arXiv subjects

Albert K. H. Kong

Publications and source records attributed to Albert K. H. Kong.

At least 73 records · Page 4Linked to original sources

arXiv2017

Gamma-ray Emission of the Kes 73/1E 1841-045 Region Observed with the Fermi Large Area Telescope

The supernova remnant Kes 73 and/or the magnetar 1E 1841-045 at its center can deposit a large amount of energy to the surroundings and is potentially responsible for particle acceleration. Using the data taken with the Fermi Large Area Telescope (LAT), we confirmed the presence of an extended source whose centroid position is highly consistent with this magnetar/supernova-remnant pair. Its emission is intense from 100 MeV to $>$100 GeV. Its LAT spectrum can be decoupled into two components which are respectively governed by two different mechanisms. According to the young age of this system, the magnetar is seemingly a necessary and sufficient source for the downward-curved spectrum below 10 GeV, as the observed $<$10 GeV flux is too high for the supernova remnant to account for. On the other hand, the supernova remnant is reasonably responsible for the hard spectrum above 10 GeV. Further studies of this region in the TeV regime is required, so that we can perform physically meaningful comparisons of the $>$10 GeV spectrum and the TeV spectrum.

astro-ph.HE↗

arXiv2017

SWIFT Detection of a 65-Day X-ray Period from the Ultraluminous Pulsar NGC 7793 P13

NGC 7793 P13 is an ultraluminous X-ray source harboring an accreting pulsar. We report on the detection of a ~65 d period X-ray modulation with Swift observations in this system. The modulation period found in the X-ray band is P=65.05+/-0.10 d and the profile is asymmetric with a fast rise and a slower decay. On the other hand, the u-band light curve collected by Swift UVOT confirmed an optical modulation with a period of P=64.24+/-0.13 d. We explored the phase evolution of the X-ray and optical periodicities and propose two solutions. A superorbital modulation with a period of ~2,700-4,700 d probably caused by the precession of a warped accretion disk is necessary to interpret the phase drift of the optical data. We further discuss the implication if this ~65d periodicity is caused by the superorbital modulation. Estimated from the relationship between the spin-orbital and orbital-superorbital periods of known disk-fed high-mass X-ray binaries, the orbital period of P13 is roughly estimated as 3-7 d. In this case, an unknown mechanism with a much longer time scale is needed to interpret the phase drift. Further studies on the stability of these two periodicities with a long-term monitoring could help us to probe their physical origins.

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arXiv2016

Discovery of a Redback Millisecond Pulsar Candidate: 3FGL J0212.1+5320

We present a multi-wavelength study of the unidentified Fermi object, 3FGL J0212.1+5320. Within the 95% error ellipse, Chandra detects a bright X-ray source (i.e., F(0.5-7keV) = 1.4e-12 erg/cm^2/s), which has a low-mass optical counterpart (M < 0.4 Msun and T ~ 6000 K). A clear ellipsoidal modulation is shown in optical/infrared at 20.87 hours. The gamma-ray properties of 3FGL J0212.1+5320 are all consistent with that of a millisecond pulsar, suggesting that it is a gamma-ray redback millisecond pulsar binary with a low-mass companion filling > 64% of the Roche-lobe. If confirmed, it will be a redback binary with one of the longest orbital periods known. Spectroscopic data taken in 2015 from the Lijiang observatory show no evidence of strong emission lines, revealing that the accretion is currently inactive (the rotation-powered pulsar state). This is consistent with the low X-ray luminosities (Lx ~ 10^32 erg/s) and the possible X-ray modulation seen by Chandra and Swift. Considering that the X-ray luminosity and the high X-ray-to-gamma-ray flux ratio (8%) are both comparable to that of the two known gamma-ray transitional millisecond pulsars, we suspect that 3FGL J0212.1+5320 could be a potential target to search for future transition to the accretion active state.

astro-ph.HE↗

arXiv2016

Chandra and Swift X-ray Observations of the X-ray Pulsar SMC X-2 During the Outburst of 2015

We report the Chandra/HRC-S and Swift/XRT observations for the 2015 outburst of the high-mass X-ray binary (HMXB) pulsar in the Small Magellanic Cloud, SMC X-2. While previous studies suggested that either an O star or a Be star in the field is the high-mass companion of SMC X-2, our Chandra/HRC-S image unambiguously confirms the O-type star as the true optical counterpart. Using the Swift/XRT observations, we extracted accurate orbital parameters of the pulsar binary through a time of arrivals (TOAs) analysis. In addition, there were two X-ray dips near the inferior conjunction, which are possibly caused by eclipses or an ionized high-density shadow wind near the companion's surface. Finally, we propose that an outflow driven by the radiation pressure from day ~10 played an important role in the X-ray/optical evolution of the outburst.

