC 5548 on timescales of up to 10 days are found to correlate with X-ray variations and delay behind them by one or two days, indicating the thermal reprocessing of X-ray emission by the central accretion flow., Comment: ApJ, March 2006, v639 issue, 24 pages, 33 figures, 10 tables

We present the results of the analysis of gamma-ray and X-ray data of GRB 050401 taken with the Swift satellite, together with a series of ground-based follow-up observations. The Swift X-ray light curve shows a clear break at about 4900 seconds after the GRB. The decay indices before and after the break are consistent with a scenario of continuous injection of radiation from the 'central engine' of the GRB to the fireball. Alternatively, this behaviour could result if ejecta are released with a range of Lorentz factors with the slower shells catching up the faster at the afterglow shock position. The two scenarios are observationally indistinguishable. The GRB 050401 afterglow is quite bright in the X-ray band but weak in the optical, with an optical to X-ray flux ratio similar to those of 'dark bursts'. We detect a significant amount of absorption in the X-ray spectrum, with N_H = (1.7 +/- 0.2) x 10^22 cm^-2 at a redshift of z=2.9, which is typical of a dense circumbust medium. Such high column density implies an unrealistic optical extinction of 30 magnitudes if we adopt the Galactic extinction law, which would not consistent with optical detection of the afterglow. This suggests that the extinction law is different from the Galactic one., Comment: 13 pages, 5 figures. MNRAS accepted

The final chapter in the long-standing mystery of the gamma-ray bursts (GRBs) centres on the origin of the short-hard class, suspected on theoretical grounds to result from the coalescence of neutron star or black hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical, and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst whose accurate position allows us to unambiguously associate it with a star-forming galaxy at redshift z=0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates., Comment: Nature in press (Oct 6 issue). 23 pages, 4 figures

Despite a rich phenomenology, gamma-ray bursts (GRBs) are divided into two classes based on their duration and spectral hardness -- the long-soft and the short-hard bursts. The discovery of afterglow emission from long GRBs was a watershed event, pinpointing their origin to star forming galaxies, and hence the death of massive stars, and indicating an energy release of about 10^51 erg. While theoretical arguments suggest that short GRBs are produced in the coalescence of binary compact objects (neutron stars or black holes), the progenitors, energetics, and environments of these events remain elusive despite recent localizations. Here we report the discovery of the first radio afterglow from a short burst, GRB 050724, which unambiguously associates it with an elliptical galaxy at a redshift, z=0.257. We show that the burst is powered by the same relativistic fireball mechanism as long GRBs, with the ejecta possibly collimated in jets, but that the total energy release is 10-1000 times smaller. More importantly, the nature of the host galaxy demonstrates that short GRBs arise from an old (>1 Gyr) stellar population, strengthening earlier suggestions, and providing support for coalescing compact object binaries as the progenitors., Comment: Accepted to Nature; revisions include broad-band afterglow model

We present a detailed analysis of the intrinsic X-ray absorption in the Seyfert 1 galaxy NGC 4151 using Chandra/HETGS data obtained 2002 May, as part of a program which included simultaneous UV spectra using HST/STIS and FUSE. NGC 4151 was in a relatively low flux state during the observations reported here, although roughly 2.5 times as bright in the 2 --10 keV band as during a Chandra observation in 2000. The soft X-ray band was dominated by emission lines, which show no discernible variation in flux between the two observations. The 2002 data show the presence of a very highly ionized absorber, in the form of H-like and He-like Mg, Si, and S lines, as well as lower ionization gas via the presence of inner-shell absorption lines from lower-ionization species of these elements. The former is too highly ionized to be radiatively accelerated in a sub-Eddington source such as NGC 4151. We find that the lower ionization gas had a column density a factor of ~ 3 higher during the 2000 observation. If due to bulk motion, we estimate that this component must have a velocity of more than 1250 km/sec transverse to our line-of-sight. We suggest that these results are consistent with a magneto-hydrodynamic flow., Comment: 42 pages, 14 figures. Accepted for publication in The Astrophysical Journal

We present results on the location, physical conditions, and geometry of the outflow in the Seyfert 1 galaxy NGC 3783 from a study of the variable intrinsic UV absorption. Based on 18 observations with HST/STIS and 6 observations with FUSE, we find: 1) The absorption from the lowest-ionization species in each of the three strong kinematic components varied inversely with the continuum flux, indicating the ionization structure responded to changes in the photoionizing flux over the weekly timescales sampled by our observations. 2) A multi- component model with an unocculted NLR and separate BLR and continuum line-of-sight covering factors predicts saturation in several lines, consistent with the lack of observed variability. 3) Column densities for the individual metastable levels are measured from the resolved C III *1175 absorption complex observed in one component. Based on our computed metastable level populations, the electron density of this absorber is ~3x10^4 cm^-3. Photoionization modeling results place it at ~25 pc from the central source. 4) Using time-dependent calculations, we are able to reproduce the detailed variability observed in this absorber, and derive upper limits on the distances for the other components of 25-50 pc. 5) The ionization parameters derived for the higher ionization UV absorbers are consistent with the modeling results for the lowest-ionization X-ray component, but with smaller total column density. They have similar pressures as the three X-ray ionization components. These results are consistent with an inhomogeneous wind model for the outflow in NGC 3783. 6) Based on the predicted emission-line luminosities, global covering factor constraints, and distances derived for the UV absorbers, they may be identified with emission- line gas observed in the inner NLR of AGNs. (abridged), Comment: 30 pages, 18 figures (7 color), emulateapj, accepted for publication in The Astrophysical Journal

