Spitzer Space Telescope Observations of Low Mass X-ray Binaries

We present preliminary results from our archival Spitzer Space Telescope program aimed at characterizing the mid-IR properties of compact objects, both isolated and in binary systems, i.e. white dwarfs, X-ray binaries, cataclysmic variables, and magnetars. Most of these sources are too faint at mid-IR wavelengths to be observable from the ground, so this study provides the very first comprehensive look at the mid-IR emission of these objects. Here we present our results for the low mass X-ray binaries. We considered all of the systems listed in the most recent catalog of Liu et al. (2007) that have known optical counterparts. The particular goals of our projects encompass: to establish the mid-IR spectral energy distribution, to search for the signatures of jets, circumbinary disks, low mass or planetary companions and debris disks, and to study the local environment of these sources.Comment: 6 pages, updated and expanded version of article to appear in Proceedings of "A Population Explosion: The Nature and Evolution of X-ray Binaries in Diverse Environments", 28 Oct - 2 Nov, St. Pete Beach, FL; eds. R.M. Bandyopadhyay, S. Wachter, D. Gelino, C.R. Gelino; AIP Conference Proceedings Serie

Fermi/Gamma-Ray Burst Monitor Observations of SGR J0501+4516 Bursts

We present our temporal and spectral analyses of 29 bursts from SGR J0501+4516, detected with the gamma-ray burst monitor on board the Fermi Gamma-ray Space Telescope during 13 days of the source's activation in 2008 (August 22- September 3). We find that the T_90 durations of the bursts can be fit with a log-normal distribution with a mean value of ~123 ms. We also estimate for the first time event durations of soft gamma repeater (SGR) bursts in photon space (i.e., using their deconvolved spectra) and find that these are very similar to the T 90 values estimated in count space (following a log-normal distribution with a mean value of ~124 ms). We fit the time-integrated spectra for each burst and the time-resolved spectra of the five brightest bursts with several models. We find that a single power law with an exponential cutoff model fits all 29 bursts well, while 18 of the events can also be fit with two blackbody functions. We expand on the physical interpretation of these two models and we compare their parameters and discuss their evolution. We show that the time-integrated and time-resolved spectra reveal that E_peak decreases with energy flux (and fluence) to a minimum of ~30 keV at F = 8.7 × 10^(–6) erg cm^(–2) s^(–1), increasing steadily afterward. Two more sources exhibit a similar trend: SGRs J1550-5418 and 1806-20. The isotropic luminosity, L iso, corresponding to these flux values is roughly similar for all sources (0.4-1.5 × 10^(40) erg s^(–1))

Mid-Infrared Circumstellar Shell Sources Discovered with Spitzer: An Obscured Population of Massive Stars?

We have discovered a large number of circular and elliptical shells around luminous central sources at 24 μm with the MIPS instrument on board the Spitzer Space Telescope. Most of these shells are not visible in the shorter wavelengths bands of IRAC or archival 2MASS and optical images. On the other hand, many of the central stars are detected in the 2MASS catalog, but lack an optical counterpart, indicating that we are dealing with a population of highly obscured objects. Our archival follow-up effort has revealed 90% of these shell sources to be previously unknown

Chandra Localizations of LMXBs: IR Counterparts and their Properties

We present new Chandra observations of the low mass X-ray binaries (LMXBs) X1624−490, X1702−429, and X1715−321 and the search for their Infrared (IR) counterparts. We also report on early results from our dedicated IR survey of LMXBs. The goal of this program is to investigate whether IR counterparts can be identified through unique IR colors and to trace the origin of the IR emission in these systems

Photometry of GX 349+2: Evidence for a 22-hour Period

The intense galactic X-ray source GX 349+2 (Sco X-2) belongs to the class of persistently bright low-mass X-ray binaries called Z-sources. GX 349+2, although observed in X-rays for more than 30 years, has only recently been optically identified with a 19th mag star. Of the six known Z-sources, only two (Sco X-1 and Cyg X-2) have been studied in the optical. It has been suggested that Z-sources as a group are characterized by evolved companions and correspondingly long orbital periods (Sco X-1, P=0.8 d; Cyg X-2, P=9.8 d). Recently Southwell et al. (1996) have presented spectroscopic observations of GX 349+2 suggesting a 14 d orbital period. We have obtained broadband photometry of the system on six consecutive nights, and find a statistically significant 21.85 +/- 0.4 h (3 sigma) period of 0.14 mag half-amplitude, superposed on erratic flickering typical of Sco X-1 type objects. As with other Z-sources, caution will be needed to insure that the variations are truly periodic, and not simply due to chaotic variability observed over a relatively short time span. Depending on the origin of the brightness variations, our proposed period could be either the orbital or half the orbital period. If our period is confirmed, then the nature of the 14 d spectroscopic variation found by Southwell et al. (1996) is unclear. There is evidence that the mass function of GX 349+2 is similar to that of Sco X-1.Comment: 17 pages including 3 tables and 5 figures; Uses AASTeX 4.0; Accepted for publication in The Astronomical Journal, volume 112, December 199

