Search

We report the detection of a transiting planet around $\pi$ Mensae (HD 39091), using data from the Transiting Exoplanet Survey Satellite (TESS). The solar-type host star is unusually bright (V=5.7) and was already known to host a Jovian planet on a highly eccentric, 5.7-year orbit. The newly discovered planet has a size of $2.04\pm 0.05$ $R_\oplus$ and an orbital period of 6.27 days. Radial-velocity data from the HARPS and AAT/UCLES archives also displays a 6.27-day periodicity, confirming the existence of the planet and leading to a mass determination of $4.82\pm 0.85$ $M_\oplus$. The star's proximity and brightness will facilitate further investigations, such as atmospheric spectroscopy, asteroseismology, the Rossiter--McLaughlin effect, astrometry, and direct imaging., Comment: Accepted for publication ApJ Letters. This letter makes use of the TESS Alert data, which is currently in a beta test phase. The discovery light curve is included in a table inside the arxiv submission

Data from the newly-commissioned \textit{Transiting Exoplanet Survey Satellite} (TESS) has revealed a "hot Earth" around LHS 3844, an M dwarf located 15 pc away. The planet has a radius of $1.32\pm 0.02$ $R_\oplus$ and orbits the star every 11 hours. Although the existence of an atmosphere around such a strongly irradiated planet is questionable, the star is bright enough ($I=11.9$, $K=9.1$) for this possibility to be investigated with transit and occultation spectroscopy. The star's brightness and the planet's short period will also facilitate the measurement of the planet's mass through Doppler spectroscopy., 10 pages, 4 figures. Submitted to ApJ Letters. This letter makes use of the TESS Alert data, which is currently in a beta test phase, using data from the pipelines at the TESS Science Office and at the TESS Science Processing Operations Center

We report on the discovery by the Swift Gamma-Ray Burst Explorer of the eighth known transient accretion-powered millisecond pulsar: SWIFT J1756.9-2508, as part of routine observations with the Swift Burst Alert Telescope hard X-ray transient monitor. The pulsar was subsequently observed by both the X-Ray Telescope on Swift and the Rossi X-Ray Timing Explorer Proportional Counter Array. It has a spin frequency of 182 Hz (5.5 ms) and an orbital period of 54.7 minutes. The minimum companion mass is between 0.0067 and 0.0086 Solar Mass, depending on the mass of the neutron star, and the upper limit on the mass is 0.030 Solar Mass (95% confidence level). Such a low mass is inconsistent with brown dwarf models. and comparison with white dwarf models suggests that the companion is a He-dominated donor whose thermal cooling has been at least modestly slowed by irradiation from the accretion flux. No X-ray bursts. dips, eclipses or quasi-periodic oscillations were detected. The current outburst lasted approx. 13 days and no earlier outbursts were found in archival data.

Using archival X-ray data, we find that the catalog location of the X-ray binary Scutum X-1 (Sct X-1) is incorrect, and that the correct location is that of the X-ray source AX J183528-0737, which is 15' to the west. Our identification is made on the basis of the 112-s pulse period for this object detected in an XMM-Newton observation, as well as spatial coincidence between AX J183528-0737 and previous X-ray observations. Based on the XMM-Newton data and archival RXTE data, we confirm secular spin-down over 17 years with period derivative Pdot~3.9e-9 s/s, but do not detect a previously reported X-ray iron fluorescence line. We identify a bright (Ks=6.55), red (J-Ks=5.51), optical and infrared counterpart to AX J183528-0737 from 2MASS, a number of mid-IR surveys, and deep optical observations, which we use to constrain the extinction to and distance of Sct X-1. From these data, as well as limited near-IR spectroscopy, we conclude that Sct X-1 is most likely a binary system comprised of a late-type giant or supergiant and a neutron star., Comment: 10 pages, 9 figures. Revised following referee's comments. Accepted to ApJ

