Search

We present the first systematic study of 50 low-redshift (0.66 < z < 1.63) iron low-ionization broad absorption-line quasars (FeLoBALQs) using SimBAL, which represents a more than five-fold increase in the number of FeLoBALQs with detailed absorption line spectral analyses. We found the outflows have a wide range of ionization parameters, − 4 ≲ log U ≲ 1.2 and densities, 2.8 ≲ log n ≲ 8 [ cm − 3 ] . The objects in our sample showed FeLoBAL gas located at a wide range of distances 0 ≲ log R ≲ 4.4 [pc], although we do not find any evidence for disk winds (with R ≪ 0.01 pc) in our sample. The outflow strength primarily depends on the outflow velocity with faster outflows found in quasars that are luminous or that have flat or redder spectral energy distributions. We found that ∼18% of the FeLoBALQs in the sample have the significantly powerful outflows needed for quasar feedback. Eight objects showed overlapping troughs in the spectra, and we identified eleven loitering outflow objects, a new class of FeLoBALQs that are characterized by low outflow velocities and high column density winds located log R ≲ 1 [pc] from the central engine. The FeLoBALs in loitering outflows objects do not show properties expected for radiatively driven winds, and these objects may represent a distinct population among FeLoBALQs. We discuss how the potential acceleration mechanisms and the origins of the FeLoBAL winds may differ for outflows at different locations in quasars.

We present continued analysis of a sample of low-redshift iron low-ionization broad absorption-line quasars (FeLoBALQs). Choi et al. (2022) presented $SimBAL$ spectral analysis of BAL outflows in 50 objects. Leighly et al. (2022) analyzed optical emission lines of 30 of those 50 objects and found that they are characterized by either a high accretion rate ($L_\mathrm{Bol}/L_\mathrm{Edd}>0.3$) or low accretion rate ($0.03, Comment: Resubmitted to ApJ after review

We report the results of analysis of the Hbeta emission-line region of a sample of thirty low-redshift (z, Resubmitted to ApJ after review

We present ultraviolet spectroscopy covering the Lyα + N v complex of six candidate low-redshift (0.9 < z < 1.5) weak emission-line quasars (WLQs) based on observations with the Hubble Space Telescope. The original systematic searches for these puzzling Type 1 quasars with intrinsically weak broad emission lines revealed an N ≈ 100 WLQ population from optical spectroscopy of high-redshift (z > 3) quasars, defined by a Lyα + N v rest-frame equivalent width (EW) threshold z < 3) WLQ candidates, however, has relied primarily on optical spectroscopy of weak broad emission lines at longer rest-frame wavelengths. With these new observations expanding existing optical coverage into the ultraviolet, we explore unifying the low- and high-z WLQ populations via EW[Lyα+N v]. Two objects in the sample unify with high-z WLQs, three others appear consistent with the intermediate portion of the population connecting WLQs and normal quasars, and the final object is consistent with typical quasars. The expanded wavelength coverage improves the number of available line diagnostics for our individual targets, allowing a better understanding of the shapes of their ionizing continua. The ratio of EW[Lyα+N v] to EW[Mg ii] in our sample is generally small but varied, favoring a soft ionizing continuum scenario for WLQs, and we find a lack of correlation between EW[Lyα+N v] and the X-ray properties of our targets, consistent with a “slim-disk” shielding gas model. We also find indications that weak absorption may be a more significant contaminant in low-z WLQ populations than previously thought.

We present a study of $\gamma$-ray emission from the core-collapse supernova remnant Cas~A in the energy range from 0.1GeV to 10TeV. We used 65 hours of VERITAS data to cover 200 GeV - 10 TeV, and 10.8 years of \textit{Fermi}-LAT data to cover 0.1-500 GeV. The spectral analysis of \textit{Fermi}-LAT data shows a significant spectral curvature around $1.3 \pm 0.4_{stat}$ GeV that is consistent with the expected spectrum from pion decay. Above this energy, the joint spectrum from \textit{Fermi}-LAT and VERITAS deviates significantly from a simple power-law, and is best described by a power-law with spectral index of $2.17\pm 0.02_{stat}$ with a cut-off energy of $2.3 \pm 0.5_{stat}$ TeV. These results, along with radio, X-ray and $\gamma$-ray data, are interpreted in the context of leptonic and hadronic models. Assuming a one-zone model, we exclude a purely leptonic scenario and conclude that proton acceleration up to at least 6 TeV is required to explain the observed $\gamma$-ray spectrum. From modeling of the entire multi-wavelength spectrum, a minimum magnetic field inside the remnant of $B_{\mathrm{min}}\approx150\,\mathrm{\mu G}$ is deduced., Comment: 33 pages, 9 Figures, 6 Tables

