arXiv subjects

Zhongxiang Wang

Publications and source records attributed to Zhongxiang Wang.

At least 73 records · Page 4Linked to original sources

arXiv2017

Identification of Candidate Millisecond Pulsars from Fermi LAT Observations II

Following our work presented in Dai et al. (2016), we report our detailed data analysis for another 38 Fermi gamma-ray un-associated sources. These sources are selected from the Fermi Large Area Telescope (LAT) third source catalog on the basis of the properties of known gamma-ray millisecond pulsars (MSPs) and for the purpose of finding likely candidate MSPs. From our analysis of the LAT data, we identify that among the 38 sources, 28 of them are single point-like sources with clean background and their spectra show significant curvature. We also conduct analysis of archival X-ray data available for 24 of the 28 sources. In the fields of 10 sources, there are at least one X-ray object, and in those of the other 14 sources, no X-ray object is detected but probably due to the X-ray observations being short. We discuss the possible MSP nature for these sources. Six of them(J0514.6-4406, J1035.7-6720, J1624.2-4041, J1744.1-7619, J1946.4-5403, and J2039.6-5618) are most likely associated with pulsars because of multi-wavelength identifications including direct radio or gamma-ray detection of pulsations. To firmly establish the associations or verify the MSP nature for other sources, deep X-ray and/or optical observations are needed.

astro-ph.HE↗

arXiv2016

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.

astro-ph.HE↗

arXiv2016

Fermi Study of gamma-ray Millisecond Pulsars: the Spectral Shape and Pulsed 25--200 GeV Emission from J0614-3329

We report our analysis of the Fermi Large Area Telescope data for 39 millisecond pulsars (MSPs) listed in the second $γ$-ray pulsar catalog. Spectra of the pulsars are obtained. We fit the spectra with a function of a power law with exponential cutoff, and find the best-fit parameters of photon index $Γ= 1.54^{+0.10}_{-0.11}$ and cutoff energy $E_{c} = 3.70^{+0.95}_{-0.70}$ GeV. This spectral shape, which includes the intrinsic differences in the spectra of the MSPs, can be used for finding candidate MSPs and unidentified types of sources detected by Fermi at high Galactic latitudes. In one of the MSPs PSR J0614-3329, we find significant pulsed emission upto 200 GeV. The result has thus added this MSP to the group of the Crab and Vela pulsars that have been detected with >50 GeV pulsed emission. Comparing the $γ$-ray spectrum of PSR J0614-3329 with those of the Crab and Vela pulsars, we discuss possible emission mechanisms for the very high-energy component.

astro-ph.HE↗

arXiv2016

Fermi Study of 5-300 GeV emission from the high-mass pulsar binary PSR B1259-63/LS 2883

We report the results from our detailed analysis of the Fermi Large Area Telescope (LAT) data for the pulsar binary PSR B1259-63/LS 2883. During the GeV flares that occurred when the pulsar was in the periastron passages, we have detected a 5--300 GeV component at $\simeq 5σ$ in emission from the binary. The detection verifies the presence of the component that has been marginally found in the previous studies of the binary. Furthermore, we have discovered that this component was present even in the quiescent state of the binary, specifically the mean anomaly phase 0.7--0.9. The component can be described by a power law with photon index $Γ\sim 1.4$, and the flux in the flares is approximately one order of magnitude higher than that in quiescence. We discuss the origin of this component. It likely arises from the inverse-Compton process: high-energy particles from the shock, due to the interaction between the winds from the pulsar and massive companion, scatter the seed photons from the companion to GeV/TeV energies. Based on this scenario, model fits to the broad-band X-ray--to--TeV spectra of the binary in the flaring and quiescent states are provided.

astro-ph.HE↗

arXiv2016

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.

