S. Laycock

X-Ray Processing of ChaMPlane Fields: Methods and Initial Results for Selected Anti-Galactic Center Fields

We describe the X-ray analysis procedure of the ongoing Chandra Multiwavelength Plane (ChaMPlane) Survey and report the initial results from the analysis of 15 selected anti-Galactic center observations (90° < l < 270°). We describe the X-ray analysis procedures for ChaMPlane using custom-developed analysis tools appropriate for Galactic sources but also of general use: optimum photometry in crowded fields using advanced techniques for overlapping sources, rigorous astrometry and 95% error circles for combining X-ray images or matching to optical/IR images, and application of quantile analysis for spectral analysis of faint sources. We apply these techniques to 15 anti-Galactic center observations (of 14 distinct fields), in which we have detected 921 X-ray point sources. We present log N-log S distributions and quantile analysis to show that in the hard band (2-8 keV) active galactic nuclei dominate the sources. Complete analysis of all ChaMPlane anti-Galactic center fields will be given in a subsequent paper, followed by papers on sources in the Galactic center and bulge regions.

A Decade in the Life of EXO 2030+375: A Multiwavelength Study of an Accreting X-Ray Pulsar

Using BATSE and RXTE observations from 1991 April to 2001 August, we have detected 71 outbursts from 82 periastron passages of EXO 2030+375, a 42 s transient X-ray pulsar with a Be star companion, including several outbursts from 1993 August to 1996 April, when the source was previously believed to be quiescent. Combining BATSE, RXTE, and EXOSAT data, we have derived an improved orbital solution. Applying this solution results in a smooth profile for the spin-up rate during the giant outburst and results in evidence for a correlation between the spin-up rate and observed flux in the brighter BATSE outbursts. Infrared and H? measurements show a decline in the density of the circumstellar disk around the Be star. This decline is followed by a sudden drop in the X-ray flux and a turnover from a spin-up trend to spin-down in the frequency history. This is the first Be/X-ray binary that shows an extended interval, about 2.5 yr, in which the global trend is spin-down, but the outbursts continue. In 1995 the orbital phase of EXO 2030+375s outbursts shifted from peaking about 6 days after periastron to peaking before periastron. The outburst phase slowly recovered to peaking at about 2.5 days after periastron. We interpret this shift in orbital phase followed by a slow recovery as evidence of a global one-armed oscillation propagating in the Be disk. This is further supported by changes in the shape of the H? profile, which are commonly believed to be produced by a reconfiguration of the Be disk. The truncated viscous decretion disk model provides an explanation for the long series of normal outbursts and the evidence of an accretion disk in the brighter normal outbursts. Long-term multiwavelength observations such as these clearly add considerably to our knowledge of Be/X-ray binaries and the relationship among optical, infrared, and X-ray observations.

Long-term properties of accretion discs in X-ray binaries — I. The variable third period in SMC X-1

Long-term X-ray monitoring data from the Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor (ASM) and the Compton Gamma Ray Observatory (CGRO) Burst And Transient Source Experiment (BATSE) reveal that the third (superorbital) period in SMC X-1 is not constant, but varies between 40 and 60 d. A dynamic power spectrum analysis indicates that the third period has been present continuously throughout the five years of ASM observations. This period changed smoothly from 60 to 45 d and then returned to its former value, on a time-scale of approximately 1600 d. During the nearly 4 yr of overlap between the CGRO and RXTE missions, the simultaneous BATSE hard X-ray data confirm and extend this variation in SMC X-1. Our discovery of such an instability in the superorbital period of SMC X-1 is interpreted in the context of recent theoretical studies of warped, precessing accretion discs. We find that the behaviour of SMC X-1 is consistent with a radiation-driven warping model.

A Long Look at the Be/X-Ray Binaries of the Small Magellanic Cloud

We have monitored 41 Be/X-ray binary systems in the Small Magellanic Cloud over ~9 yr using PCA RXTE data from a weekly survey program. The resulting light curves were analyzed in search of orbital modulations with the result that 10 known orbital ephemerides were confirmed and refined, while 10 new ones where determined. A large number of X-ray orbital profiles are presented for the first time, showing similar characteristics over a wide range of orbital periods. Lastly, three pulsars, SXP 46.4, SXP 89.0, and SXP 165, were found to be misidentifications of SXP 46.6, SXP 91.1, and SXP 169, respectively.

Chandra Multiwavelength Plane (ChaMPlane) Survey: An Introduction

We introduce the Chandra Multiwavelength Plane (ChaMPlane) survey, designed to measure or constrain the populations of low-luminosity (LX 1031 ergs s-1) accreting white dwarfs, neutron stars, and stellar mass black holes in the Galactic plane and bulge. ChaMPlane incorporates two surveys, X-ray (Chandra) and optical (NOAO 4 m Mosaic imaging), and a follow-up spectroscopy and IR identification program. The survey has now extended through the first 6 yr of Chandra data using serendipitous sources detected in 105 distinct ACIS-I and ACIS-S fields observed in 154 pointings and covered by 65 deep Mosaic images in V, R, I, and Hα. ChaMPlane incorporates fields with Galactic latitude 12° and selected to be devoid of bright point or diffuse sources, with exposure time 20 ks and (where possible) minimum NH. We describe the scientific goals and introduce the X-ray and optical/IR processing and databases. We derive preliminary constraints on the space density or luminosity function of cataclysmic variables (CVs) from the X-ray/optical data for 14 fields in the Galactic anticenter. The lack of ChaMPlane CVs in these anticenter fields suggests that their space density is ~3 times below the value (3 × 10-5 pc-3) found for the solar neighborhood by previous X-ray surveys. Companion papers describe the X-ray and optical processing in detail, optical spectroscopy of ChaMPlane sources in selected anticenter fields, and IR imaging results for the Galactic center field. An appendix introduces the ChaMPlane Virtual Observatory (VO) for online access to the X-ray and optical images and source catalogs for ready display and further analysis.