We describe observations of the seventh accretion-powered millisecond pulsar, HETE J1900.1-2455 made with the Rossi X-ray Timing Explorer during the year of activity that followed its discovery in 2005 June. We detected intermittent pulsations at a peak fractional amplitude of 3%, but only in the first two months of the outburst. On three occasions during this time we observed an abrupt increase in the pulse amplitude, approximately coincident with the time of a thermonuclear burst, followed by a steady decrease on a timescale of approx. 10 d. HETE J1900.1-2455 has shown the longest active period by far for any transient accretion-powered millisecond pulsar, comparable instead to the outburst cycles for other transient X-ray binaries. Since the last detection of pulsations, HETE J1900.1-2455 has been indistinguishable from a low-accretion rate, non-pulsing LMXB; we hypothesize that other, presently active LMXBs may have also been detectable initially as millisecond X-ray pulsars., Comment: 5 pages, 3 figures, accepted by ApJL following minor revisions suggested by referee

We report High Energy Transient Explorer 2 (HETE-2) Wide Field X-ray Monitor/French Gamma Telescope observations of XRF030723 along with observations of the XRF afterglow made using the 6.5m Magellan Clay telescope and the Chandra X-ray Observatory. The observed peak energy E_pk_obs of the nu F_nu burst spectrum is found to lie within (or below) the WXM 2-25 keV passband at 98.5% confidence, and no counts are detected above 30 keV. Our best fit value is E_pk_obs=8.4+3.5/-3.4 keV. The ratio of X-ray to Gamma-ray flux for the burst follows a correlation found for GRBs observed with HETE-2, and the duration of the burst is similar to that typical of long-duration GRBs. If we require that the burst isotropic equivalent energy E_iso and E_pk_rest satisfy the relation discovered by Amati et al. (2002), a redshift of z=0.38+0.36/-0.18 can be determined, in agreement with constraints determined from optical observations. We are able to fit the X-ray afterglow spectrum and to measure its temporal fade. Although the best-fit fade is shallower than the concurrent fade in the optical, the spectral similarity between the two bands indicates that the X-ray fade may actually trace the optical fade. If this is the case, the late time rebrightening observed in the optical cannot be due to a supernova bump. We interpret the prompt and afterglow X-ray emission as arising from a jetted GRB observed off-axis and possibly viewed through a complex circumburst medium due to a progenitor wind., Comment: 14 pages, 9 figures, to appear in ApJ

On 2001 September 21 at 05:15:50.56 UT, the French Gamma Telescope (FREGATE) on the High Energy Transient Explorer (HETE) detected a bright gamma-ray burst (GRB). The burst was also seen by the X-detector on the Wide-field X-ray Monitor (WXM) instrument and was therefore well localized in the X-direction; however, the burst was outside the fully coded field of view of the WXM Y-detector, and therefore information on the Y-direction of the burst was limited. Cross-correlation of the HETE and Ulysses time histories yielded an Interplanetary Network (IPN) annulus that crosses the HETE error strip at an ~45° angle. The intersection of the HETE error strip and the IPN annulus produces a diamond-shaped error region for the location of the burst having an area of 310 arcmin2. Based on the FREGATE and WXM light curves, the duration of the burst is characterized by t90 = 34.2 s in the WXM 4-25 keV energy range, and 23.8 and 21.8 s in the FREGATE 6-40 and 32-400 keV energy ranges, respectively. The fluence of the burst in these same energy ranges is 4.8 × 10-6, 5.5 × 10-6, and 11.4 × 10-6 ergs cm-2, respectively. Subsequent optical and radio observations by ground-based observers have identified the afterglow of GRB 010921 and determined an apparent redshift of z = 0.450.

We present an analysis of the nine type I X-ray bursts that were observed from KS 1731-260 with RXTE. We find that the bursts divide naturally into two populations: ``fast bursts'' occur on the Banana Branch when the accretion rate is high and exhibit short decay times, high peak fluxes, and radius expansion episodes. ``Slow bursts'' occur in the Island State at lower accretion rates, have lower peak fluxes, higher fluences, longer decay times, and show no evidence of radius expansion. All five of the fast bursts, and none of the four slow bursts, show coherent oscillations near 524 Hz. We perform in-burst phase connection of the burst pulsations, which allows us to unambiguously characterize their frequency evolution. That evolution exhibits a variety of behaviors, including a sharp spin-down during one burst. Applying our phase models, we find that the pulsations are spectrally harder than the burst emission, with the strength of the pulsations increasing monotonically with photon energy. Coherently summing all of the burst signals, we find upper limits of ~5% relative to the strength of the main pulse on any modulation at 1/2 or 3/2 the main pulse frequency. We discuss our results in the context of models in which the burst pulsations are produced by a hot spot on the surface of a spinning neutron star., Comment: Version to be published in ApJ v542, Oct 20, 2000. Only minor changes. 24 pages including 8 figures