Blazars are known for their variability on a wide range of timescales at all wavelengths. Most studies of TeV gamma-ray blazars focus on short timescales, especially during flares. With a decade of observations from the Fermi-LAT and VERITAS, we present an extensive study of the long-term multi-wavelength radio-to-gamma-ray flux-density variability, with the addition of a couple of short-time radio-structure and optical polarization observations of the blazar 1ES 1215+303 (z=0.130), with a focus on its gamma-ray emission from 100 MeV to 30 TeV. Multiple strong GeV gamma-ray flares, a long-term increase in the gamma-ray and optical flux baseline and a linear correlation between these two bands are observed over the ten-year period. Typical HBL behaviors are identified in the radio morphology and broadband spectrum of the source. Three stationary features in the innermost jet are resolved by VLBA at 43.1, 22.2, and 15.3 GHz. We employ a two-component synchrotron self-Compton model to describe different flux states of the source, including the epoch during which an extreme shift in energy of the synchrotron peak frequency from infrared to soft X-rays is observed., Comment: Accepted for publication in the Astrophysical Journal

Clustering measurements of obscured and unobscured quasars show that obscured quasars reside in more massive dark matter halos than their unobscured counterparts. These results are inconsistent with simple unified (torus) scenarios, but might be explained by models in which the distribution of obscuring material depends on Eddington ratio or galaxy stellar mass. We test these possibilities by constructing simple physical models to compare to observed AGN populations. We find that previously observed relationships between obscuration and Eddington ratio or stellar mass are not sufficient reproduce the observed quasar clustering results ($\langle \log M_{\text{halo}}/M_{\odot} \rangle = 12.94 ^{+ 0.10}_{- 0.11}$ and $\langle \log M_{\text{halo}}/M_{\odot} \rangle = 12.49 ^{+ 0.08}_{- 0.08}$ for obscured and unobscured populations, respectively) while maintaining the observed fraction of obscured quasars (30-65$\%$). This work suggests that evolutionary models, in which obscuration evolves on the typical timescale for black hole growth, are necessary to understand the observed clustering of mid-IR selected quasars., 14 pages, 10 figures, accepted for publication in ApJ

We introduce SimBAL, a novel spectral-synthesis procedure that uses large grids of ionic column densities generated by the photoionization code Cloudy and a Bayesian model calibration to forward-model broad absorption line quasar spectra. We used SimBAL to analyze the HST COS spectrum of the low-redshift BALQ SDSS J085053.12+445122.5. SimBAL analysis yielded velocity-resolved information about the physical conditions of the absorbing gas. We found that the ionization parameter and column density increase, and the covering fraction decreases as a function of velocity. The total log column density is 22.9 (22.4) [cm^-2] for solar (Z=3Z_\odot) metallicity. The outflow lies 1--3 parsecs from the central engine, consistent with the estimated location of the torus. The mass outflow rate is 17--28 M_\odot yr^-1, the momentum flux is consistent with L_Bol/c, and the ratio of the kinematic to bolometric luminosity is 0.8--0.9%. The outflow velocity is similar to the escape velocity at the absorber's location, and force multiplier analysis indicates that part of the outflow could originate in resonance-line driving. The location near the torus suggests that dust scattering may play a role in the acceleration, although the lack of reddening in this UV-selected object indictes a relatively dust-free line of sight. The low accretion rate (0.06 L_Edd) and compact outflow suggests that SDSS~J0850+4451 might be a quasar past its era of feedback, although since its mass outflow is about 8 times the accretion rate, the wind is likely integral to the accretion physics of the central engine., Comment: Resubmitted to ApJ after responding to referee report

Extended extragalactic radio sources have traditionally been classified into FR I and II types, based on the ratio r of the separation S between the brightest regions on either sides of the host galaxy and the total size T of the radio source. Here we examine the distribution of various physical properties as a function of r of 1040 luminous radio galaxies (RGs) at z0.8 AND OIII5007 line luminosity >10^6 Lsun are markedly different on average from the rest of the RGs, in the sense that they are hosted in lower mass galaxies, live in relatively sparse environments, and have higher accretion rates onto the central SMBH. Thus these objects and the rest of RGs form a well-defined dichotomy. Motivated by the stark differences in the nuclear emission line properties of the RG subsamples, we suggest that the accretion rate onto the SMBH may play the primary role in creating the different morphologies. At relatively high accretion rates, the accretion system may produce powerful jets that create the "classical double" morphology (roughly corresponding to the LD sources with r>0.8 and emission lines); at lower accretion rates the jets from a radiatively inefficient accretion flow generate radio lobes without apparent "hot spots" at the edge (corresponding to the majority of LD sources). At slightly lower accretion rates AND in galaxies with dense galactic structure, sources with prominent jets result. It is possible that while the high accretion rate systems could affect sub-Mpc scale environments, the jets from lower accretion rate systems may efficiently suppress activity within the host galaxies., Comment: ApJ, accepted. 20 pages, 16 figures, 4 tables. Image quality of Figures 1 & 2 degraded due to size limits. Table 4 is available and kept up-to-date at http://member.ipmu.jp/yen-ting.lin/RG/index.html. Higher resolution version available at http://member.ipmu.jp/yen-ting.lin/Data/ytlin.pdf