astro-ph.HE↗

arXiv2016

Transient X-ray source population in the Magellanic-type galaxy NGC 55

We present the spectral and temporal properties of 15 candidate transient X-ray sources detected in archival {\it XMM-Newton} and {\it Chandra} observations of the nearby Magellanic-type, SB(s)m galaxy NGC 55. On the basis of an X-ray color classification scheme, the majority of the sources may be identified as X-ray binaries (XRBs), and six sources are soft, including a likely supernova remnant. We perform a detailed spectral and variability analysis of the data for two bright candidate XRBs. Both sources displayed strong short-term X-ray variability, and their X-ray spectra and hardness ratios are consistent with those of XRBs. These results, combined with their high X-ray luminosities ($\sim 10^{38}~\rm erg~s^{-1}$), strongly suggest that they are black hole (BH) binaries. Seven less luminous sources have spectral properties consistent with those of neutron star or BH XRBs in both normal and high-rate accretion modes, but one of them is the likely counterpart to a background galaxy (because of positional coincidence). From the spectral analysis, we find that the six soft sources are candidate super soft sources (SSSs), with dominant emission in the soft (0.3--2 keV) X-ray band. Archival {\it Hubble Space Telescope} optical images for 7 sources are available, and the data suggest that most of them are likely to be high-mass XRBs. Our analysis has revealed the heterogeneous nature of the transient population in NGC 55 (6 high-mass XRBs, 1 low-mass XRBs, 6 SSSs, 1 active galactic nucleus), helping establish the similarity of the X-ray properties of this galaxy to those of other Magellanic-type galaxies.

astro-ph.HE↗

arXiv2016

Identification of Candidate Millisecond Pulsars from Fermi LAT Observations

We report our detailed data analysis for 39 $γ$-ray sources selected from the 992 unassociated sources in the \textit{Fermi} Large Area Telescope (LAT) third source catalog. The selection criteria, which were set for finding candidate millisecond pulsars (MSPs), are non-variables with curved spectra and $>$5$^{\circ}$ Galactic latitudes. From our analysis, 24 sources were found to be point-like sources not contaminated by background or nearby unknown sources. Three of them, J1544.6$-$1125, J1625.1$-$0021, and J1653.6$-$0158, have been previously studied, indicating that they are likely MSPs. The spectra of J0318.1+0252 and J2053.9+2922 do not have properties similar to that of known $γ$-ray MSPs, and we thus suggest that they are not MSPs. Analysis of archival X-ray data for most of the 24 sources were also conducted. Four sources were found with X-ray objects in their error circles, and 16 with no detection. The ratios between the $γ$-ray fluxes and X-ray fluxes or flux upper limits are generally lower than those of the known $γ$-ray MSPs, suggesting that if the $γ$-ray sources are MSPs, none of the X-ray objects are the counterparts. Deep X-ray or radio observations of these sources are needed in order to identify their MSP nature.

astro-ph.HE↗

arXiv2015

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

We report our discovery of orbitally modulated $γ$-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$3$σ$ 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 $γ$-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 $γ$-ray emission from them.

astro-ph.HE↗

arXiv2015

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 source's 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 source's 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.

astro-ph.HE↗

arXiv2015

Possible Fermi Detection of the Accreting Millisecond Pulsar Binary SAX J1808.4-3658

We report the Fermi Large Area Telescope (LAT) detection of a $γ$-ray source at the position of SAX J1808.4$-$3658. This transient low-mass X-ray binary contains an accreting millisecond puslar, which is only seen during its month-long outbursts and likely switches to be rotation powered during its quiescent state. Emission from the $γ$-ray source can be described by a power law with an exponential cutoff, the characteristic form for pulsar emission. Folding the source's 2.0--300 GeV photons at the binary orbital period, a weak modulation is seen (with an H-test value of $\sim$17). In addition, three sets of archival XMM-Newton data for the source field are analyzed, and we find only one X-ray source with 3--4$σ$ flux variations in the 2$σ$ error circle of the $γ$-ray source. However based on the X-ray properties, this X-ray source is not likely a background AGN, the major class of Fermi sources detected by LAT. These results support the possible association between the $γ$-ray source and SAX J1808.4$-$3658 and thus the scenario that the millisecond pulsar is rotation powered in the quiescent state. Considering a source distance of 3.5 kpc for SAX J1808.4$-$3658, the 0.1--300 GeV luminosity is 5.7$\times 10^{33}$ erg s$^{-1}$, implying a $γ$-ray conversion efficiency of 63\% for the pulsar in this binary.