We present optical, near-IR, and radio follow up of sixteen Swift bursts, including our discovery of nine afterglows and a redshift determination for three. These observations, supplemented by data from the literature, provide an afterglow recovery rate of 60% in the optical/near-IR, much higher than in previous missions (BeppoSAX, HETE-2, INTEGRAL, and IPN). The optical/near-IR afterglows of Swift events are on average 1.7 mag fainter at t=12 hr than those of previous missions. The X-ray afterglows are similarly fainter compared to those of pre-Swift bursts. In the radio the limiting factor is the VLA threshold and the detection rate for Swift bursts is similar to that for past missions. The redshift distribution of pre-Swift bursts peaked at z~1, whereas the five Swift bursts with measured redshifts are distributed evenly between 1.3 and 3.2. From these results we conclude that (i) the pre-Swift distributions were biased in favor of bright events and low redshift events, (ii) the higher sensitivity and accurate positions of Swift result in a better representation of the true burst redshift and brightness distributions (which are higher and dimmer, respectively), and (iii) as many as 1/3 of the bursts can be optically dark, as a result of a high redshift and/or dust extinction. We remark that the apparent lack of low redshift, low luminosity Swift bursts, and the lower event rate compared to pre-launch estimates (90 vs. 150 per year), are the result of a threshold that is similar to that of BATSE. In view of these inferences, afterglow observers may find it advisable to make significant changes in follow up strategies of Swift events. [abridged], Comment: Submitted to ApJ; 15 pages, 8 figures, 1 table

We present the results from a {\it Hubble Space Telescope/ACS} study of the supernovae associated with gamma-ray bursts 040924 ($z=0.86$) and 041006 ($z=0.71$). We find evidence that both GRBs were associated with a SN 1998bw-like supernova dimmed by $\sim 1.5$ and $\sim 0.3$ magnitudes, respectively, making GRB 040924 the faintest GRB-associated SN ever detected. We study the luminosity dispersion in GRB/XRF-associated SNe and compare to local Type Ibc supernovae from the literature. We find significant overlap between the two samples, suggesting that GRB/XRF-associated SNe are not necessarily more luminous nor produce more $^{56}$Ni than local SNe. Based on the current (limited) datasets, we find that the two samples may share a similar $^{56}$Ni production mechanism., Comment: ApJ accepted (in press). Revised version. High-resolution figures available at http://www.astro.caltech.edu/~ams/GRB-SNe.html

We present the results from an Hubble Space Telescope/ACS search for supernovae associated with X-ray flashes 020903, 040701, 040812 and 040916. We find strong evidence that XRF 020903 (z=0.25) was associated with a SN 1998bw-like supernova and confirm this using optical spectroscopy at t ~ 25 days. We find no evidence, however, for SN 1998bw-like supernovae associated with the other three events. In the case of XRF 040701 (z=0.21), we rule out even a faint supernova similar to SN 2002ap, using template light-curves for several local Type Ic supernovae. For the two cases in which the redshift is not known, XRFs 040812 and 040916, we derive robust redshift limits assuming they were accompanied by supernovae similar to SN 1998bw and compare these limits with photometric redshift constraints provided by their host galaxies. We supplement this analysis with results for three additional events (XRFs 011030, 020427 and 030723) and discuss the observed diversity of supernovae associated with X-ray flashes and gamma-ray bursts. We conclude that XRF-SNe exist, but can be significantly fainter than SN 1998bw, possibly consistent with the observed spread in local Type Ibc supernovae., Comment: Submitted to ApJ, 22 pages, 11 figures. High resolution figures available at http://www.astro.caltech.edu/~ams/XRF-SNe.html

We study the low-contrast Fe II emission blends in the ultraviolet (1250--2200A) and optical (4000--6000A) spectra of the Seyfert 1 galaxy NGC 5548 and show that these features vary in flux and that these variations are correlated with those of the optical continuum. The amplitude of variability of the optical Fe II emission is 50% - 75% that of Hbeta and the ultraviolet Fe II emission varies with an even larger amplitude than Hbeta. However, accurate measurement of the flux in these blends proves to be very difficult even using excellent Fe II templates to fit the spectra. We are able to constrain only weakly the optical Fe II emission-line response timescale to a value less than several weeks; this upper limit exceeds all the reliably measured emission-line lags in this source so it is not particularly meaningful. Nevertheless, the fact that the optical Fe II and continuum flux variations are correlated indicates that line fluorescence in a photoionized plasma, rather than collisional excitation, is responsible for the Fe II emission. The iron emission templates are available upon request., Comment: 34 pages including 12 figures and 2 tables. Accepted for publication by ApJ (tentatively in vol. 626 June 10, 2005)