MMT Observations of the Black Hole Candidate XTE J1118+480 near and in Quiescence*

We report on the analysis of new and previously published MMT optical spectra of the black hole binary XTE J1118+480 during the decline from the 2000 outburst to true quiescence. From cross-correlation with template stars, we measure the radial velocity of the secondary to derive a new spectroscopic ephemeris. The observations acquired during approach to quiescence confirm the earlier reported modulation in the centroid of the double-peaked Hα emission line. In addition, our data combined with the results presented by Zurita et al. (2002) provide support for a modulation with a periodicity in agreement with the expected precession period of the accretion disk of ~52 days. Doppler images during the decline phase of the Hα emission line show evidence for a hot spot and emission from the gas stream: the hot spot is observed to vary its position, which may be due to the precession of the disk. The data available during quiescence show that the centroid of the Hα emission line is offset by about -100 km s-1 from the systemic velocity, which suggests that the disk continues to precess. An Hα tomogram reveals emission from near the donor star after subtraction of the ringlike contribution from the accretion disk, which we attribute to chromospheric emission. No hot spot is present, suggesting that accretion from the secondary has stopped (or decreased significantly) during quiescence. Finally, a comparison is made with the black hole XRN GRO J0422+32: we show that the Hα profile of this system also exhibits a behavior consistent with a precessing disk.

Long-Term Behavior of X-Ray Pulsars in the Small Magellanic Cloud

Results of a 4 yr X-ray monitoring campaign of the Small Magellanic Cloud using the Rossi X-ray Timing Explorer (RXTE) are presented. This large data set makes possible detailed investigation of a significant sample of SMC X-ray binaries. Eight new X-ray pulsars were discovered, and a total of 20 different systems were detected. Spectral and timing parameters were obtained for 18. In the case of 10 pulsars, repeated outbursts were observed, allowing determination of candidate orbital periods for these systems. We also discuss the spatial and pulse-period distributions of the SMC pulsars.

RADIAL DISTRIBUTION OF X-RAY POINT SOURCES NEAR THE GALACTIC CENTER

We present the log N-log S and spatial distributions of X-ray point sources in seven Galactic bulge (GB) fields within 4? from the Galactic center (GC). We compare the properties of 1159 X-ray point sources discovered in our deep (100 ks) Chandra observations of three low extinction Window fields near the GC with the X-ray sources in the other GB fields centered around Sgr B2, Sgr C, the Arches Cluster, and Sgr A* using Chandra archival data. To reduce the systematic errors induced by the uncertain X-ray spectra of the sources coupled with field-and-distance-dependent extinction, we classify the X-ray sources using quantile analysis and estimate their fluxes accordingly. The result indicates that the GB X-ray population is highly concentrated at the center, more heavily than the stellar distribution models. It extends out to more than 14 from the GC, and the projected density follows an empirical radial relation inversely proportional to the offset from the GC. We also compare the total X-ray and infrared surface brightness using the Chandra and Spitzer observations of the regions. The radial distribution of the total infrared surface brightness from the 3.6 band ?m images appears to resemble the radial distribution of the X-ray point sources better than that predicted by the stellar distribution models. Assuming a simple power-law model for the X-ray spectra, the closer to the GC the intrinsically harder the X-ray spectra appear, but adding an iron emission line at 6.7 keV in the model allows the spectra of the GB X-ray sources to be largely consistent across the region. This implies that the majority of these GB X-ray sources can be of the same or similar type. Their X-ray luminosity and spectral properties support the idea that the most likely candidate is magnetic cataclysmic variables (CVs), primarily intermediate polars (IPs). Their observed number density is also consistent with the majority being IPs, provided the relative CV to star density in the GB is not smaller than the value in the local solar neighborhood.

Three new X‐ray pulsars detected in the Small Magellanic Cloud and the positions of two other known pulsars determined

Three new X-ray pulsars have been detected in the Small Magellanic Cloud (SMC) and the positions of two others have been determined, with archive Chandra data. A series of five observations of the SMC took place between 2002 May and October. Analysis of these data has revealed three previously unknown X-ray pulsars at pulse periods of 34, 503 and 138 s. The position of pulsar XTE J0052−725, which was originally detected by RXTE on 2002 June 19, was also accurately determined and a previously detected 7.78-s RXTE pulsar was identified as the source SMC X-3. Ke yw ords: stars: emission-line, Be ‐ Magellanic Clouds ‐ X-rays: binaries.

The Discovery of an Outburst and Pulsed X-Ray Flux from SMC X-2 Using the Rossi X-Ray Timing Explorer

Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor observations of SMC X-2 show that the source experienced an outburst in 2000 JanuaryndashApril reaching a peak luminosity of greater than sim1038 ergs s-1. RXTE Proportional Counter Array observations during this outburst reveal the presence of pulsations with a 2.37 s period. However, optical photometry of the optical counterpart shows that the source was still significantly fainter than it was more than a half a year after the outburst in the 1970s when SMC X-2 was discovered.