The clustering of quasars on small scales yields fundamental constraints on models of quasar evolution and the buildup of supermassive black holes. This paper describes the first systematic survey to discover high redshift binary quasars. Using color-selection and photometric redshift techniques, we searched 8142 deg^2 of SDSS imaging data for binary quasar candidates, and confirmed them with follow-up spectroscopy. Our sample of 27 high redshift binaries (24 of them new discoveries) at redshifts 2.9 < z < 4.3 with proper transverse separations 10 kpc < R_{\perp} < 650 kpc increases the number of such objects known by an order of magnitude. Eight members of this sample are very close pairs with R_{\perp} < 100 kpc, and of these close systems four are at z > 3.5. The completeness and efficiency of our well-defined selection algorithm are quantified using simulated photometry and we find that our sample is ~ 50% complete. Our companion paper uses this knowledge to make the first measurement of the small scale clustering (R < 1 Mpc/h comoving) of high-redshift quasars. High redshift binaries constitute exponentially rare coincidences of two extreme (M >~ 10^9 Msun) supermassive black holes. At z ~ 4 there is about one close binary per 10 Gpc^3, thus these could be the highest sigma peaks, the analogs of superclusters, in the early Universe., Submitted to ApJ

Interacting binaries typically have separations in the milli-arcsecond regime and hence it has been challenging to resolve them at any wavelength. However, recent advances in optical interferometry have improved our ability to discern the components in these systems and have now enabled the direct determination of physical parameters. We used the Navy Prototype Optical Interferometer to produce for the first time images resolving all three components in the well-known Algol triple system. Specifically, we have separated the tertiary component from the binary and simultaneously resolved the eclipsing binary pair, which represents the nearest and brightest eclipsing binary in the sky. We present revised orbital elements for the triple system, and we have rectified the 180-degree ambiguity in the position angle of Algol C. Our directly determined magnitude differences and masses for this triple star system are consistent with earlier light curve modeling results., Accepted to Astrophysical Journal Letters, 15 pages, 2 eps figures.

We use the multi-epoch spectra of 362 quasars from the Sloan Digital Sky Survey Reverberation Mapping project to investigate the dependence of the blueshift of CIV relative to MgII on quasar properties. We confirm that high-blueshift sources tend to have low CIV equivalent widths (EWs), and that the low-EW sources span a range of blueshift. Other high-ionization lines, such as HeII, also show similar blueshift properties. The ratio of the line width (measured as both the full-width at half maximum and the velocity dispersion) of CIV to that of MgII increases with blueshift. Quasar variability might enhance the connection between the CIV blueshift and quasar properties (e.g., EW). The variability of the MgII line center (i.e., the wavelength that bisects the cumulative line flux) increases with blueshift. In contrast, the CIV line center shows weaker variability at the extreme blueshifts. Quasars with the high-blueshift CIV lines tend to have less variable continuum emission, when controlling for EW, luminosity, and redshift. Our results support the scenario that high-blueshift sources tend to have large Eddington ratios., 18 pages, 25 figures, Accepted for publication in ApJ

The 3D Radioastronomical Approach to Doppler tomography has been used to study the H$\alpha$ emission sources in U Coronae Borealis. These 3D tomograms provide greater resolution than the projected 2D version and highlight the jet-like gas flows in the $V_z$ direction transverse to the orbital plane. In this paper, the 3D tomograms are compared at two distinct epochs when U CrB was in the disk-like state (1993 data) and the stream-like state (1994 data). Both states display a prominent emission source, the circumprimary bulge, which is produced when the gas stream strikes the photosphere of the mass-gainer. This source is detected within $V_z$ = $\pm$150 km s{$^{-1}$}, and demonstrates that the bulge is not confined to the orbital plane although it achieves maximum strength near $V_z$=0 km s{$^{-1}$}. Other emission sources include the stream-star and stream-disk shocks and a Localized Region (LR) where the circling disk material strikes the incoming gas stream. The LR has $V_z$ velocities of 200 to 500 km s{$^{-1}$} in the disk-like state. The disk emission is seen over a range of $V_z$ velocities, and there is evidence that the disk is inclined to the orbital plane or may have two arms. The gas stream flows along its predicted trajectory in the stream-like state, and a comparison with the disk-like state suggests that the gas stream has a higher density than the disk in both states of this binary., Comment: 17 pages, 9 figures, submitted to ApJ

We study the spectral energy distributions and evolution of a large sample of optically selected quasars from the Sloan Digital Sky Survey (SDSS) that were observed in 323 Chandra images analyzed by the Chandra Multiwavelength Project (ChaMP). Our highest-confidence matched sample includes 1135 X-ray detected quasars in the redshift range 0.2 3 QSOs detected, we find no evidence for evolution out to z~5 for either the X-ray photon index Gamma or for the ratio of optical/UV to X-ray flux alpha_ox. About 10% of detected QSOs are obscured (Nh>1E22), but the fraction might reach ~1/3 if most non-detections are absorbed. We confirm a significant correlation between alpha_ox and optical luminosity, but it flattens or disappears for fainter AGN alone. Gamma hardens significantly both towards higher X-ray luminosity, and for relatively X-ray loud quasars. These trends may represent a relative increase in non-thermal X-ray emission, and our findings thereby strengthen analogies between Galactic black hole binaries and AGN.