We present a power spectrum analysis of the final 2dF Galaxy Redshift Survey, employing a direct Fourier method. The sample used comprises 221,414 galaxies with measured redshifts. We investigate in detail the modelling of the sample selection. A new angular mask is derived, based on revisions to the photometric calibration. The redshift selection function is determined by dividing the survey according to rest-frame colour, and deducing a self-consistent treatment of k-corrections and evolution for each population. The covariance matrix for the power-spectrum estimates is determined using two different approaches to the construction of mock surveys which are used to demonstrate that the input cosmological model can be correctly recovered. We are confident that the 2dFGRS power spectrum can be used to infer the matter content of the universe. On large scales, our estimated power spectrum shows evidence for the `baryon oscillations' that are predicted in CDM models. Fitting to a CDM model, assuming a primordial $n_{s}=1$ spectrum, $h=0.72$ and negligible neutrino mass, the preferred parameters are $\Omega_{M} h = 0.168 \pm 0.016$ and a baryon fraction $\Omega_{b} /\Omega_{M} = 0.185\pm0.046$ (1$\sigma$ errors). The value of $\Omega_{M} h$ is $1\sigma$ lower than the $0.20 \pm 0.03$ in our 2001 analysis of the partially complete 2dFGRS. This shift is largely due to the signal from the newly-sampled regions of space, rather than the refinements in the treatment of observational selection. This analysis therefore implies a density significantly below the standard $\Omega_{M} =0.3$: in combination with CMB data from WMAP, we infer $\Omega_{M} =0.231\pm 0.021$. (Abridged.), Comment: Updated to match the in press MNRAS version. P(k) data and covariance matrix available at http://www.mso.anu.edu.au/2dFGRS/Public/Release/PowSpec

We present a catalog of 450 high signal-to-noise microlensing events observed by the MACHO collaboration between 1993 and 1999. The events are distributed throughout our fields and, as expected, they show clear concentration toward the Galactic center. No optical depth is given for this sample since no blending efficiency calculation has been performed, and we find evidence for substantial blending. In a companion paper we give optical depths for the sub-sample of events on clump giant source stars, where blending is not a significant effect. Several events with sources that may belong to the Sagittarius dwarf galaxy are identified. For these events even relatively low dispersion spectra could suffice to classify these events as either consistent with Sagittarius membership or as non-Sagittarius sources. Several unusual events, such as microlensing of periodic variable source stars, binary lens events, and an event showing extended source effects are identified. We also identify a number of contaminating background events as cataclysmic variable stars., Comment: 34 pages, 9 figures + 3 example lightcurves, all 564 lightcurves will be available at http://wwwmacho.mcmaster.ca, submitted to ApJ, see companion paper by Popowski et al

Using 7 years of MACHO survey data, we present a new determination of the optical depth to microlensing towards the Galactic bulge. We select the sample of 62 microlensing events (60 unique) on clump giant sources and perform a detailed efficiency analysis. We use only the clump giant sources because these are bright bulge stars and are not as strongly affected by blending as other events. Using a subsample of 42 clump events concentrated in just 4.5 square degrees, we find tau = 2.17^{+0.47}_{-0.38} x 10^{-6} at (l,b) = (1.50, -2.68), somewhat smaller than found in most previous MACHO studies, but in excellent agreement with recent theoretical predictions. We also present the optical depth in each of the 19 fields in which we detected events, and find limits on optical depth for fields with no events. The errors in optical depth in individual fields are dominated by Poisson noise. We measure optical depth gradients (1.06 +/- 0.71) x 10^{-6} deg^{-1} and (0.29 +/- 0.43) x 10^{-6} deg^{-1} in the galactic latitude b and longitude l directions, respectively. Finally, we discuss the possibility of anomalous duration distribution of events in the field 104 centered on (l,b) = (3.11, -3.01) as well as investigate spatial clustering of events in all fields., Comment: 42 pages, 15 figures (6 of them in color), 12 tables; results unchanged, discussion of blending strengthened including addition of new appendix; to be published in ApJ, Vol. 630, Sept. 10, 2005 issue

We use the 2dF Galaxy Redshift Survey to measure the dependence of the bJ-band galaxy luminosity function on large-scale environment, defined by density contrast in spheres of radius 8h-1Mpc, and on spectral type, determined from principal component analysis. We find that the galaxy populations at both extremes of density differ significantly from that at the mean density. The population in voids is dominated by late types and shows, relative to the mean, a deficit of galaxies that becomes increasingly pronounced at magnitudes brighter than M_bJ-5log10h <-18.5. In contrast, cluster regions have a relative excess of very bright early-type galaxies with M_bJ-5log10h < -21. Differences in the mid to faint-end population between environments are significant: at M_bJ-5log10h=-18 early and late-type cluster galaxies show comparable abundances, whereas in voids the late types dominate by almost an order of magnitude. We find that the luminosity functions measured in all density environments, from voids to clusters, can be approximated by Schechter functions with parameters that vary smoothly with local density, but in a fashion which differs strikingly for early and late-type galaxies. These observed variations, combined with our finding that the faint-end slope of the overall luminosity function depends at most weakly on density environment, may prove to be a significant challenge for models of galaxy formation., Comment: 14 pages, 12 figures, accepted MNRAS

