Reba M. Bandyopadhyay

VISTA Variables in the Via Lactea (VVV): The public ESO near-IR variability survey of the Milky Way

Original article can be found at: http://www.sciencedirect.com/science/journal/13841076 Copyright Elsevier B.V.

The United Kingdom Infrared Telescope Infrared Deep Sky Survey First Data Release

The First Data Release (DR1) of the UKIRT Infrared Deep Sky Survey (UKIDSS) took place on 2006 July 21. UKIDSS is a set of five large near–infrared surveys, covering a complementary range of areas, depths, and Galactic latitudes. DR1 is the first large release of survey-quality data from UKIDSS and includes 320 deg of multicolour data to (Vega) K = 18, complete (depending on the survey) in three to five bands from the set ZYJHK, together with 4 deg of deep JK data to an average depth K = 21. In addition the release includes a similar quantity of data with incomplete filter coverage. In JHK, in regions of low extinction, the photometric uniformity of the calibration is better than 0.02mag. in each band. The accuracy of the calibration in ZY remains to be quantified, and the same is true of JHK in regions of high extinction. The median image FWHM across the dataset is 0.82. We describe changes since the Early Data Release in the implementation, pipeline and calibration, quality control, and archive procedures. We provide maps of the areas surveyed, and summarise the contents of each of the five surveys in terms of filters, areas, and depths. DR1 marks completion of 7 per cent of the UKIDSS 7-year goals.

A CATALOG OF X-RAY POINT SOURCES FROM TWO MEGASECONDS OF CHANDRA OBSERVATIONS OF THE GALACTIC CENTER

We present a catalog of 9017 X-ray sources identified in Chandra observations of a 2 ◦ × 0. 8 field around the Galactic center. This enlarges the number of known X-ray sources in the region by a factor of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Ms of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities of 4 × 10 32 erg s −1 (0.5–8.0 keV; 90% confidence) over an area of 1 deg 2 , and up to an order of magnitude more sensitive in the deepest exposure (1.0 Ms) around Sgr A ∗ . The positions of 60% of our sources are accurate to <1 �� (95% confidence), and 20% have positions accurate to < 0. 5. We search for variable sources, and find that 3% exhibit flux variations within an observation, and 10% exhibit variations from observation-to-observation. We also find one source, CXOUGC J174622.7−285218, with a periodic 1745 s signal (1.4% chance probability), which is probably a magnetically accreting cataclysmic variable. We compare the spatial distribution of X-ray sources to a model for the stellar distribution, and find 2.8σ evidence for excesses in the numbers of X-ray sources in the region of recent star formation encompassed by the Arches, Quintuplet, and Galactic center star clusters. These excess sources are also seen in the luminosity distribution of the X-ray sources, which is flatter near the Arches and Quintuplet than elsewhere in the field. These excess point sources, along with a similar longitudinal asymmetry in the distribution of diffuse iron emission that has been reported by other authors, probably have their origin in the young stars that are prominent at l ≈ 0. 1.

Infrared spectroscopy of low-mass X-ray binaries – II

Using CGS4 on UKIRT, we have obtained 2.00–2.45 μ m infrared spectra of a number of low-mass X-ray binaries including Sco X-1, Sco X-2 and GX1 3+1. Sco X-1 shows emission lines only, supporting our previous conclusion that the spectral type of the evolved secondary must be earlier than G5. Emission lines are also seen in the spectrum of Sco X-2, confirming the identity of the IR counterpart. We report the detection of CO bands in GX1 3+1, and estimate the most likely spectral type of the secondary to be K5III. We also find P Cygni-type profiles in the Brackett γ lines of Sco X-1 and GX1 3+1, indicating the presence of high-velocity outflows in these systems. We present spectra of candidate IR counterparts to several other elusive X-ray binaries. Finally, implications for the nature and classification of these systems are discussed.

“Soft X-ray transient” outbursts which are not soft

Abstract We have accumulated multiwavelength (X-ray, optical, radio) lightcurves for the eight black hole X-ray binaries which have been observed to enter a supposed ‘soft X-ray transient’ outburst, but remained in the low/hard state throughout the outburst. Comparison of the lightcurve morphologies, spectral behaviour, properties of the quasi-periodic oscillations and the radio jet provides the first study of such objects as a sub-class of X-ray transients. However, rather than assuming that these hard state X-ray transients are different from the ‘canonical’ soft X-ray transient, we prefer to consider the possibility that new analysis of both soft and hard state X-ray transients in a spectral context will provide a model capable of explaining the outburst mechanisms of (almost) all black hole X-ray binaries.