astro-ph.HE↗

arXiv2016

Studying the SGR 1806-20/Cl* 1806-20 region using the \emph{Fermi} Large Area Telescope

The region around SGR 1806-20 and its host stellar cluster Cl* 1806-20 is a potentially important site of particle acceleration. The soft $γ-$ray repeater and Cl* 1806-20, which also contains several very massive stars including a luminous blue variable hypergiant LBV 1806-20, are capable of depositing a large amount of energy to the surroundings. Using the data taken with the \emph{Fermi} Large Area Telescope (LAT), we identified an extended LAT source to the south-west of Cl* 1806-20. The centroid of the 1-50~GeV emission is consistent with that of HESS J1808-204 (until now unidentified). The LAT spectrum is best-fit by a broken power-law with the break energy $E_\mathrm{b}=297\pm15$ MeV. The index above $E_\mathrm{b}$ is $2.60\pm0.04$, and is consistent with the flux and spectral index above 100 GeV for HESS J1808-204, suggesting an association between the two sources. Meanwhile, the interacting supernova remnant SNR G9.7-0.0 is also a potential contributor to the LAT flux. A tentative flux enhancement at the MeV band during a 45-day interval (2011 Jan 21 - 2011 Mar 7) is also reported. We discuss possible origins of the extended LAT source in the context of both leptonic and hadronic scenarios.

astro-ph.HE↗

arXiv2016

Discovery of an X-ray Emitting Contact Binary System 2MASS J11201034$-$2201340

We report the detection of orbital modulation, a model solution, and X-ray properties of a newly discovered contact binary, 2MASS J11201034$-$2201340. We serendipitously found this X-ray point source outside the error ellipse when searching for possible X-ray counterparts of $γ$-ray millisecond pulsars among the unidentified objects detected by the {\it Fermi Gamma-ray Space Telescope}. The optical counterpart of the X-ray source (unrelated to the $γ$-ray source) was then identified using archival databases. The long-term CRTS survey detected a precise signal with a period of $P=0.28876208(56)$ days. A follow-up observation made by the SLT telescope of Lulin Observatory revealed the binary nature of the object. Utilizing archived photometric data of multi-band surveys, we construct the spectral energy distribution, which is well fitted by a K2V spectral template. The fitting result of the orbital profile using the Wilson--Devinney code suggests that 2MASS J11201034-2201340 is a short-period A-type contact binary and the more massive component has a cool spot. The X-ray emission was first noted in observations made by \swift\, then further confirmed and characterized by an \xmm\ observation. The X-ray spectrum can be described by a power law or thermal Bremsstrahlung. Unfortunately, we could not observe significant X-ray orbital modulation. Finally, according to the spectral energy distribution, this system is estimated to be 690 pc from Earth with a calculated X-ray intensity of $(0.7-1.5)\times 10^{30}$ erg s$^{-1}$, which is in the expected range of an X-ray emitting contact binary.

astro-ph.SR↗

arXiv2015

X-Ray afterglow of SWIFT J1644+57: a Compton echo?

Swift, Chandra and XMM have found a weak but nearly constant X-ray component from Swift J1644+57 that appeared at ~500 days and was visible at least until ~ 1400 days after the stellar capture, which cannot be explained by standard tidal disruption theories. We suggest that this X-ray afterglow component may result from Thomson scattering between the primary X-rays and its surrounding plasma, i.e. the Compton echo effect. Similar phenomena has also been observed from molecular clouds in our Galactic Center, which were caused by the past activity of Srg A*. If this interpretation of Swift J1644+57 afterglow is correct, this is the first Compton Echo effect observed in the cosmological distances.

astro-ph.HE↗

arXiv2015

Revisiting the ultraluminous supersoft source in M101: an optically thick outflow model