Reverberation mapping is a proven technique that is used to measure the size of the broad emission-line region and central black hole mass in active galactic nuclei. More ambitious reverberation mapping programs that are well within the capabilities of Hubble Space Telescope could allow us to determine the nature and flow of line-emitting gas in active nuclei and to assess accurately the systematic uncertainties in reverberation-based black hole mass measurements., Comment: 10 pages, 6 figures. To appear in Planets to Cosmology: Essential Science in Hubble's Final Years, ed. M. Livio (Cambridge: CUP), in press (2005)

We present improved black hole masses for 35 active galactic nuclei (AGNs) based on a complete and consistent reanalysis of broad emission-line reverberation-mapping data. From objects with multiple line measurements, we find that the highest precision measure of the virial product is obtained by using the cross-correlation function centroid (as opposed to the cross-correlation function peak) for the time delay and the line dispersion (as opposed to full width half maximum) for the line width and by measuring the line width in the variable part of the spectrum. Accurate line-width measurement depends critically on avoiding contaminating features, in particular the narrow components of the emission lines. We find that the precision (or random component of the error) of reverberation-based black hole mass measurements is typically around 30%, comparable to the precision attained in measurement of black hole masses in quiescent galaxies by gas or stellar dynamical methods. Based on results presented in a companion paper by Onken et al., we provide a zero-point calibration for the reverberation-based black hole mass scale by using the relationship between black hole mass and host-galaxy bulge velocity dispersion. The scatter around this relationship implies that the typical systematic uncertainties in reverberation-based black hole masses are smaller than a factor of three. We present a preliminary version of a mass-luminosity relationship that is much better defined than any previous attempt. Scatter about the mass-luminosity relationship for these AGNs appears to be real and could be correlated with either Eddington ratio or object inclination., Comment: 61 pages, including 8 Tables and 16 Figures. Accepted for publication in The Astrophysical Journal

Interpretation of the \ion{Fe}{2}(UV)/\ion{Mg}{2} emission ratios from quasars has a major cosmological motivation. Both Fe and Mg are produced by short-lived massive stars. In addition, Fe is produced by accreting white dwarf supernovae somewhat after star formation begins. Therefore, we expect that the Fe/Mg ratio will gradually decrease with redshift. We have used data from the Sloan Digital Sky Survey to explore the dependence of the \ion{Fe}{2}(UV)/\ion{Mg}{2} ratio on redshift and on luminosity in the redshift range of $0.75< z< 2.20$, and we have used predictions from our 830-level model for the \ion{Fe}{2} atom in photoionization calculations to interpret our findings. We have split the quasars into several groups based upon the value of their \ion{Fe}{2}(UV)/\ion{Mg}{2} emission ratios, and then checked to see how the fraction of quasars in each group varies with the increase of redshift. We next examined the luminosity dependence of the \ion{Fe}{2}(UV)/\ion{Mg}{2} ratio, and we found that beyond a threshold of \ion{Fe}{2}(UV)/\ion{Mg}{2} =~ 5, and $M_{2500} < -25\rm mag$, the \ion{Fe}{2}(UV)/\ion{Mg}{2} ratio increases with luminosity, as predicted by our model. We interpret our observed variation of the \ion{Fe}{2}(UV)/\ion{Mg}{2} ratio with redshift as a result of the correlation of redshift with luminosity in a magnitude limited quasar sample., Comment: ApJL accepted

We present some preliminary results from an on-going study of the evolution of stellar populations in rich clusters of galaxies. This sample contains core line-strength measurements from 183 galaxies with b_J <= 19.5 from four clusters with ~0.04. Using predictions from stellar population models to compare with our measured line strengths we can derive relative luminosity-weighted mean ages and metallicities for the stellar populations in each of our clusters. We also investigate the Mgb'-sigma and Hbeta_G'-sigma scaling relations. We find that, consistent with previous results, Mgb' is correlated with sigma, the likely explanation being that larger galaxies are better at retaining their heavier elements due to their larger potentials. Hbeta', on the other hand, we find to be anti-correlated with sigma. This result implies that the stellar populations in larger galaxies are older than in smaller galaxies., Comment: 3 pages, 2 figures, to appear in the Proceedings of IAU Colloquium 195: "Outskirts of Galaxy Clusters: intense life in the suburbs", Torino Italy, March 12-16 2004

The Burst and Transient Source Experiment (BATSE), aboard the Compton Gamma Ray Observatory (CGRO), provided a record of the low-energy gamma-ray sky (20-1000 keV) between 1991 April and 2000 May (9.1y). Using the Earth Occultation Technique to extract flux information, a catalog of sources using data from the BATSE large area detectors has been prepared. The first part of the catalog consists of results from the monitoring of 58 sources, mostly Galactic. For these sources, we have included tables of flux and spectral data, and outburst times for transients. Light curves (or flux histories) have been placed on the world wide web. We then performed a deep-sampling of 179 objects (including the aforementioned 58 objects) combining data from the entire 9.1y BATSE dataset. Source types considered were primarily accreting binaries, but a small number of representative active galaxies, X-ray-emitting stars, and supernova remnants were also included. The deep sample results include definite detections of 83 objects and possible detections of 36 additional objects. The definite detections spanned three classes of sources: accreting black hole and neutron star binaries, active galaxies and supernova remnants. Flux data for the deep sample are presented in four energy bands: 20-40, 40-70, 70-160, and 160-430 keV. The limiting average flux level (9.1 y) for the sample varies from 3.5 to 20 mCrab (5 sigma) between 20 and 430 keV, depending on systematic error, which in turn is primarily dependent on the sky location. To strengthen the credibility of detection of weaker sources (5-25 mCrab), we generated Earth occultation images, searched for periodic behavior using FFT and epoch folding methods, and critically evaluated the energy-dependent emission in the four flux bands., Comment: 64 pages, 17 figures, abstract abridged, Accepted by ApJS

