Searching for Debris Disks around Isolated Pulsars

Different pieces of observational evidence suggest the existence of disks around isolated neutron stars. Such disks could be formed from supernova fallback when neutron stars are born in core-collapse supernova explosions. Efforts have been made to search for disks around different classes of pulsars, which include millisecond pulsars, young neutron star classes (magnetars, central compact objects, and X-ray dim isolated neutron stars), and regular radio pulsars. We review the main results from observations at wavelengths of from optical to sub-millimeter/millimeter.Comment: 23 pages, 3 figures, based on an invited talk at the Cosmic Dust VI conference, significant comments from referees incorporated, accepted for publication in Planetary and Space Scienc

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 detection of the Galactic low-mass X-ray binaries. Among 187 binaries catalogued in Liu et al. (2007), we find 13 counterparts and two candidate counterparts. For the 13 counterparts, two (4U~0614+091 and GX~339$-$4) have already been confirmed by previous studies to have a jet and one (GRS~1915+105) to have a candidate circumbinary disk, from which the detected infrared emission arose. Having collected the broad-band optical and near-infrared data in literature and constructed flux density spectra for the other 10 binaries, we identify that three (A0620$-$00, XTE J1118+480, and GX 1+4) are candidate circumbinary disk systems, four (Cen X-4, 4U 1700+24, 3A 1954+319, and Cyg X-2) had thermal emission from their companion stars, and three (Sco X-1, Her X-1, and Swift J1753.5$-$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 the both.Comment: 9 pages, 5 figures, submitted to Ap

Searching for γ\gamma-ray Counterparts to Very Faint X-Ray Transient Neutron Star binaries

Very faint X-ray transients (VFXTs) are a group of X-ray binaries with low luminosities, the peak X-ray luminosities during their outbursts being only 10$^{34}$--10$^{36}$ erg\,s$^{-1}$. Using the $\gamma$-ray data obtained with the Large Area Telescope (LAT) onboard {\it Fermi Gamma-Ray Space Telescope}, we investigate their possible nature of containing rotation-powered pulsars, or more specifically being transitional millisecond pulsars (MSPs). Among more than 40 known VFXTs, we select 12 neutron star systems. We analyze the LAT data for the fields of the 12 VFXTs in 0.2--300 GeV energy range, but do not find any counterparts likely detected by {\it Fermi}. We obtain the luminosity upper limits for the 12 sources. While the distances to the sources are largely uncertain, the upper limits are comparable to the luminosities of two transitional systems PSR J1023$-$0038 and XSS J12270$-$4859. From our study, we conclude that no evidence is found at $\gamma$-rays for the suggestion that some of VFXTs could contain rotation-powered MSPs (or be transitional MSP systems).Comment: 14 pages, 3 figure

Fermi Observation of the transitional pulsar binary XSS J12270-4859

Because of the disappearance of its accretion disk since the time period around 2012 November--December, XSS J12270-4859 has recently been identified as, in addition to PSR J1023+0038, another transitional millisecond pulsar binary. We have carried out detailed analysis of the Fermi Large Area Telescope data for the binary. While both spectra before and after the disk-disappearance transition are well described by an exponentially cut-off power law, typical for pulsars' emission in the Fermi's 0.2-300 GeV, a factor of 2 flux decrease related to the transition is detected. A weak orbital modulation is seen, but only detectable in the after-transition data, same to that found at X-rays. In the long-term light curve of the source before the transition, a factor of 3 flux variations are seen. Comparing to the properties of J1023+0038, we disucss the implications from these results. We suggest that since the modulation is aligned with that at X-rays in orbital phase, it possibly arises due to the occultation of the gamma-ray emitting region by the companion. The origin of the variations in the long-term light curve is not clear, because the source field also contains unidentified radio or X-ray sources and their contamination can not be excluded. Multi-wavelength observations of the source field will help identify the origin of the variations by detecting any related flux changes from the in-field sources.Comment: 12 pages, 5 figures, 2 tables, submitted to Ap

