Zhongxiang Wang

MULTIBAND STUDIES OF THE OPTICAL PERIODIC MODULATION IN THE X-RAY BINARY SAX J1808.4–3658 DURING ITS QUIESCENCE AND 2008 OUTBURST

We report on time-resolved optical imaging of the X-ray binary SAX J1808.4-3658 during its quiescent state and 2008 outburst. The binary, containing an accretion-powered millisecond pulsar, has a large sinusoidal-like modulation in its quiescent optical emission. We employ a Markov chain Monte Carlo technique to fit our multi-band light curve data in quiescence with an irradiated star model, and derive a tight constraint of 50(-5)(+6) deg on the inclination angle i of the binary system. The pulsar and its companion are constrained to have masses of 0.97(-0.22)(+0.31) M-circle dot and 0.04(-0.01)(+0.02) M-circle dot (both 1 sigma ranges), respectively. The dependence of these results on the measurements of the companions projected radial velocity is discussed. We also find that the accretion disk had nearly constant optical fluxes over a similar to 500 day period in the quiescent state our data covered, but started brightening 1.5 months before the 2008 outburst. Variations in modulation during the outburst were detected in our four observations made 7-12 days after the start of the outburst, and a sinusoidal-like modulation with 0.2 mag amplitude changed to have a smaller amplitude of 0.1 mag. The modulation variations are discussed. We estimate the albedo of the companion during its quiescence and the outburst, which was approximately 0 and 0.8 (for isotropic emission), respectively. This large difference probably provides additional evidence that the neutron star in the binary turns on as a radio pulsar in quiescence.

THE LIKELY ORBITAL PERIOD OF THE ULTRACOMPACT LOW-MASS X-RAY BINARY 2S 0918-549

We report the discovery of the likely orbital period of the ultracompact low-mass X-ray binary (LMXB) 2S 0918-549. Using time-resolved optical photometry carried out with the 8-m Gemini South Telescope, we obtained a 2.4-hr long, Sloan r light curve of 2S 0918-549 and found a periodic, sinusoidal modulation at 17.4+/-0.1 min with a semiamplitude of 0.015+/-0.002 mag, which we identify as the binary period. In addition to 4U 0513-40 in the globular cluster NGC 1851 and the Galactic disk source 4U 1543-624, 2S 0918-549 is the third member of the ultracompact LMXBs that have orbital periods around 18 min. Our result verifies the suggestion of 2S 0918-549 as an ultracompact binary based on its X-ray and optical spectroscopic properties. Given that the donor in 2S 0918-549 has been suggested to be either a C-O or He white dwarf, its likely mass and radius are around 0.024--0.029 M_sun and 0.03--0.032 R_sun, respectively, for the former case and 0.034--0.039 M_sun and 0.033--0.035 R_sun for the latter case. If the optical modulation arises from X-ray heating of the mass donor, its sinusoidal shape suggests that the binary has a low inclination angle, probably around 10 deg.

WISE DETECTION OF THE GALACTIC LOW-MASS X-RAY BINARIES

We report on the results from our search for the Wide-field Infrared Survey Explorer (WISE) detection of the Galactic low-mass X-ray binaries (LMXBs). Among 187 binaries cataloged in Liu et al., we find 13 counterparts and 2 candidate counterparts. For the 13 counterparts, 2 (4U 0614+091 and GX 339–4) have already been confirmed by previous studies to have a jet and 1 (GRS 1915+105) to have a candidate circumbinary disk, from which the detected infrared emission arose. Having collected the broadband optical and near-infrared data in the literature and constructed flux density spectra for the other 10 binaries, we identify that 3 (A0620–00, XTE J1118+480, and GX 1+4) are candidate circumbinary disk systems, 4 (Cen X-4, 4U 1700+24, 3A 1954+319, and Cyg X-2) had thermal emission from their companion stars, and 3 (Sco X-1, Her X-1, and Swift J1753.5–0127) are peculiar systems with the origin of their infrared emission rather uncertain. We discuss the results and WISE counterparts brightness distribution among the known LMXBs, and suggest that more than half of the LMXBs would have a jet, a circumbinary disk, or both.

THE LIKELY FERMI DETECTION OF THE SUPERNOVA REMNANT RCW 103

We report on the results from our gamma-ray analysis of the supernova remnant (SNR) RCW 103 region. The data were taken with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. An extended source is found at a position consistent with that of RCW 103 and its emission was only detected above 1 GeV (10 significance), with a power-law spectrum with a photon index of 2.0 +/- 0.1. We obtain its 1-300 GeV spectrum and the total flux gives a luminosity of 8.3 x 10(33) erg s(-1) at a source distance of 3.3 kpc. Given the positional coincidence and property similarities of this source with other SNRs, we identify it as the likely Fermi gamma-ray counterpart to RCW 103. Including radio measurements of RCW 103, the spectral energy distribution (SED) is modeled by considering emission mechanisms based on both hadronic and leptonic scenarios. We find that models in the two scenarios can reproduce the observed SED, while in the hadronic scenario the existence of SNR-molecular cloud interactions is suggested as a high density of the target protons is required.