We have studied a sample of 415 associated (z_ab z_em; relative velocity with respect to QSO 3000km/s), so as to understand their origin. From the analysis of the composite spectra, as well as from the comparison of measured equivalent widths in individual spectra, we conclude that the associated Mg II absorbers have higher apparent ionization, measured by the strength of the C IV absorption lines compared to the Mg II absorption lines, than the intervening absorbers. The ionization so measured appears to be related to apparent ejection velocity, being lower as the apparent ejection velocity is more and more positive. There is clear evidence, from the composite spectra, for SMC like dust attenuation in these systems; the 2175AA absorption feature is not present. The extinction is almost twice that observed in the similarly selected sample of intervening systems. We reconfirm that QSOs with non-zero FIRST radio flux are intrinsically redder than the QSOs with no detection in the FIRST survey. The incidence of associated Mg II systems in QSOs with non-zero FIRST radio flux is 1.7 times that in the QSOs with no detection in the FIRST survey. The associated absorbers in radio-detected QSOs which comprise about 12% of our sample, cause 3 times more reddening than the associated absorbers in radio-undetected QSOs. This excess reddening possibly suggests an intrinsic nature for the associated absorbers in radio-detected QSOs., Accepted for publication in ApJ

We present spectroscopy of binary quasar candidates, with component separations of 3'' ≤ Δ θ < 6'', selected from Data Release 4 of the Sloan Digital Sky Survey (SDSS DR4) using kernel density estimation (KDE). Of our 27 new quasar pair observations, 10 are binary quasars, which doubles the number of known g < 21 binaries with 3'' ≤ Δ θ < 6'' separations. Several of our observed binaries are wide-separation lens candidates that merit additional higher resolution spectroscopy, as well as deep imaging to search for lensing galaxies. Our candidates are initially selected by UV excess (u − g < 1), but are otherwise selected irrespective of the relative colors of the quasar pair, and we thus use them to suggest optimal color similarity and photometric redshift approaches for targeting binary quasars or projected quasar pairs. We find that a third or more of all binary quasars have quite dissimilar components on the basis of a typical color similarity criterion (χ2color 20). From a sample that is complete on proper scales of 23.7 h−1 kpc < Rprop < 29.9 h−1 kpc , we determine the projected quasar correlation function to be p = 24.0-10.8+16.9, which is 2 σ lower than recent estimates. We argue that our low p estimates may indicate redshift evolution in the quasar correlation function from z ~ 1.9 to z ~ 1.4 on scales of Rprop ~ 25 h−1 kpc. The size of this evolution broadly tracks quasar clustering on larger scales, consistent with merger-driven models of quasar origin. Spectroscopy of all of our DR4 KDE binary quasar candidates should be sufficient to detect quasar clustering evolution at Rprop ~ 25 h−1 kpc for z < 2.5 in a single homogeneous sample.

We report the discovery of a cluster-scale lensed quasar, SDSS J1029+2623, selected from the Sloan Digital Sky Survey. The lens system exhibits two lensed images of a quasar at z_s=2.197. The image separation of 22.5" makes it the largest separation lensed quasar discovered to date. The similarity of the optical spectra and the radio loudnesses of the two components support the lensing hypothesis. Images of the field show a cluster of galaxies at z_l~0.55 that is responsible for the large image separation. The lensed images and the cluster light center are not collinear, which implies that the lensing cluster has a complex structure.

The reconstruction of three-dimensional (3D) Doppler tomograms based on observational data has been accomplished for the first time. The distribution of the Hα emission intensity I(Vx,Vy,Vz) of the interacting Algol binary system U Coronae Borealis has been restored in 3D velocity space with resolutions of 30 km s-1 in Vx and Vy and 110 km s-1 in Vz. The reconstruction was based on 47 Hα spectra from 1994 by applying the developed radioastronomical approach (RA) to few-projections tomography. The comparison between the previous 2D and our 3D Doppler tomograms shows similarities with the main structural features of the gas flows in the orbital plane. Specifically, the gas stream along the ballistic trajectory and equatorial emission centered on the primary star are displayed on the 3D Doppler tomogram as distinct emission sources. A high-velocity stream (Vz ~ 200 km s-1) with strong emission intensity has also been discovered moving in the direction across the orbital plane of the system.

著者人数: 18名, Accepted: 2006-04-25, 資料番号: SA1000374000

We present the results of a search for metal absorption lines in the spectra of background QSOs whose sightlines pass close to foreground QSOs. We detect MgII(2796,2803) absorption in Sloan Digital Sky Survey (SDSS) spectra of four z>1.5 QSOs whose lines of sight pass within 26-98 kpc of lower redshift (z~0.5-1.5) QSOs. The 100% [4/4 pairs] detection of MgII in the background QSOs is clearly at odds with the incidence of associated (z_abs ~ z_em) systems -- absorbers which exist towards only a few percent of QSOs. Although the quality of our foreground QSO spectra is not as high as the SDSS data, absorption seen towards one of the background QSOs clearly does not show up at the same strength in the spectrum of the corresponding foreground QSO. This implies that the absorbing gas is distributed inhomogeneously around the QSO, presumably as a direct consequence of the anisotropic emission from the central AGN. We discuss possible origins for the MgII lines, including: absorption by gas from the foreground QSO host galaxy; companion galaxies fuelling the QSO through gravitational interactions; and tidal debris left by galaxy mergers or interactions which initiated the QSO activity. No single explanation is entirely satisfactory, and we may well be seeing a mixture of phenomena., Accepted for publication in ApJ Letters. 5 pages