Utilizing observations obtained with the Rossi X-ray Timing Explorer, we examine correlations between the properties of 0.08-22 Hz variable frequency QPOs and the X-ray spectral parameters for the black hole candidates XTE J1550-564 and GRO J1655-40. The spectra were fitted to a model including a multi-temperature blackbody disk and a power-law component. We find that the QPO frequency and amplitude are well correlated with the spectral parameters, although the correlations found for XTE J1550-564 are generally opposite to those for GRO J1655-40. There is one exeption: Both sources exhibit a general increase in the QPO frequency as the disk flux increases (or as the mass accretion rate through the disk increases). In addition, these QPOs are observed only when the power-law component contributes more than 20% of the 2-20 keV flux, which indicates that both the disk and the power-law components are linked to the QPO phenomenon.

We present results from the analysis of X-ray energy spectra and quasi-periodic oscillations (QPOs) from a set of observations that samples a broad range of time variability in GRS 1915+105. We first demonstrate that the frequency and integrated amplitude of a 0.5-10 Hz QPO is correlated with the apparent temperature of the accretion disk for the majority of observations. We then show that the behavior of GRS 1915+105 exhibits two distinct modes of accretion. In the first mode, the QPO is present between 0.5 and 10 Hz and variability in the source luminosity is dominated by the power-law component. In the second mode, the QPO is absent and the changes in the luminosity are dominated by thermal emission from the accretion disk. We find that the color radius and temperature of the inner accretion disk are empirically related by Rcol ∝ T + const. We discuss these results in terms of ongoing efforts to explain the origin of both the QPOs and the hard X-ray component in the spectrum of GRS 1915+105.

We have investigated 52 Rossi X-Ray T iming Explorer pointed observations of GRO J1655(40 span- ning the X-ray outburst that commenced on 1996 April 25 and lasted for 16 months. Our X-ray timing analyses reveal four types of quasi-periodic oscillations (QPOs): three with relatively stable central fre- quencies near 300, 9, and 0.1 Hz, and a fourth QPO that varied over the range 14¨28 Hz. The 300 and 0.1 Hz QPOs appear only when the power-law component dominates the X-ray spectrum and the esti- mated unabsorbed X-ray luminosity is above At lower luminosity the thermal D0.2L Edd .( L X D 0.1L Edd ), component dominates the spectrum; the disk appears somewhat cooler (D1.3 keV), and its inner radius appears larger. In this state only two of the QPOs are observed: the broad and spectrally ii soft ˇˇ 9 Hz QPO and the narrow, ii hard ˇˇ QPO that varies from 14 to 28 Hz as the hard —ux decreases. At still lower luminosities the power-law component contributes less than 30% of the total lumi- (L X \ 0.1L Edd ), nosity, the inner disk appears both larger and cooler, the 9 Hz QPO vanishes, and only a very weak (rms 0.3%) and narrow QPO at 28 Hz remains. The 300 Hz QPO is likely to be analogous to the stationary QPO at 67 Hz seen in the microquasar GRS 1915)105. We discuss four models of these high-frequency QPOs that depend on eUects due to general relativity. The models suggest that these rapid QPOs may eventually provide a measure of the mass and rotation of the accreting black hole. The 9 Hz QPO displays a spectrum consistent with a thermal origin, but this frequency is not uniquely con- sistent with any of the natural timescales associated with the disk. The mechanism for the 14¨28 Hz QPOs appears to be linked to the power-law component, similar to the 1¨10 Hz QPOs in GRS 1915)105. Thus these low-frequency QPOs have the potential to lead us to the origin of the energetic electrons that radiate the power-law spectral component. Finally, we show data for GRO J1655(40 and GRS 1915)105 as each source teeters between relative stability and a state of intense oscillations at 0.1 Hz. A comparison of the respective spectral parameters allows us to speculate that the black hole mass in GRS 1915)105 is very large, possibly D100 M _ . Subject headings: black hole physicsstars: individual (GRO J1655(40) ¨ stars: oscillations ¨ X-rays: stars