A Chandra Catalog of X-Ray Sources in the Central 150 pc of the Galaxy

We present the catalog of X-ray sources detected in a shallow Chandra survey of the inner 2° × 08 of the Galaxy and in two deeper observations of the Radio Arches and Sgr B2. The catalog contains 1352 objects that are highly absorbed (NH 4 × 1022 cm-2) and are therefore likely to lie near the Galactic center (D ≈ 8 kpc), and 549 less absorbed sources that lie within 6 kpc of Earth. On the basis of the inferred luminosities of the X-ray sources and the expected numbers of various classes of objects, we suggest that the sources with LX 1033 ergs s-1 that comprise ≈90% of the catalog are cataclysmic variables and that the ≈100 brighter objects are accreting neutron stars and black holes, young isolated pulsars, and Wolf-Rayet and O (WR/O) stars in colliding-wind binaries. We find that the spatial distribution of X-ray sources matches that of the old stellar population observed in the infrared, which supports our suggestion that most of the X-ray sources are old cataclysmic variables. However, we find that there is an apparent excess of ≈10 bright sources in the Radio Arches region. That region is already known to be the site of recent star formation, so we suggest that the bright sources in this region are young high-mass X-ray binaries, pulsars, or WR/O star binaries. We briefly discuss some astrophysical questions that this catalog can be used to address.

USA Experiment and RXTE Observations of a Variable Low-Frequency Quasi-periodic Oscillation in XTE J1118+480

The USA experiment on ARGOS and RXTE have exensively observed the X-ray transient XTE J1118+480 during its recent outburst in 2000 April--June. We present detailed monitoring of the evolution of a low frequency QPO which drifts from 0.07 Hz to 0.15 Hz during the outburst. We examine possible correlations of the QPO frequency with the flux and spectral characteristics of the source, and compare this QPO to low frequency QPOs observed in other black hole candidates.Using the USA Experiment on the Advanced Research and Global Observation Satellite and the Rossi X-Ray Timing Explorer, we have extensively observed the X-ray transient XTE J1118+480 during its recent outburst in 2000 April-June. We present detailed monitoring of the evolution of a low-frequency quasi-periodic oscillation (QPO) that drifts from 0.07 to 0.15 Hz during the outburst. We examine possible correlations of the QPO frequency with the flux and spectral characteristics of the source, and we compare this QPO with low-frequency QPOs observed in other black hole candidates.

Eclipse Timings of the Low-Mass X-Ray Binary EXO 0748–676. III. Orbital Period Jitter Observed with the Unconventional Stellar Aspect Experiment and the Rossi X-Ray Timing Explorer

We present seven eclipse timings of the low-mass X-ray binary (LMXB) EXO 0748-676 obtained with the Unconventional Stellar Aspect (USA) Experiment during 1999-2000, as well as 122 eclipse timings obtained with the Rossi X-Ray Timing Explorer (RXTE) during 1996-2000. According to our analysis, the mean orbital period has increased by ~8 ms between the pre-RXTE era (1985-1990) and the RXTE/USA era (1996-2000). This corresponds to an orbital period derivative of Porb/orb ~ 2 × 107 yr. However, neither a constant orbital period derivative nor any other simple ephemeris provides an acceptable fit to the data; individual timings of eclipse centers have residuals of up to 15 or more seconds away from our derived smooth ephemerides. When we consider all published eclipse timing data, including those presented here, a model that includes observational measurement error, cumulative period jitter, and underlying period evolution is found to be consistent with the timing data. We discuss several physical mechanisms for LMXB orbital evolution in an effort to account for the change in orbital period and the observed intrinsic jitter in the mideclipse times.

Disk Diffusion Propagation Model for the Outburst of XTE J1118+480

We present a linear diffusion model for the evolution of the double-peaked outburst in the transient source XTE J1118+480. The model treats the two outbursts as episodic mass deposition at the outer radius of the disk followed by evolution of disk structure according to a diffusion process. We demonstrate that light curves with fast-rise, exponential decay profile are a general consequence of the diffusion process. Deconvolution of the light curve proves to be feasible and gives an input function specifying mass deposition at the outer disk edge as well as the total mass of the disk, both as functions of time. The derived evolution of total disk mass can be correlated with the observed evolution of the ~0.1 Hz QPO in the source reported by Wood et al.

The Superluminal Source GRS 1915+105: A High-Mass X-Ray Binary?

We obtained K-band (2.0-2.5 μm) infrared spectra of the visually obscured superluminal source GRS 1915+105 at three different epochs of 1995, at times when it was detected as a hard X-ray source. The three spectra exhibit H I Brγ (2.167 μm) and He I (2.058 μm) emission lines. The spectral morphology, absolute magnitude, and time variability of the infrared counterpart are consistent with the class of high-mass X-ray binaries that contain late Oe and early Be stars as mass donors. In GRS 1915+105 we have not detected the Doppler-shifted emission lines observed in the classic stellar source of relativistic jets, SS 433. We discuss why it may be more difficult in GRS 1915+105 than in SS 433 to detect emission lines from the jets.