We report the discovery of two doubly-imaged quasars, SDSS J100128.61+502756.9 and SDSS J120629.65+433217.6, at redshifts of 1.838 and 1.789 and with image separations of 2.86'' and 2.90'', respectively. The objects were selected as lens candidates from the Sloan Digital Sky Survey (SDSS). Based on the identical nature of the spectra of the two quasars in each pair and the identification of the lens galaxies, we conclude that the objects are gravitational lenses. The lenses are complicated; in both systems there are several galaxies in the fields very close to the quasars, in addition to the lens galaxies themselves. The lens modeling implies that these nearby galaxies contribute significantly to the lens potentials. On larger scales, we have detected an enhancement in the galaxy density near SDSS J100128.61+502756.9. The number of lenses with image separation of ~3'' in the SDSS already exceeds the prediction of simple theoretical models based on the standard Lambda-dominated cosmology and observed velocity function of galaxies., Comment: 24 pages, 9 figures, accepted for publication in ApJ

Extensive radio data of two Algol systems and two RS CVn binaries were re-analyzed. We found evidence for a new periodicity that we interpret as a superhump in Algol, in which it may have been expected according to its semi-detached nature and low binary mass ratio. This is the first detection of the superhump phenomenon in the radio and the first observation of superhumps in Algol systems. According to our result, the accretion disk in Algol precesses in spite of its non-Keplerian nature and therefore this phenomenon is not restricted to the classical Keplerian disks in compact binaries.We propose that in Algol systems with short orbital periods, the disk is magnetically active as well as the secondary star. The magnetic field in the disk originates from amplification of the seed field in the magnetized material transferred from the secondary. The disk and stellar fields interact with each other, with reconnection of the field lines causing flares and particle acceleration. Relativistic particles are trapped in the field and directed toward the polar regions of the secondary star because of the dipole structure of its magnetic field. Our proposed model for the magnetic activity in Algol systems provides a simple explanation to the observed properties of Algol in the radio wavelengths, and to the presence of quiescent gyrosynchrotron emission near the polar region of the secondary star, where electrons are difficult to be confined if the field lines are open as in normal single magnetic stars. We propose that the superhump variation in the radio is generated by enhanced reconnection when the elongated side of the elliptic disk is the closest to the cool star. This leads to flares and enhancement in particle acceleration and is manifested as stronger gyrosynchrotron radiation., Comment: 8 pages, 5 figures, ApJ, accepted, uses apjfonts.sty and emulateapj5.sty, full abstract in paper

In an ongoing infrared imaging survey of quasars at Keck Observatory, we have discovered that the z=1.285 quasar SDSS J233646.2-010732.6 comprises two point sources with a separation of 1.67". Resolved spectra show that one component is a standard quasar with a blue continuum and broad emission lines; the other is a broad absorption line (BAL) quasar, specifically, a BAL QSO with prominent absorption from MgII and metastable FeII, making it a member of the ``FeLoBAL'' class. The number of known FeLoBALs has recently grown dramatically from a single example to more than a dozen, including a gravitationally lensed example and the binary member presented here, suggesting that this formerly rare object may be fairly common. Additionally, the presence of this BAL quasar in a relatively small separation binary adds to the growing evidence that the BAL phenomenon is not due to viewing a normal quasar at a specific orientation, but rather that it is an evolutionary phase in the life of many, if not all, quasars, and is particularly associated with conditions found in interacting systems.

We report on exploratory Chandra observations of the three highest redshift quasars known (z = 5.82, 5.99, and 6.28), all found in the Sloan Digital Sky Survey. These data, combined with a previous XMM-Newton observation of a z = 5.74 quasar, form a complete set of color-selected, z > 5.7 quasars. X-ray emission is detected from all of the quasars at levels that indicate that the X-ray to optical flux ratios of z ~ 6 optically selected quasars are similar to those of lower redshift quasars. The observations demonstrate that it will be feasible to obtain quality X-ray spectra of z ~ 6 quasars with current and future X-ray missions.

A sample of 4208 objects with magnitude 15 < g* < 22 and colors of main-sequence A stars have been selected from 370 deg(2) of Sloan Digital Sky Survey (SDSS) commissioning observations. The data is from two long, narrow stripes, each with an opening angle of greater than 60 degrees, at Galactic latitudes 36 degrees < \b\ < 63 degrees on the celestial equator. Relative photometric calibrations good to 2% and consistent absolute photometry allows this uniform sample to be treated statistically over the large area. An examination of the sample's distribution shows that these stars trace considerable substructure in the halo. Large overdensities of A-colored stars in the north at (l, b, R) = (350 degrees, 50 degrees, 46 kpc) and in the south at (157, -58, 33 kpc) and extending over tens of degrees are present in the halo of the Milky Way. Ivezic ct al. have detected the northern structure from a sample of RR Lyrae stars in the SDSS. Using photometry to separate the stars by surface gravity, both structures are shown to contain a sequence of low surface gravity stars consistent with identification as a blue horizontal branch (BHB). Both structures also contain a population of high surface gravity stars 2 mag fainter than the BHB stars, consistent with their identification as blue stragglers (BSs). The majority of the high surface gravity stars in the Galactic halo may be BS stars like these. A population of F stars associated with the A star excess in the southern structure is detected (the F stars in the northern structure at 46 kpc would be too faint for the SDSS to detect). From the numbers of detected BHB stars, lower limits to the implied mass of the structures are 6 x 10(6) M-circle dot and 2 x 10(6) M-circle dot, although one does not yet know the full spatial extent of the structures. The fact that two such large clumps have been detected in a survey of only 1% of the sky indicates that such structures are not uncommon in the halo. Simple spheroidal parameters are fit to a complete sample of the remaining unclumped BHB stars and yield (at r < 40 kpc) a fit to a halo distribution with flattening (c/a = 0.65 +/- 0.2) and a density falloff exponent of alpha = -3.2 +/- 0.3.