We have investigated the X-ray timing properties of XTE J1550-564 during 60 Rossi X-Ray Timing Explorer Proportional Counter Array observations made between 1998 September 18 and November 28. We detect quasi-periodic oscillations (QPOs) near 185 Hz during four time intervals. The QPO widths (FWHM) are ~50 Hz, and the rms amplitudes are ~1% of the mean flux at 2-30 keV. This is the third Galactic black hole candidate known to exhibit a transient X-ray timing signature above 50 Hz, following the 67 Hz QPO in GRS 1915+105 and the 300 Hz QPO in GRO J1655-40. However, unlike the previous cases, which appear to show stationary frequencies, the QPO frequency in XTE J1550-564 must vary by at least ~10% to be consistent with observations. The occurrences and properties of the QPO were insensitive to large changes in the X-ray intensity (1.5-6.8 crab). However, the QPO appearance was accompanied by changes in the energy spectrum, namely, an increase in the temperature and a decrease in the normalization of the thermal component. The QPO is also closely related to the hard X-ray power-law component of the energy spectrum since the fractional amplitude of the QPO increases with photon energy. The fast QPOs in accreting black hole binaries are thought to be effects of general relativity; however, the relevance of the specific physical models that have been proposed remains largely uncertain. Low-frequency QPOs in the range 3-13 Hz were often observed. Occasionally, at high luminosity, the rms QPO amplitude was ~15% of the flux, a level previously reached only by GRS 1915+105. These extraordinary oscillations have a coherence parameter (ν/Δν) in the range 4-12 and are tied to the power-law component in the energy spectrum.

Excellent coverage of the 1998 outburst of the X-ray Nova XTE J1550-564 was provided by the Rossi X-ray Timing Explorer. XTE J1550-564 exhibited an intense (6.8 Crab) flare on 1998 September 19 (UT), making it the brightest new X-ray source observed with RXTE. We present a spectral analysis utilizing 60 Proportional Counter Array spectra from 2.5-20 keV spanning 71 days, and a nearly continuous All Sky Monitor light curve. The spectra were fit to a model including multicolor blackbody disk and power-law components. XTE J1550-564 is observed in the very high, high/soft, and intermediate canonical outburst states of Black Hole X-ray Novae.

We present Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor observations of the X-ray binary Circinus X-1 that illustrate the variety of intensity profiles associated with the 16.55 day flaring cycle of the source. We also present eight observations of Cir X-1 made with the RXTE Proportional Counter Array over the course of a cycle wherein the average intensity of the flaring state decreased gradually over ~12 days. Fourier power density spectra for these observations show a narrow quasi-periodic oscillation (QPO) peak that shifts in frequency between 6.8 and 32 Hz, as well as a broad QPO peak that remains roughly stationary at ~4 Hz. We identify these as Z-source horizontal and normal branch oscillations (HBOs/NBOs), respectively. Color-color and hardness-intensity diagrams (CDs/HIDs) show curvilinear tracks for each of the observations. The properties of the QPOs and very low frequency noise allow us to identify segments of these tracks with Z-source horizontal, normal, and flaring branches that shift location in the CDs and HIDs over the course of the 16.55 day cycle. These results contradict a previous prediction, based on the hypothesis that Cir X-1 is a high- atoll source, that HBOs should never occur in this source.

We present the results from RXTE observations of a new X-ray transient, GRS 1737-31, near the Galactic center during an outburst. Over a span of two weeks, the source was seen to vary significantly on time scales down to a few seconds. The rapid flares in the light curves bear resemblance to those observed for Cygnus X-1, a well-known black hole candidate (BHC). The power-density spectrum is also very typical of BHCs in the HARD (or LOW) state: it is flat below a characteristic break frequency of ~0.03 Hz, and becomes roughly a 1/f power law above. The observed X-ray spectrum can be characterized by a simple power law with a photon index of ~1.7 over a broad energy range 2-200 keV, which is remarkably similar to that of Cyg X-1 in the HARD state. The similarities to Cyg X-1 make GRS 1737-31 a likely BHC. However, unlike most transient BHCs, GRS 1737-31 does not seem to reach a ``soft state'' even during an outburst. We discuss this phenomenon in light of the comparison to a class of hard X--ray BHCs including transients GRO J0422+32, V404Cyg and GRO J1719-24, and persistent sources 1E1740.7-2942 and GRS 1758-258.