NEAR-INFRARED IMAGING OF WHITE DWARFS WITH CANDIDATE DEBRIS DISKS

We have carried out JHKs imaging of 12 white dwarf debris disk candidates from the WIRED Sloan Digital Sky Survey Data Release 7 catalog, aiming to confirm or rule out disks among these sources. On the basis of positional identification and the flux density spectra, we find that seven white dwarfs have excess infrared emission, but mostly at Wide-field Infrared Survey Explorer W1 and W2 bands. Four are due to nearby red objects consistent with background galaxies or very low mass dwarfs, and one exhibits excess emission at JHKs consistent with an unresolved L0 companion at the correct distance. While our photometry is not inconsistent with all seven excesses arising from disks, the stellar properties are distinct from the known population of debris disk white dwarfs, making the possibility questionable. In order to further investigate the nature of these infrared sources, warm Spitzer imaging is needed, which may help resolve galaxies from the white dwarfs and provide more accurate flux measurements.

SUPERNOVA REMNANT KESTEVEN 27: INTERACTION WITH A NEIGHBOR HI CLOUD VIEWED BY FERMI

We report on the likely detection of gamma-ray emission from the supernova remnant (SNR) Kesteven 27 (Kes 27). We analyze 5.7 yr Fermi Large Area Telescope data of the SNR region and find an unresolved source at a position consistent with the radio brightness peak and the X-ray knot of Kes 27, which is located in the eastern region of the SNR and caused by the interaction with a nearby HI cloud. The sources emission is best fit with a power-law spectrum with a photon index of 2.5 +/- 0.1 and a >0.2 GeV luminosity of 5.8*10^34 erg s^-1 assuming a distance of 4.3 kpc, as derived from radio observations of the nearby HI cloud. Comparing the properties of the source with that of other SNRs that are known to be interacting with nearby high-density clouds, we discuss the origin of the sources emission. The spectral energy distribution of the source can be described by a hadronic model that considers the interaction of energetic protons, escaping from the shock front of Kes 27, with a high-density cloud.

THE LIKELY FERMI DETECTION OF THE SUPERNOVA REMNANT SN 1006

We report the likely detection of gamma-ray emission from the northeast shell region of the historical supernova remnant (SNR) SN 1006. Having analyzed 7 years of Fermi LAT Pass 8 data for the region of SN 1006, we found a GeV gamma-ray source detected with 4 sigma significance. Both the position and spectrum of the source match those of HESS J1504-418 respectively, which is TeV emission from SN 1006. Considering the source as the GeV gamma-ray counterpart to SN~1006, the broadband spectral energy distribution is found to be approximately consistent with the leptonic scenario that has been proposed for the TeV emission from the SNR. Our result has likely confirmed the previous study of the SNRs with TeV shell-like morphology: SN 1006 is one of them sharing very similar peak luminosity and spectral shape.

SERENDIPITOUS DISCOVERY OF A CANDIDATE DEBRIS DISK AROUND THE DA WHITE DWARF SDSS J114404.74+052951.6

We report our serendipitous discovery of a candidate debris disk around the recently identified DA white dwarf (WD) SDSS J114404.74+052951.6. The Ca II infrared (IR) triplet, while weakly detected in the sources optical spectrum, shows a double-peak like profile, and near-IR excesses are also detected from broadband imaging. The two features are commonly seen in the WD gaseous debris disks, and thus indicate the existence of such a disk around the DA WD. We further analyze the emission lines and broadband IR excesses and estimate that the debris disk may have a size of 20-50 R-WD and an inclination angle of 82 degrees. The estimated temperature and cooling age for this WD are 23,027 K and similar to 21 Myr, respectively, making it possibly one of the hottest and youngest among candidates of WD debris-disk systems. These properties are also in line with the likely conditions for having a sufficiently bright debris disk. Further observations of the source are warranted in order to determine the debris disks properties and search for more absorption features of heavy elements.

FERMI VARIABILITY STUDY OF THE CANDIDATE PULSAR BINARY 2FGL J0523.3–2530

The Fermi source 2FGL~J0523.3

Search for gamma-ray emission from four accreting millisecond pulsars with Fermi/LAT

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