An extensive collection of spectroscopic observations of the interacting binary CX Dra spanning a 23 year interval have been analyzed. This study includes a re—nement of the orbital solution of CX Dra ; equivalent width measurements that show short-, medium-, and long-term behavior of the diUerence pro- —les ; a calculation of the Balmer decrement ; velocity maps based on the velocity curves of the Ha and He I diUerence emission peaks ; trailed spectrograms of the Ha ,H b ,H e I ,and Si II lines ; and Doppler tomograms at these four wavelengths. The main conclusions are : 1. The circumstellar environment in the system changes in cycles of hundreds of days. The length of the cycles is variable. These cycles may be part of a ii super ˇˇ 4000 day cycle. 2. The equivalent widths of the diUerence Ha and He I j6678 lines are modulated with the orbital period of 6.696 days. The corresponding phase diagrams indicate a great deal of scatter, but the modula- tion is quite conspicuous when long data strings are used. The pattern of the phase diagram suggests a permanent presence of the emission in Ha ,H e I j6678, and Si II j6371 lines. 3. The radial velocities of the Ha emission peak follow an S-wave. The resulting velocity map shows that the source of the single-peaked emission lies close to the L1 point, roughly between the primary and L1 point. 4. Doppler tomograms constructed for Ha observed and diUerence pro—les show that the emission comes from a region of low velocity, a gas stream, and an accretion disk. The Hb emission arises from a region that is cospatial with the Ha source. The Doppler tomograms for He I j6678 and Si II j6371 lines suggest that emission in this gas also originates from a locus of a disk around the primary star. 5. The model based on the equivalent widths of the diUerence pro—les, the trailed spectrograms, and Doppler tomograms of the Ha ,H e I j6678, Si II j6371, and Hb lines suggests that the main source of the Ha emission is about halfway between the stars at a distance of 0.49a from the primary star, and that the He I and Si II emission sources arise from an accretion disk centered on the primary star. Subject headings : accretion, accretion disksbinaries : closecircumstellar matter ¨ stars : emission-line, Bestars : individual (CX Dra)

Archival IUE spectra of U Sge have been used to study hot (D105 K) circumstellar gas in this system. The observed spectra contain several UV resonance lines, of which the Si IV lines (jj1394 and 1405) are the strongest. These lines are observed in absorption, so we must remove the photospheric absorption line contribution to the spectra in order to see the emission produced by the circumstellar gas. We have developed a robust method for creating such diUerence spectra from IUE data. In the observed spectra we see a variation in the line pro—les and line centers with an orbital phase suggesting the presence of circumstellar gas. The residual emission seen in the diUerence spectra is strongest between phases / \ 0.3 and / \ 0.7, with a strength of up to 0.2 of the continuum —ux. This is con—rmed by the presence of an emission region visible in the Doppler tomogram of the j1394 line. This is consistent with the location of an Ha ii absorption zone ˇˇ seen in tomograms of U Sge and U CrB, and with theoretical predictions of a high temperature accretion region. Subject headings : accretion, accretion disksbinaries : eclipsingcircumstellar matter ¨ stars : individual (U Sagittae) ¨ ultraviolet : stars

Two-dimensional hydrodynamic simulations of mass transfer in short-period Algol-type binaries were performed using the numerical code Virginia Hydrodynamics 1. This code uses the piecewise parabolic method with a Lagrangian remap. Our version of the code also accounts for radiative cooling and collisional ionization and excitation processes. The purpose of performing the simulations was to study the Hα emission from circumstellar gas in the Algols. Using observational evidence from the literature to constrain the gas stream properties, hydrodynamic maps of the Hα emissivity in the two systems β Per (P = 2.87 days) and TT Hya (P = 6.95 days) were made in both Cartesian and velocity coordinates from the simulation data. The velocity maps were then compared to Doppler tomograms constructed from observed Hα line emission in these systems. Since the tomograms cannot be directly transformed to maps of emission in spatial coordinates, the simulated Cartesian maps enabled us to interpret the dynamical processes that produce the features observed in the Doppler tomograms. We find that the simulations produce asymmetric accretion structures with many features similar to those found in the Doppler tomograms of Algol systems.