We report on quasi-periodic oscillations (QPOs) in the black hole candidate GRS 1915+105 seen in 31 observations made by the Proportional Counter Array on board the Rossi X-Ray Timing Explorer. We distinguish three different types: a QPO with constant centroid frequency of 67 Hz, a dynamic low-frequency (10-3-10 Hz) QPO with a large variety of amplitudes and widths, and high-amplitude "sputters" at frequencies of 10-3-10-1 Hz that are among the most extreme X-ray variations ever seen. We discuss the 67 Hz QPO on the assumption that it arises in the inner accretion disk of a black hole binary. If this QPO represents the rotation frequency of the innermost stable orbit around a nonrotating black hole, then the implied mass is 33 M☉. An alternative interpretation of this QPO as g-mode oscillations in the inner disk (Nowak et al. 1996) implies a black hole mass of 10 M☉. Four selected QPOs at lower frequencies (0.067-1.8 Hz) are tracked continuously in five energy bands. The QPO-folded profiles are mostly sinusoidal. Remarkably, all four investigated QPOs are broadened in frequency by a random walk in oscillation phase. At higher photon energies the QPO profiles show larger amplitude and increasing phase lag. Our QPO profile analysis shows that the phase delays are not caused by scattering effects, and we discuss a more direct relation between these QPOs and the origin of the hard X-ray spectrum. There are at least three general shapes of the broadband power continuum, with a typical persistence time scale of several weeks. The combined characteristics of the power spectra, light curves, and energy spectra during the period between 1996 February 21 and 1996 August 15 are interpreted as a succession of four different emission states. None of these states appears identical to any of the canonical states of black hole binaries.

A highly coherent 523.92+-0.05 Hz periodic X-ray signal has been observed during a type I X-ray burst from the low-mass X-ray binary system KS 1731-260 with the PCA on RXTE. The spectral evolution of the burst indicates photospheric-radius expansion and contraction. The 524 Hz signal occurred at the end of the contraction phase, lasted for ~2 s, was highly coherent (Q >~ 900), and had a pulse fraction (ratio of sinusoidal amplitude to mean count rate) of 6.2+-0.6%. KS 1731-260 is one of only three systems that have exhibited high-coherence millisecond oscillations during X-ray bursts and the first reported where the pulsations are associated with photospheric contraction. These coherent signals may be interpreted as a direct indication of the neutron star spin.

The Galactic superluminal motion source GRS 1915+105 was observed with the Rossi X-Ray Timing Explorer satellite at several occasions during its ongoing active state. The observed X-ray intensity changes drastically on a variety of timescales ranging from subseconds to days. In particular, the source exhibits quasi-periodic brightness sputters with varying duration and repetition timescale. These episodes occur occasionally, while the more common X-ray intensity variations are faster with much smaller amplitudes. The spectrum during the brightness sputters is remarkably different from the spectrum of the mean high-state emission. We argue that such sputtering episodes are possibly caused by a major accretion disk instability. Based on the coincidence in time of two radio flares following the observed X-ray sputtering episodes, we speculate that superluminal ejections (as observed from GRS 1915+105 during earlier activity periods) are related to episodes of large-amplitude X-ray variations.

We discuss our recent Rossi X-Ray Timing Explorer (RXTE) observations of GRO J1744-28, which discovered quasi-periodic oscillations (QPOs) of the intensities in the energy band 3-12 keV during observations starting on 1996 January 18.77 UT. We have found that the power spectrum in the frequency band 5-1000 Hz consists of two red-noise components that can be characterized by two power laws, each with an index of -5/3 and a QPO peak centered at 40 Hz. We suggest that the power peak is due to a newly discovered form of turbulence in the accretion column of super-Eddington accretion-powered pulsars driven by photon bubble instabilities. These instabilities give rise to strong power peaks at frequencies characteristic of photon diffusion and bubble coalescence in the highly nonlaminar accretion column resulting in photon bubble oscillations (PBOs). The relationship between the rms amplitude of the PBOs and the intensity is in qualitative agreement with observations. Our calculations also suggest that the observed high-frequency red-noise component with a -5/3 power-law index from 40 to 600 Hz is the first evidence of photon bubble turbulence in the accretion column of an X-ray pulsar. Recent RXTE observations of Sco X-1 have found high-frequency QPOs at 1100 and 830 Hz. We show that PBOs at these frequencies are a natural consequence of photon bubble instabilities. We also show that the rms amplitudes of the calculated PBOs at these frequencies are consistent with the observations. We predict that further RXTE observations of Sco X-1 should reveal additional PBOs at 2000 and 2600 Hz as well as a broadband continuum spectrum with a -5/3 power law, extending from 3000 to several times 104 Hz.