It is well known that the clustering of galaxies depends on galaxy type.Such relative bias complicates the inference of cosmological parameters from galaxy redshift surveys, and is a challenge to theories of galaxy formation and evolution. In this paper we perform a joint counts-in-cells analysis on galaxies in the 2dF Galaxy Redshift Survey, classified by both colour and spectral type, eta, as early or late type galaxies. We fit three different models of relative bias to the joint probability distribution of the cell counts, assuming Poisson sampling of the galaxy density field. We investigate the nonlinearity and stochasticity of the relative bias, with cubical cells of side 10Mpc \leq L \leq 45Mpc (h=0.7). Exact linear bias is ruled out with high significance on all scales. Power law bias gives a better fit, but likelihood ratios prefer a bivariate lognormal distribution, with a non-zero `stochasticity' - i.e. scatter that may result from physical effects on galaxy formation other than those from the local density field. Using this model, we measure a correlation coefficient in log-density space (r_LN) of 0.958 for cells of length L=10Mpc, increasing to 0.970 by L=45Mpc. This corresponds to a stochasticity sigma_b/bhat of 0.44\pm0.02 and 0.27\pm0.05 respectively. For smaller cells, the Poisson sampled lognormal distribution presents an increasingly poor fit to the data, especially with regard to the fraction of completely empty cells. We compare these trends with the predictions of semianalytic galaxy formation models: these match the data well in terms of overall level of stochasticity, variation with scale, and fraction of empty cells., Comment: 24 pages, 13 figures, LaTex, minor changes to match version accepted to MNRAS. Version with high resolution and colour figures available from http://www.ast.cam.ac.uk/~vw/stocbias.html

We present an analysis of the relative bias between early- and late-type galaxies in the Two-degree Field Galaxy Redshift Survey (2dFGRS). Our analysis examines the joint counts in cells between early- and late-type galaxies, using approximately cubical cells with sides ranging from 7h^{-1}Mpc to 42h^{-1}Mpc. We measure the variance of the counts in cells using the method of Efstathiou et al. (1990), which we find requires a correction for a finite volume effect. We fit lognormal models to the one-point density distribution and develop methods of dealing with biases in the recovered variances resulting from this technique. We directly fit deterministic models for the joint density distribution function, f(delta_E,delta_L), to the joint counts in cells using a maximum likelihood technique. Our results are consistent with a scale invariant relative bias factor on all scales studied. Linear bias is ruled out on scales less than l=28h^{-1}Mpc. A power-law bias model is a significantly better fit to the data on all but the largest scales studied; the relative goodness of fit of this model as compared to that of the linear bias model suggests that any nonlinearity is negligible for l>~40h^{-1}Mpc, consistent with the expectation from theory that the bias should become linear on large scales. (abridged), Comment: 19 pages, 17 figures, submitted to MNRAS

We measure the clustering of galaxy groups in the 2dFGRS Percolation-Inferred Galaxy Group (2PIGG) catalogue. The 2PIGG sample has 29,000 groups with at least two members. The clustering amplitude of the full 2PIGG catalogue is weaker than that of 2dFGRS galaxies, in agreement with theoretical predictions. We have subdivided the 2PIGG catalogue into samples that span a factor of 25 in median total luminosity. Our correlation function measurements span an unprecedented range of clustering strengths, connecting the regimes probed by groups fainter than L* galaxies and rich clusters. There is a steady increase in clustering strength with group luminosity; the most luminous groups are ten times more strongly clustered than the full 2PIGG catalogue. We demonstrate that the 2PIGG results are in very good agreement with the clustering of groups expected in the LCDM model., Comment: 16 pages. Submitted to MNRAS

The construction of a catalogue of galaxy groups from the 2-degree Field Galaxy Redshift Survey (2dFGRS) is described. Groups are identified by means of a friends-of-friends percolation algorithm which has been thoroughly tested on mock versions of the 2dFGRS generated from cosmological N-body simulations. The tests suggest that the algorithm groups all galaxies that it should be grouping, with an additional 40% of interlopers. About 55% of the ~190000 galaxies considered are placed into groups containing at least two members of which ~29000 are found. Of these, ~7000 contain at least four galaxies, and these groups have a median redshift of 0.11 and a median velocity dispersion of 260km/s. This 2dFGRS Percolation-Inferred Galaxy Group (2PIGG) catalogue represents the largest available homogeneous sample of galaxy groups. It is publicly available on the WWW., Comment: 15 pages, 8 figures (1 in colour), MNRAS, 348, 866 (as before)