During moonlit nights, observations with ground-based Cherenkov telescopes at very high energies (VHE, $E>100$ GeV) are constrained since the photomultiplier tubes (PMTs) in the telescope camera are extremely sensitive to the background moonlight. Observations with the VERITAS telescopes in the standard configuration are performed only with a moon illumination less than 35$\%$ of full moon. Since 2012, the VERITAS collaboration has implemented a new observing mode under bright moonlight, by either reducing the voltage applied to the PMTs (reduced-high-voltage configuration, RHV), or by utilizing UV-transparent filters. While these operating modes result in lower sensitivity and increased energy thresholds, the extension of the available observing time is useful for monitoring variable sources such as blazars and sources requiring spectral measurements at the highest energies. In this paper we report the detection of $\gamma$-ray flaring activity from the BL Lac object 1ES 1727+502 during RHV observations. This detection represents the first evidence of VHE variability from this blazar. The integral flux is $(1.1\pm0.2)\times10^{-11}\mathrm{cm^{-2}s^{-1}}$ above 250 GeV, which is about five times higher than the low-flux state. The detection triggered additional \veritas\ observations during standard dark-time. Multiwavelength observations with the FLWO 48" telescope, and the Swift and Fermi satellites are presented and used to produce the first spectral energy distribution (SED) of this object during $\gamma$-ray flaring activity. The SED is then fitted with a standard synchrotron-self-Compton model, placing constraints on the properties of the emitting region and of the acceleration mechanism at the origin of the relativistic particle population in the jet., Comment: 11 pages, 3 figures, accepted for publication in ApJ

SDSS J015957.64+003310.5 is an X-ray selected, z = 0.31 active galactic nucleus (AGN) from the Stripe 82X survey that transitioned from a Type 1 quasar to a Type 1.9 AGN between 2000 and 2010. This is the most distant AGN, and first quasar, yet observed to have undergone such a dramatic change. We re-observed the source with the double spectrograph on the Palomar 5 m telescope in 2014 July and found that the spectrum is unchanged since 2010. From fitting the optical spectra, we find that the AGN flux dropped by a factor of 6 between 2000 and 2010 while the broad Hα emission faded and broadened. Serendipitous X-ray observations caught the source in both the bright and dim state, showing a similar 2–10 keV flux diminution as the optical while lacking signatures of obscuration. The optical and X-ray changes coincide with g-band magnitude variations over multiple epochs of Stripe 82 observations. We demonstrate that variable absorption, as might be expected from the simplest AGN unification paradigm, does not explain the observed photometric or spectral properties. We interpret the changing state of J0159+0033 to be caused by dimming of the AGN continuum, reducing the supply of ionizing photons available to excite gas in the immediate vicinity around the black hole. J0159+0033 provides insight into the intermittency of black hole growth in quasars, as well as an unprecedented opportunity to study quasar physics (in the bright state) and the host galaxy (in the dim state), which has been impossible to do in a single sources until now.

The astrophysical journal 800(1), 61 - (2015). doi:10.1088/0004-637X/800/1/61, We present the results of 71.6 hr of observations of the Geminga pulsar (PSR J0633+1746) with the VERITAS very-high-energy gamma-ray telescope array. Data taken with VERITAS between 2007 November and 2013 February were phase-folded using a Geminga pulsar timing solution derived from data recorded by the XMM- Newton and Fermi-LAT space telescopes. No significant pulsed emission above 100 GeV is observed, and we report upper limits at the 95% confidence level on the integral flux above 135 GeV (spectral analysis threshold) of 4.0 × 10$^{–1}$3 s$^{–1}$ cm$^{–2}$ and 1.7 × 10$^{–13}$ s$^{–1}$ cm$^{–2}$ for the two principal peaks in the emission profile. These upper limits, placed in context with phase-resolved spectral energy distributions determined from 5 yr of data from the Fermi-Large Area Telescope (LAT), constrain possible hardening of the Geminga pulsar emission spectra above ~50 GeV., Published by Univ.11032, Chicago, Ill. [u.a.]

We have used two-dimensional back-projection Doppler tomography as a tool to examine the influence of gravitational and magnetic phenomena in interacting binaries that undergo mass transfer from a magnetically active star onto a non-magnetic main-sequence star. This multitiered study of over 1300 time-resolved spectra of 13 Algol binaries involved calculations of the predicted dynamical behavior of the gravitational flow and the dynamics at the impact site, analysis of the velocity images constructed from tomography, and the influence on the tomograms of orbital inclination, systemic velocity, orbital coverage, and shadowing. The H? tomograms revealed eight sources: chromospheric emission, a gas stream along the gravitational trajectory, a star-stream impact region, a bulge of absorption or emission around the mass-gaining star, a Keplerian accretion disk, an absorption zone associated with hotter gas, a disk-stream impact region, and a hot spot where the stream strikes the edge of a disk. We described several methods used to extract the physical properties of the emission sources directly from the velocity images, including S-wave analysis, the creation of simulated velocity tomograms from hydrodynamic simulations, and the use of synthetic spectra with tomography to sequentially extract the separate sources of emission from the velocity image. In summary, the tomography images have revealed results that cannot be explained solely by gravitational effects: chromospheric emission moving with the mass-losing star, a gas stream deflected from the gravitational trajectory, and alternating behavior between stream state and disk state. Our results demonstrate that magnetic effects cannot be ignored in these interacting binaries.