The All-Sky Monitor on the Rossi X-ray Timing Explorer has been monitoring the sky in the 1.5 - 12 keV band since late February. The instrument consists of three coded-aperture cameras which can be rotated to view different regions by a motorized drive assembly. Intensities of ~100 known sources are obtained via least-square fits of shadow patterns to the data and compiled to form x-ray light curves. Six orbital periodicities and four long-term periodicities, all previously known, have been detected in these light curves. Searches for additional sources have also been conducted. X-ray light curves for the Crab Nebula, Cyg X-1, 4U 1705-44, GRO J1655-40, and SMC X-1 are reported. They illustrate the quality of the results and the range of observed phenomena.

The newly discovered source GRO J1744-28 is remarkable in many respects. It is a low-mass X-ray binary system that shows 2.1 Hz coherent pulsations. It also produces type II X-ray bursts similar to those seen in the Rapid Burster. In this Letter we report results from a series of observations with the Rossi X-Ray Timing Explorer (RXTE) during which GRO J1744-28 showed quasi-periodic X-ray brightness oscillations (QPOs) at 20, 40, and 60 Hz. Their fractional rms amplitudes are 0.19%, 5.9%, and 0.42%, respectively. The QPO centroid frequency and its rms amplitude as a function of source brightness in the band of 2-60 keV indicate that these QPOs are of a different kind from the beat-frequency QPOs that have been observed among Z sources as well as X-ray pulsars. We discuss possible interpretations of these QPOs in the context of neutron star g-mode oscillations and photon bubble oscillations.

In this Letter, we provide an introduction to the main features seen in a series of observations of the bursting pulsar GRO J1744-28. The observations were made from 1996 January through May with the proportional counter array on the Rossi X-Ray Timing Explorer spacecraft. In the 2-10 keV band, GRO J1744-28 emitted large bursts of ~10 s duration at a rate of about 2 per hour. The peak flux during these bursts was ~6-40 times greater than that in the quiescent, or nonbursting, periods. For the earliest bursts, the inferred peak luminosity approaches 100 times the Eddington limit, which is suggestive of some kind of beaming mechanism. A range of smaller bursts and quasi-periodic oscillation features were also seen. All this activity was superposed on an almost perfect sinusoidal modulation at a frequency of 2.14 Hz with an amplitude of ~10% of the nonbursting flux. The source's persistent flux declined in a roughly linear trend from late January until mid-May, by which time its intensity was confused with several other sources.

We have carried out a study of Cir X-1 through (1) detailed sampling over a single 16.5 day intensity cycle with the Rossi X-Ray Timing Explorer (RXTE) proportional counter array and (2) complementary monitoring over six such cycles with the RXTE all-sky monitor (ASM). We report here the temporal and spectral evolution at nonflaring phases, during which Cir X-1 remained unusually bright (~1.0 crab) and relatively steady. The Fourier power density spectrum of the source was observed to vary with strong correlations among low-frequency flat-topped power (1-10 Hz), a quasi-periodic oscillation (QPO) peak centered at 1.3-12 Hz, and a broad QPO peak centered at ~20 Hz up to ~100 Hz. As orbital phase increased within the cycle, the rms amplitude of the flat-topped power generally (but not monotonically) decreased while the QPO features generally evolved toward higher frequency. The 1.5-12 keV spectrum was observed to generally harden during the nonflaring phases of each of the six 16.5 day periods monitored by the ASM.