astro-ph.HE↗

arXiv2015

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 $9σ$ in the 0.3--8 keV energy band. The \xmm{} spectrum is consistent with a power law with photon index $Γ= 3.17^{+0.22}_{-0.20}$, 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 ($Γ\sim 2.5$--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.

astro-ph.HE↗

arXiv2014

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.

astro-ph.HE↗

arXiv2014

Fermi Variability Study of the Candidate Pulsar Binary 2FGL~J0523.3-2530

The Fermi source 2FGL~J0523.3$-$2530 has recently been identified as a candidate millisecond pulsar binary with an orbital period of 16.5 hrs. We have carried out detailed studies of the source's emission properties by analyzing data taken with the Fermi Large Area Telescope in the 0.2--300 GeV energy range. Long-term, yearly variability from the source has been found, with a factor of 4 flux variations in 1--300 GeV. From spectral analysis, we find an extra spectral component at 2--3 GeV that causes the source brightening. While no orbital modulations have been found from the Fermi data over the whole period of 2008--2014, orbital modulation in the source's $>$2 GeV emission is detected during the last 1.5 yrs of the Fermi observation. Our results support the millisecond pulsar binary nature of 2FGL~J0523.3$-$2530. Multi-wavelength observations of the source are warranted in order to find any correlated flux variations and thus help determine the origin of the long-term variability, which currently is not understood.

astro-ph.HE↗

arXiv2014

Searching for $γ$-Ray Pulsars among Fermi Unassociated Sources: 2FGL J1906.5+0720

We report the results from our analysis of the \textit{Fermi} Large Area Telescope data of the \textit{Fermi} unassociated source 2FGL J1906.5$+$0720, which is a high-ranked candidate pulsar. In order to better study our target, we first update the ephemeris for PSR J1907$+$0602, which is used to help remove any possible contamination due to strong emission from this nearby pulsar. From our analysis, 2FGL J1906.5$+$0720 is confirmed to have a significant low energy cutoff at $\sim$ 1~GeV in its emission (14$σ$--18$σ$ significance), consistent with those seen in young pulsars. We search for pulsations but no spin frequency signals are found in a frequency range of 0.5--32 Hz. No single model can fully describe the source's overall \fermi\ $γ$-ray spectrum, and the reason for this is the excess emission detected at energies of $\geq$4 GeV. The high-energy component possibly indicates emission from a pulsar wind nebula, when considering 2FGL J1906.5$+$0720 as a young pulsar. We conclude that 2FGL J1906.5$+$0720 is likely a pulsar based on the emission properties we have obtained, and observations at other energies are needed in order to confirm its pulsar nature.

astro-ph.HE↗

arXiv2014

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.

astro-ph.HE↗

arXiv2014

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.

astro-ph.SR↗

arXiv2013

Near-Infrared Imaging of White Dwarfs with Candidate Debris Disks

We have carried out $JHK_s$ imaging of 12 white dwarf debris disk candidates from the WIRED SDSS DR7 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 WISE 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 $JHK_s$ 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.

astro-ph.SR↗

arXiv2013

The likely Fermi Detection of the Supernova Remnant RCW 103

We report on the results from our $γ$-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$σ$ significance), having a power-law spectrum with a photon index of 2.0$\pm$0.1. We obtain its 1--300 GeV spectrum, and the total flux gives a luminosity of 8.3$\times 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 $γ$-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 interaction is suggested as a high density of the target protons is required.

astro-ph.HE↗