Z. X. Wang

DISCOVERY OF A 693.5 s PERIOD IN THE X-RAY BINARY 4U 1820 – 30: A SUPERHUMP INTERPRETATION

The X-ray source 4U 1820-30 in the globular cluster NGC 6624 is known as the most compact binary among the identified X-ray binaries (XRBs). Having an orbital period of 685.0 s, the source consists of a neutron star (NS) primary and likely a 0.06-0.08 M ☉ white dwarf (WD) secondary. Here, we report on far-ultraviolet (FUV) observations of this XRB, made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. From our Fourier spectral analysis of the FUV timing data, we obtain a period of 693.5 ± 1.3 s, which is significantly different from the orbital period. The light curve folded at this period can be described by a sinusoid, with a fractional semiamplitude of 6.3% and the phase zero (maximum of the sinusoid) at MJD 50886.015384 ± 0.000043 (TDB). While the discovered FUV period may be consistent with a hierarchical triple system model that was previously considered for 4U 1820-30, we suggest that it could instead be the indication of superhump modulation, which arises from an eccentric accretion disk in the binary. The X-ray and FUV periods would be the orbital and superhump periods, respectively, indicating a 1% superhump excess and a WD/NS mass ratio around 0.06. Considering 4U 1820-30 as a superhump source, we discuss the implications.

RADIO OBSERVATIONS OF GRB 100418a: TEST OF AN ENERGY INJECTION MODEL EXPLAINING LONG-LASTING GRB AFTERGLOWS

We present the results of our radio observational campaign of gamma-ray burst (GRB) 100418a, for which we used the Australia Telescope Compact Array, the Very Large Array, and the Very Long Baseline Array. GRB 100418a was a peculiar GRB with unusual X-ray and optical afterglow profiles featuring a plateau phase with a very shallow rise. This observed plateau phase was believed to be due to a continued energy injection mechanism that powered the forward shock, giving rise to an unusual and long-lasting afterglow. The radio afterglow of GRB 100418a was detectable several weeks after the prompt emission. We conducted long-term monitoring observations of the afterglow and attempted to test the energy injection model advocating that the continuous energy injection is due to shells of material moving at a wide range of Lorentz factors. We obtained an upper limit of γ < 7 for the expansion rate of the GRB 100418a radio afterglow, indicating that the range-of-Lorentz factor model could only be applicable for relatively slow-moving ejecta. A preferred explanation could be that continued activity of the central engine may have powered the long-lasting afterglow.

DISCOVERY OF GAMMA-RAY ORBITAL MODULATION IN THE BLACK WIDOW PSR J1311−3430

We report our discovery of orbitally modulated

Near-IR observations of the anomalous X-ray pulsar CXO J164710.2-455216

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Candidate Near-IR Counterpart to the X-ray Transient Pulsar GRO J1744-28

-ray emission from the black widow system PSR J1311-3430. We analyze the \textit{Fermi} Large Area Telescope data during the offpulse phase interval of the pulsar, and find the orbital modulation signal at a

Spitzer Detection of a Debris Disk around an Isolated Young Neutron Star

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Superhumps and an extreme mass ratio in the ultracompact low-mass X-ray binary 4U 1820-30

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Discovery of the Orbital Period of the Ultracompact Low-Mass X-Ray Binary 4U 1543-624

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Optical/UV Spectroscopy of Ultracompact Low-Mass X-Ray Binaries

confidence level. Further spectral analysis shows no significant differences for the spectra obtained during the bright and faint orbital phase ranges. A simple sinusoid-like function can describe the modulation. Given these properties, we suggest that the intrabinary

Keck and Magellan Observations of Neutron Star X-Ray Transients in Quiescence

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