INTEGRAL and New Classes of High-Mass X-ray Binaries

The gamma-ray observatory INTEGRAL, launched in October 2002, produces a wealth of discoveries and new results on compact high energy Galactic objects, nuclear gamma-ray line emission, diffuse line and continuum emission, cosmic background radiation, AGN and high energy transients. Two important serendipitous discoveries made by the INTEGRAL mission are new classes of X-ray binaries, namely the highly-obscured high-mass X-ray binaries, and the super-giant fast transients. In this paper I will review the current status of these discoveries.Comment: 3 pages, 1 figure, submitted; Proceedings "The nature and evolution of X-ray binaries in diverse environments", St Petersburg/FL, USA, 28 Oct - 02 Nov 200

Discovery of Twin Wolf-Rayet Stars Powering Double Ring Nebulae

We have spectroscopically discovered a pair of twin, nitrogen-type, hydrogen-rich, Wolf-Rayet stars (WN8-9h) that are both surrounded by circular, mid-infrared-bright nebulae detected with the Spitzer Space Telescope and MIPS instrument. The emission is probably dominated by a thermal continuum from cool dust, but also may contain contributions from atomic line emission. There is no counterpart at shorter Spitzer/IRAC wavelengths, indicating a lack of emission from warm dust. The two nebulae are probably wind-swept stellar ejecta released by the central stars during a prior evolutionary phase. The nebulae partially overlap on the sky and we speculate on the possibility that they are in the early stage of a collision. Two other evolved massive stars have also been identified within the area subtended by the nebulae, including a carbon-type Wolf-Rayet star (WC8) and an O7-8 III-I star, the latter of which appears to be embedded in one of the larger WN8-9h nebulae. The derived distances to these stars imply that they are coeval members of an association lying 4.9 (1.2) kpc from Earth, near the intersection of the Galaxy's Long Bar and the Scutum-Centaurus spiral arm. This new association represents an unprecedented display of complex interactions between multiple stellar winds, outflows, and the radiation fields of evolved massive stars.Comment: Accepted to ApJ Letters on Friday, September 3, 2010; 15 pages, 4 figure

The WIRED Survey. IV. New Dust Disks from the McCook & Sion White Dwarf Catalog

We have compiled photometric data from the Wide-field Infrared Survey Explorer All Sky Survey and other archival sources for the more than 2200 objects in the original McCook & Sion Catalog of Spectroscopically Identified White Dwarfs. We applied color-selection criteria to identify 28 targets whose infrared spectral energy distributions depart from the expectation for the white dwarf photosphere alone. Seven of these are previously known white dwarfs with circumstellar dust disks, five are known central stars of planetary nebulae, and six were excluded for being known binaries or having possible contamination of their infrared photometry. We fit white dwarf models to the spectral energy distributions of the remaining ten targets, and find seven new candidates with infrared excess suggesting the presence of a circumstellar dust disk. We compare the model dust disk properties for these new candidates with a comprehensive compilation of previously published parameters for known white dwarfs with dust disks. It is possible that the current census of white dwarfs with dust disks that produce an excess detectable at K-band and shorter wavelengths is close to complete for the entire sample of known WDs to the detection limits of existing near-IR all-sky surveys. The white dwarf dust disk candidates now being found using longer wavelength infrared data are drawn from a previously underrepresented region of parameter space, in which the dust disks are overall cooler, narrower in radial extent, and/or contain fewer emitting grains.Comment: accepted for publication in The Astrophysical Journal; 34 pages, 5 figures, 5 tables; added missing reference in Section 2 (p. 7

X-Ray Binaries and the Dynamical States of Globular Clusters

We summarize and discuss recent work (Fregeau 2007) that presents the confluence of three results suggesting that most Galactic globular clusters are still in the process of core contraction, and have not yet reached the thermal equilibrium phase driven by binary scattering interactions: that 1) the three clusters that appear to be overabundant in X-ray binaries per unit encounter frequency are observationally classified as "core-collapsed," 2) recent numerical simulations of cluster evolution with primordial binaries show that structural parameters of clusters in the binary-burning phase agree only with "core-collapsed" clusters, and 3) a cluster in the binary-burning phase for the last few Gyr should have about 5 times more dynamically formed X-ray sources than if it were in the core contraction phase for the same time.Comment: Conference proceedings from "A Population Explosion: The Nature and Evolution of X-ray Binaries in Diverse Environments," 28 Oct - 2 Nov, St. Petersburg Beach, FL. 4 page