High-dimensional, large-sample astrophysical databases of galaxy clusters, such as the Chandra Deep Field South COMBO-17 database, provide measurements on many variables for thousands of galaxies and a range of redshifts. Current understanding of galaxy formation and evolution rests sensitively on relationships between different astrophysical variables; hence an ability to detect and verify associations or correlations between variables is important in astrophysical research. In this paper, we apply a recently defined statistical measure called the distance correlation coefficient which can be used to identify new associations and correlations between astrophysical variables. The distance correlation coefficient applies to variables of any dimension; it can be used to determine smaller sets of variables that provide equivalent astrophysical information; it is zero only when variables are independent; and it is capable of detecting nonlinear associations that are undetectable by the classical Pearson correlation coefficient. Hence, the distance correlation coefficient provides more information than the Pearson coefficient. We analyze numerous pairs of variables in the COMBO-17 database with the distance correlation method and with the maximal information coefficient. We show that the Pearson coefficient can be estimated with higher accuracy from the corresponding distance correlation coefficient than from the maximal information coefficient. For given values of the Pearson coefficient, the distance correlation method has a greater ability than the maximal information coefficient to resolve astrophysical data into highly concentrated V-shapes, which enhances classification and pattern identification. These results are observed over a range of redshifts beyond the local universe and for galaxies from elliptical to spiral., Comment: 11 pages, 6 figures, 4 tables; Astrophysical Journal, accepted, in press

We report on a blind survey for extragalactic radio variability that was carried out by comparing two epochs of data from the FIRST survey with a third epoch from a new 1.4 GHz survey of SDSS Stripe 82. The three epochs are spaced seven years apart and have an overlapping area of 60 deg^2. We uncover 89 variable sources down to the millijansky level, 75 of which are newly-identified, and we find no evidence for transient phenomena. This new sample of variable sources allows us to infer an upper limit to the mean characteristic timescale of AGN radio variability of 14 years. We find that only 1% of extragalactic sources have fractional variability f_var >3, while 44% of Galactic sources vary by this much. The variable sample contains a larger fraction of quasars than a comparable non-variable control sample, though the majority of the variable sources appear to be extended galaxies in the optical. This implies that either quasars are not the dominant contributor to the variability of the sample, or that the deep optical data allow us to detect the host galaxies of some low-z quasars. We use the new, higher resolution data to report on the morphology of the variable sources. Finally, we show that the fraction of sources that are variable remains constant or increases at low flux densities. This may imply that next generation radio surveys with telescopes like the Australian Square Kilometer Array Pathfinder and MeerKAT will see a constant or even increasing fraction of variable sources down into the submillijansky regime., Published in ApJ

Doppler tomography of Hα spectra of four Algol-type binaries TT Hya, SW Cyg, U CrB, and U Sge has revealed quasi-stable accretion disks around the mass gaining star in each system. In the tomograms, these accretion structures are nearly symmetric distributions of gas between the stellar surface and the Roche lobe around the mass gainer. The disk is most stable in TT Hya and SW Cyg, which have orbital periods greater than 4.5 days. However, the morphology of the accretion structures in the shorter period systems, U CrB and U Sge, can change from a disklike distribution to a streamlike distribution. During a streamlike epoch of these systems, the gas stream in the Doppler tomogram closely follows the predicted free fall path from the inner Lagrangian point toward the mass gainer. However, during the disklike stage, the intensity of the disk is comparable to that of the gas stream. These tomograms display the first maps of accretion disks in the Algol-type class of interacting binaries.

AU Mon is a long-period (11.113 d) Algol-type binary system with a persistent accretion disk that is apparent as double-peaked H-alpha emission. We present previously unpublished optical spectra of AU Mon which were obtained over several years with dense orbital phase coverage. We utilize these data, along with archival UV spectra, to model the temperature and structure of the accretion disk and the gas stream. Synthetic spectral profiles for lines including H-alpha, H-beta, and the Al III and Si IV doublets were computed with the Shellspec program. The best match between the model spectra and the observations is obtained for an accretion disk of inner/outer radius 5.1/23 R_sun, thickness of 5.2 R_sun, density of 1.0e-13 g/cm^3, and maximum temperature of 14000 K, along with a gas stream at a temperature of ~8000 K transferring ~2.4e-9 M_sun/yr. We show H-alpha Doppler tomograms of the velocity structure of the gas, constructed from difference profiles calculated through sequentially subtracting contributions from the stars and accretion structures. The tomograms provide independent support for the Shellspec modeling, while also illustrating that residual emission at sub-Keplerian velocities persists even after subtracting the disk and stream emission. Spectral variability in the H-alpha profile beyond that expected from either the orbital or the long-period cycle is present on both multi-week and multi-year timescales, and may reflect quasi-random changes in the mass transfer rate or the disk structure. Finally, a transient UV spectral absorption feature may be modeled as an occasional outflow launched from the vicinity of the disk-stream interaction region., Comment: 16 pages, 12 figures, 3 tables, emulateapj. Accepted to ApJ