The 2dFGRS Percolation-Inferred Galaxy Group (2PIGG) catalogue of ~29000 objects is used to study the luminous content of galaxy systems of various sizes. Mock galaxy catalogues constructed from cosmological simulations are used to gauge the accuracy with which intrinsic group properties can be recovered. A Schechter function is found to be a reasonable fit to the galaxy luminosity functions in groups of different mass in the real data. The characteristic luminosity L* is larger for more massive groups. However, the mock data show that the shape of the recovered luminosity function is expected to differ from the true shape, and this must be allowed for when interpreting the data. The variation of halo mass-to-light ratio with group size is studied in both these wavebands. A robust trend of increasing M/L with increasing group luminosity is found in the 2PIGG data. From groups with L_bj=10^{10}Lsol to those 100 times more luminous, the typical bj-band M/L increases by a factor of 5, whereas the rf-band M/L grows by a factor of 3.5. These trends agree well with the simulations, which also predict a minimum M/L on a scale corresponding to the Local Group. Our data indicate that if such a minimum exists, then it must occur at L<~10^{10}Lsol, below the range accurately probed by the 2PIGG catalogue. According to the mock data, the bj M/Ls of the largest groups are expected to be approximately 1.1 times the global value. Assuming that this correction applies to the real data yields an estimate of Omega_m=0.26+/-0.03 (statistical)., Comment: Same as before

We measure moments of the galaxy count probability distribution function in the two-degree field galaxy redshift survey (2dFGRS). The survey is divided into volume limited subsamples in order to examine the dependence of the higher order clustering on galaxy luminosity. We demonstrate the hierarchical scaling of the averaged p-point galaxy correlation functions, xibar_p, up to p=6. The hierarchical amplitudes, S_p = xibar_p/xibar_2^{p-1}, are approximately independent of the cell radius used to smooth the galaxy distribution on small to medium scales. On larger scales we find the higher order moments can be strongly affected by the presence of rare, massive superstructures in the galaxy distribution. The skewness S_3 has a weak dependence on luminosity, approximated by a linear dependence on log luminosity. We discuss the implications of our results for simple models of linear and non-linear bias that relate the galaxy distribution to the underlying mass., Comment: 14 pages, 10 figures. Slight revisions to match version published in MNRAS

We use the two-degree field Galaxy Redshift Survey (2dFGRS) to test the hierarchical scaling hypothesis: namely, that the $p$-point galaxy correlation functions can be written in terms of the two point correlation function or variance. This scaling is expected if an initially Gaussian distribution of density fluctuations evolves under the action of gravitational instability. We measure the volume averaged $p$-point correlation functions using a counts in cells technique applied to a volume limited sample of 44,931 $L_*$ galaxies. We demonstrate that $L_{*}$ galaxies display hierarchical clustering up to order $p=6$ in redshift space. The variance measured for $L_{*}$ galaxies is in excellent agreement with the predictions from a $\Lambda$-cold dark matter N-body simulation. This applies to all cell radii considered, $0.3<(R/h^{-1}{\rm Mpc})<30$. However, the higher order correlation functions of $L_*$ galaxies have a significantly smaller amplitude than is predicted for the dark matter for $R<10h^{-1}$Mpc. This disagreement implies that a non-linear bias exists between the dark matter and $L_*$ galaxies on these scales. We also show that the presence of two rare, massive superclusters in the 2dFGRS has an impact on the higher-order clustering moments measured on large scales., Comment: 5 pages, 4 figures. Slight revisions to match version published in MNRAS

We measure the redshift space reduced void probability function (VPF) for 2dFGRS volume limited galaxy samples covering the absolute magnitude range M_bJ-5logh=-18 to -22. Theoretically, the VPF connects the distribution of voids to the moments of galaxy clustering of all orders, and can be used to discriminate clustering models in the weakly non-linear regime. The reduced VPF measured from the 2dFGRS is in excellent agreement with the paradigm of hierarchical scaling of the galaxy clustering moments. The accuracy of our measurement is such that we can rule out, at a very high significance, popular models for galaxy clustering, including the lognormal distribution. We demonstrate that the negative binomial model gives a very good approximation to the 2dFGRS data over a wide range of scales, out to at least 20h-1Mpc. Conversely, the reduced VPF for dark matter in a LambdaCDM universe does appear to be lognormal on small scales but deviates significantly beyond \approx 4h-1Mpc. We find little dependence of the 2dFGRS reduced VPF on galaxy luminosity. Our results hold independently in both the north and south Galactic pole survey regions., Comment: 10 pages, 6 figures. Dark matter comparison added. Accepted MNRAS