X-ray outbursts from a new transient in NGC 55

We report the outbursts from a newly discovered X-ray transient in the Magellanic-type, SB(s)m galaxy NGC 55. The transient source, XMMU J001446.81-391123.48, was undetectable in the 2001 \xmm{} and 2004 \chandra{} observations, but detected in a 2010 \xmm{} observation at a significance level of $9\sigma$ in the 0.3--8 keV energy band. The \xmm{} spectrum is consistent with a power law with photon index $\Gamma = 3.17^{+0.22}_{-0.20}$, but is better fit with a $kT_{in} = 0.70\pm0.06$\,keV disk blackbody. The luminosity was $\sim 10^{38}$\,erg\,s$^{-1}$, and the source displayed strong short-term X-ray variability. These results, combined with the hardness ratios of its emission, strongly suggest an X-ray binary nature for the source. The follow-up studies with \swift{} XRT observations revealed that the source exhibited recurrent outbursts with period about a month. The XRT spectra can be described by a power law ($\Gamma\sim 2.5$--2.9) or a disk blackbody ($kT_{in}\sim 0.8$--1.0\,keV), and the luminosity was in a range of 10$^{38}$--10$^{39}$\,erg\,s$^{-1}$, with no evidence showing any significant changes of the spectral parameters in the observations. Based on the X-ray spectral and temporal properties, we conclude that XMMU J001446.81-391123.48 is a new transient X-ray binary in NGC 55, which possibly contains a black hole primary.Comment: Accepted for publication in MNRAS; Matched with accepted version; 6 Pages, 5 Figures, 2 Table

Transient X-ray Sources in the Magellanic-type Galaxy NGC 4449

We report the identification of seven transient X-ray sources in the nearby Magellanic-type galaxy NGC 4449 using the archival multi-epoch X-ray observations conducted with {\it Chandra}, {\it XMM-Newton} and {\it Swift} telescopes over year 2001--2013. Among them, two sources are classified as supersoft X-ray sources (SSSs) because of their soft X-ray color and rest of the sources are X-ray binaries (XRBs). Transient SSSs spectra can be fitted with a blackbody of effective temperature $\sim 80-105$ eV and luminosities were $\simeq 10^{37} - 10^{38} {\rm~erg\ s}^{-1}$ in 0.3--8 keV. These properties are consistent with the widely accepted model for SSSs, an accreting white dwarf with the steady nuclear burning on its surface, while the SSS emission has also been observed in many post-nova systems. Detailed analysis of one sufficiently bright SSS revealed the strong short-term variability, possibly showing a 2.3 hour periodic modulation, and long-term variability, detectable over 23 years with different X-ray telescopes before year 2003. The X-ray properties of four other transients are consistent with neutron star or black hole binaries in their hard state, while the remaining source is most likely an XRB with a quasi-soft X-ray spectrum. Analysis of archival {\it Hubble Space Telescope} image data was also conducted, and multiple massive stars were found as possible counterparts. We conclude that the X-ray transient properties in NGC 4449 are similar to those in other Magellanic-type galaxies.Comment: Accepted for publication in ApJ, 10 Pages, 4 Tables, 3 Figure

Discovery of Gamma-Ray Orbital Modulation in the Black Widow PSR J1311-3430

We report our discovery of orbitally modulated $\gamma$-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 $\sim$3$\sigma$ 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 $\gamma$-ray emission arises from the region close to the companion and the modulation is caused by the occultation of the emitting region by the companion, similar to that is seen in the transitional millisecond pulsar binary (MSP) PSR J1023+0038. Considering the X-ray detection of intrabinary shock emission from eclipsing MSP binaries recently reported, this discovery further suggests the general existence of intrabinary $\gamma$-ray emission from them.Comment: 5 pages, 4 figures, accepted for publication in ApJ