The Murchison Widefield Array (MWA) has collected hundreds of hours of Epoch of Reionization (EoR) data and now faces the challenge of overcoming foreground and systematic contamination to reduce the data to a cosmological measurement. We introduce several novel analysis techniques such as cable reflection calibration, hyper-resolution gridding kernels, diffuse foreground model subtraction, and quality control methods. Each change to the analysis pipeline is tested against a two dimensional power spectrum figure of merit to demonstrate improvement. We incorporate the new techniques into a deep integration of 32 hours of MWA data. This data set is used to place a systematic-limited upper limit on the cosmological power spectrum of $\Delta^2 \leq 2.7 \times 10^4$ mK$^2$ at $k=0.27$ h~Mpc$^{-1}$ and $z=7.1$, consistent with other published limits, and a modest improvement (factor of 1.4) over previous MWA results. From this deep analysis we have identified a list of improvements to be made to our EoR data analysis strategies. These improvements will be implemented in the future and detailed in upcoming publications., Comment: 21 pages, 16 figures, accepted to ApJ

In this paper, we explore for the first time the relative magnitudes of three fundamental sources of uncertainty, namely, foreground contamination, thermal noise, and sample variance, in detecting the H I power spectrum from the epoch of reionization (EoR). We derive limits on the sensitivity of a Fourier synthesis telescope to detect EoR based on its array configuration and a statistical representation of images made by the instrument. We use the Murchison Widefield Array (MWA) configuration for our studies. Using a unified framework for estimating signal and noise components in the H I power spectrum, we derive an expression for and estimate the contamination from extragalactic point-like sources in three-dimensional k-space. Sensitivity for EoR H I power spectrum detection is estimated for different observing modes with MWA. With 1000 hr of observing on a single field using the 128 tile MWA, EoR detection is feasible (S/N >1 for k [< over ~] 0.8 Mpc[superscript –1]). Bandpass shaping and refinements to the EoR window are found to be effective in containing foreground contamination, which makes the instrument tolerant to imaging errors. We find that for a given observing time, observing many independent fields of view does not offer an advantage over a single field observation when thermal noise dominates over other uncertainties in the derived power spectrum., National Science Foundation (U.S.) (Grant AST-0457585), National Science Foundation (U.S.) (Grant PHY-0835713), National Science Foundation (U.S.) (Grant CAREER-0847753), National Science Foundation (U.S.) (Grant AST-0908884), United States. Air Force Office of Scientific Research (Grant FA9550-0510247), Smithsonian Astrophysical Observatory, MIT School of Science

We present the first spectroscopic images of solar radio transients from the prototype for the Murchison Widefield Array (MWA), observed on 2010 March 27. Our observations span the instantaneous frequency band 170.9-201.6 MHz. Though our observing period is characterized as a period of `low' to `medium' activity, one broadband emission feature and numerous short-lived, narrowband, non-thermal emission features are evident. Our data represent a significant advance in low radio frequency solar imaging, enabling us to follow the spatial, spectral, and temporal evolution of events simultaneously and in unprecedented detail. The rich variety of features seen here reaffirms the coronal diagnostic capability of low radio frequency emission and provides an early glimpse of the nature of radio observations that will become available as the next generation of low frequency radio interferometers come on-line over the next few years., Comment: 15 pages, 5 figures, accepted for publication in Astrophysical Journal Letters. Movies for figures 4 and 5 available at http://www.mwatelescope.org/info/mwa_proto.html

Table 1 contained one error. The time of ascending node T asc was incorrect due to a numerical conversion error in the preparation of the original table text. The correct value is T asc = 54264.78707(6). The correct value was used in all of the analysis leading up to the paper. As T asc is a purely fiducial reference time; the scientific conclusions of the paper are unchanged. We thank A. Patruno for bringing this error to our attention.

Eight observations of Cir X-1 with SAS 3, each lasting 1-6 days, have yielded a variety of new phenomena, viz., a luminous state of steady emission, rapid large-intensity dips, an extremely rapid X-ray transition, and bright flares. Through searches for periodic X-ray pulsations were carried out on data trains of duration up to 6 days; upper limits for pulsations with periods greater than 250 microsec range down to 0.3%. Aperiodic variability with characteristic times of 0.4-1.0 sec was observed but is not well characterized by a simple shot noise model. No millisecond bursts were observed during 40,000 sec in three separate observations. Spectral parameters derived before and after several X-ray transitions indicate that the transitions are not due to absorption of X-rays by intervening gas. Models previously proposed for the Cir X-1 system do not easily provide explanations for all the complex phenomena reported herein.