(Abridged) We present densely sampled BVRI light curves of the optical transient associated with the gamma-ray burst GRB 030329, the result of a coordinated observing campaign conducted at five observatories. Augmented with published observations of this GRB, the compiled optical dataset contains 2687 photometric measurements, obtained between 78 minutes and 79 days after the burst. We show that the underlying supernova 2003dh evolved faster than, and was probably somewhat fainter than the type Ic SN 1998bw, associated with GRB 980425. We find that our data can be described by a broken power-law decay perturbed by a complex variable component. The early- and late-time decay slopes are determined to be ~1.1 and ~2, respectively. Assuming this single power-law model, we constrain the break to lie between ~3 and ~8 days after the burst. This simple, singly-broken power-law model, derived only from the analysis of our optical observations, may also account for available multi-band data, provided that the break happened ~8 days after the burst. The more complex double-jet model of Berger et al. provides a comparable fit to the optical, X-ray, mm and radio observations of this event. We detect a significant change in optical colors during the first day. Our color analysis is consistent with a cooling break frequency sweeping through the optical band during the first day. The light curves of GRB 030329 reveal a rich array of variations, superposed over the mean power-law decay. We find that the early variations are asymmetric, with a steep rise followed by a relatively slower (by a factor of about two) decline. The variations maintain a similar time scale during the first four days, and then get significantly longer., Comment: 14 pages, 12 figures, accepted for publication in ApJ with minor changes. See the GRB030329 Data Treasury at http://wise-obs.tau.ac.il/GRB030329/

The most intense monitoring observations yet made were carried out on the Seyfert 1 galaxy NGC 4151 in the optical and near-infrared wave-bands. A lag from the optical light curve to the near-infrared light curve was measured. The lag-time between the V and K light curves at the flux minimum in 2001 was precisely 48+2-3 days, as determined by a cross-correlation analysis. The correlation between the optical luminosity of an active galactic nucleus (AGN) and the lag-time between the UV/optical and the near-infrared light curves is presented for NGC 4151 in combination with previous lag-time measurements of NGC 4151 and other AGNs in the literature. This correlation is interpreted as thermal dust reverberation in an AGN, where the near-infrared emission from an AGN is expected to be the thermal re-radiation from hot dust surrounding the central engine at a radius where the temperature equals to that of the dust sublimation temperature. We find that the inner radius of the dust torus in NGC 4151 is $\sim $ 0.04 pc corresponding to the measured lag-time, well outside the broad line region (BLR) determined by other reverberation studies of the emission lines., Comment: Accepted for publication in ApJ Letters, 13 pages, 3 figures; Corrected typos

We present the intrinsic spectral energy distribution (SED) of the NLS1 Arakelian 564, constructed with contemporaneous data obtained during a multi-wavelength, multi-satellite observing campaign in 2000 and 2001. We compare it with that of the NLS1 Ton S180 and with those obtained for BLS1s to infer how the relative accretion rates vary among the Sy1 population. Although the peak of the SED is not well constrained, most of the energy is emitted in the 10-100 eV regime, constituting roughly half of the emitted energy in the optical/X-ray ranges. This is consistent with a primary spectral component peaking in the extreme UV/soft X-ray band, and disk-corona models, hence high accretion rates. Indeed, we estimate that \dot{m}~1. We examine the emission lines in its spectrum, and we constrain the physical properties of the line-emitting gas through photoionization modeling. The line-emitting gas is characterized by log n~11 and log U~0, and is stratified around log U~0. Our estimate of the radius of the H\beta-emitting region ~10 \pm 2 lt-days is consistent with the radius-luminosity relationships found for Sy1 galaxies. We also find evidence for super-solar metallicity in this NLS1. We show that the emission lines are not good diagnostics for the underlying SEDs and that the absorption line studies offer a far more powerful tool to determine the ionizing continuum of AGNs, especially if comparing the lower- and higher-ionization lines., Comment: 15 pages, 10 figures; accepted for publication in The Astrophysical Journal, LaTeX emulateapj.sty

Fourier coefficents have been derived for the $V$ and $R$ light curves of 785 overtone RR Lyrae variables in 16 MACHO fields near the bar of the LMC. The $\phi_{31}$ and $R_{21}$ coefficients have been compared with those of the first overtone RR Lyrae variables in the Galactic globular clusters NGC 6441, M107, M5, M3, M2, $\omega$ Centauri and M68. The results indicate that many of the LMC variables have properties similar to the ones in M2, M3, M5 and the Oosterhoff type I variables in $\omega$ Cen, but they are different from the Oosterhoff type II variables in $\omega$ Cen. Equations derived from hydrodynamic pulsation models have been used to calculate the luminosity and temperature for the 330 bona fide first-overtone variables. The results indicate that they have $\log L$ in the range 1.6 to $1.8\lsun$ and $\log T_{eff}$ between 3.85 and 3.87. Based on these temperatures, a mean color excess $E(V-R) =0.08$ mag, equivalent to $E(B-V)=0.14$ mag, has been estimated for these 330 stars. The 80 M5-like variables (selected according to their location in the $\phi_{31}-\log P$ plot) are used to determine a LMC distance. After correcting for the effects of extinction and crowding, a mean apparent magnitude $=18.99 \pm 0.02$ (statistical) $\pm 0.16$ (systematic) has been estimated for these 80 stars. Combining this with a mean absolute magnitude $M_V=0.56\pm 0.06$ for M5-like stars derived from Baade-Wesselink analyses, main sequence fitting, Fourier parameters and the trigonometric parallax of RR Lyrae, we derive an LMC distance modulus $\mu=18.43\pm 0.06$ (statistical) $\pm 0.16$ (systematic) mag. The large systematic error arises from the difficulties of correcting for interstellar extinction and for crowding., Comment: 51 pages, 17 figures, 12 tables, accepted to AJ

Many past studies of cosmological gamma-ray bursts (GRBs) have been limited because of the large distance to typical GRBs, resulting in faint afterglows. There has long been a recognition that a nearby GRB would shed light on the origin of these mysterious cosmic explosions, as well as the physics of their fireballs. However, GRBs nearer than z=0.2 are extremely rare, with an estimated rate of localisation of one every decade. Here, we report the discovery of bright optical afterglow emission from GRB 030329. Our prompt dissemination and the brilliance of the afterglow resulted in extensive followup (more than 65 telescopes) from radio through X-ray bands, as well as measurement of the redshift, z=0.169. The gamma-ray and afterglow properties of GRB 030329 are similar to those of cosmological GRBs (after accounting for the small distance), making this the nearest known cosmological GRB. Observations have already securely identified the progenitor as a massive star that exploded as a supernova, and we anticipate futher revelations of the GRB phenomenon from studies of this source., Comment: 13 pages, 4 figures. Original text

We present a detailed spectral analysis of the data obtained from NGC 3783 during the period 2000-2001 using Chandra. This analysis leads us to the following results. 1) NGC 3783 fluctuated in luminosity by a factor ~1.5 during individual observations (~170 ks duration). These fluctuations were not associated with significant spectral variations. 2) On a longer time scale (20-120 days), we found the source to exhibit two very different spectral shapes. The main difference between these can be well-described by the appearance and disappearance of a spectral component that dominates the underlying continuum at the longest wavelengths. The spectral variations are not related to the brightening or the fading of the continuum at short wavelengths in any simple way. 3) The appearance of the soft continuum component is consistent with being the only spectral variation, and there is no need to invoke changes in the opacity of the absorbers. 4) Photoionization modeling indicates that a combination of three ionized absorbers, each split into two kinematic components, can explain the strengths of almost all the absorption lines and bound-free edges. All three components are thermally stable and seem to have the same gas pressure. 5) The only real discrepancy between our model and the observations concerns the range of wavelengths absorbed by the iron M-shell UTA feature. This most likely arises as the result of our underestimation of the poorly-known dielectronic recombination rates appropriate for these ions. 6) The lower limit on the distance of the absorbing gas in NGC 3783 is between 0.2 and 3.2 pc. The assumption of pressure equilibrium imposes an upper limit of about 25 pc on the distance of the least-ionized component from the central source. (abridged), Comment: 16 pages, 12 figures (9 in color), emulateapj5, accepted for publication in The Astrophysical Journal

We have frequency analyzed 6391 variables classified earlier as fundamental mode RR Lyrae (RR0) stars in the MACHO database on the Large Magellanic Cloud (LMC). The overwhelming majority of these variables have been proved to be indeed RR0 stars, whereas the remaining ones have fallen in one of the following categories: single- and double-mode Cepheids, binaries, first overtone and double-mode RR Lyrae stars and non-classified variables. Special attention has been paid to the Blazhko stars. We found altogether 731 Blazhko variables showing either a doublet or an equidistant triplet pattern at the main pulsation component in their frequency spectra. The incidence rate of the Blazhko variables among the RR0 stars in the LMC is 11.9%, which is three times higher than their rate among the first overtone RR Lyrae stars. From the frequency spectra we found that variables with larger modulation amplitudes at the higher frequency side of the main pulsation component are three times more frequent than the ones showing the opposite amplitude pattern. A search made for a modulation component with the Blazhko period in the average brightness of the individual variables showed the existence of such a modulation with an overall amplitude of ~0.006 mag. On the other hand, a similar search for quadruple modulation patterns around the main pulsation component have failed to clearly detect such components at the ~0.004 mag level. This means that the amplitudes of the quadruple components should be, on the average, at least ten times smaller than those of the triplet components. This finding and the existence of Blazhko variables with highly asymmetric modulation amplitudes not only question the validity of the magnetic oblique rotator model, but also puts stringent constraints on models based on mode coupling theories., Comment: 14 pages, 15 figures, 6 tables, to appear in ApJ (Vol. 598, November 20, 2003)

We report an intrinsic absorber with decreasing outflow velocity in the Seyfert 1 galaxy NGC 3783. This is the first detection of a change in radial velocity in an outflow associated with a Seyfert galaxy. These results are based on measurements from 18 observations with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope, obtained between 2000 February and 2002 January. In two intervals separated by ~13 and 9 months, the absorption lines in the kinematic component with highest outflow velocity exhibited mean redward velocity shifts of ~35 and 55 km/s, respectively. The rate of velocity decrease was 2.2 +/- 0.6 times more rapid in the second interval. No variations in absorption velocities were detected in the other kinematic components. We explore potential interpretations of the observed velocity shifts: radial deceleration of the UV absorber due to a change in either the speed or direction of motion of the outflow, and the evolution of a continuous flow across our line of sight to the emission source., Comment: 17 pages, 5 figures, accepted for publication in the Astrophysical Journal