The M101 galaxy contains the best-known example of an ultraluminous supersoft source (ULS), dominated by a thermal component at kT ~ 0.1 keV. The origin of the thermal component and the relation between ULSs and standard (broad-band spectrum) ultraluminous X-ray sources (ULXs) are still controversial. We re-examined the X-ray spectral and timing properties of the M101 ULS using archival Chandra and XMM-Newton observations. We show that the X-ray time-variability and spectral properties are inconsistent with standard disk emission. The characteristic radius R_{bb} of the thermal emitter varies from epoch to epoch between ~10,000 km and ~100,000 km; the colour temperature kT_{bb} varies between ~50 eV and ~140 eV; and the two quantities scale approximately as R_{bb} ~ T_{bb}^{-2}. In addition to the smooth continuum, we also find (at some epochs) spectral residuals well fitted with thermal plasma models and absorption edges: we interpret this as evidence that we are looking at a clumpy, multi-temperature outflow. We suggest that at sufficiently high accretion rates and inclination angles, the super-critical, radiatively driven outflow becomes effectively optically thick and completely thermalizes the harder X-ray photons from the inner part of the inflow, removing the hard spectral tail. We develop a simple, spherically symmetric outflow model and show that it is consistent with the observed temperatures, radii and luminosities. A larger, cooler photosphere shifts the emission peak into the far-UV and makes the source dimmer in X-rays but possibly ultraluminous in the UV. We compare our results and interpretation with those of Liu et al. (2013).

astro-ph.HE↗

arXiv2014

An Accreting White Dwarf near the Chandrasekhar Limit in the Andromeda Galaxy

The iPTF detection of the most recent outburst of the recurrent nova system RX J0045.4+4154 in the Andromeda Galaxy has enabled the unprecedented study of a massive ($M>1.3\ M_\odot$) accreting white dwarf (WD). We detected this nova as part of the near daily iPTF monitoring of M31 to a depth of $R\approx 21$\,mag and triggered optical photometry, spectroscopy and soft X-ray monitoring of the outburst. Peaking at an absolute magnitude of $M_R=-6.6$ mag, and with a decay time of 1 mag per day, it is a faint and very fast nova. It shows optical emission lines of He/N and expansion velocities of 1900 to 2600 km s$^{-1}$ 1--4 days after the optical peak. The {\it Swift} monitoring of the X-ray evolution revealed a supersoft source (SSS) with $kT_{\rm eff}\approx 90-110\ {\rm eV}$ that appeared within 5 days after the optical peak, and lasted only 12 days. Most remarkably, this is not the first event from this system, rather it is a recurrent nova with a time between outbursts of approximately 1 year, the shortest known. Recurrent X-ray emission from this binary was detected by ROSAT in 1992 and 1993, and the source was well characterized as a $M>1.3\ M_\odot$ WD SSS. Based on the observed recurrence time between different outbursts, the duration and effective temperature of the SS phase, MESA models of accreting WDs allow us to constrain the accretion rate to $\dot M>1.7\times10^{-7}\ {M_{\odot}\ {\rm yr}}^{-1}$ and WD mass $>1.30\ M_{\odot}$. If the WD keeps $30\%$ of the accreted material, it will take less than a Myr to reach core densities high enough for carbon ignition (if made of C/O) or electron capture (if made of O/Ne) to end the binary evolution.

astro-ph.SR↗

arXiv2013

Combined Analysis of Hubble and VLT Photometry of the Intermediate Mass Black Hole ESO 243-49 HLX-1

In this paper we present a combined analysis of data obtained with the Hubble Space Telescope (HST), Very Large Telescope (VLT), and Swift X-ray telescope (XRT) of the intermediate mass black hole ESO 243-49 HLX-1 that were taken 2 months apart between September and November 2010. Previous separate analyses of these data found that they were consistent with an irradiated accretion disc with contribution from either a very young or very old stellar population, and also indicated that the optical flux of the HLX-1 counterpart could be variable. Such variability could only be attributed to a varying accretion disc, so simultaneous analysis of all data sets should break the degeneracies in the model fits. We thus simultaneously fit the broad-band spectral energy distribution (SED) from near-infrared through to X-ray wavelengths of the two epochs of data with a model consisting of an irradiated accretion disc and a stellar population. We show that this combined analysis rules out an old stellar population, finding that the SED is dominated by emission from an accretion disc with moderate reprocessing in the outer disc around an intermediate mass black hole imbedded in a young (20 Myr) stellar cluster with a mass of 1E5 Msun. We also place an upper limit on the mass of an additional hidden old stellar population of 1E6 Msun. However, optical r-band observations of HLX-1 obtained with the Gemini-South telescope covering part of the decay from a later X-ray outburst are consistent with constant optical flux, indicating that the observed variability between the HST and VLT observations could be spurious caused by differences in the background subtraction applied to the two optical data sets. In this scenario the contribution of the stellar population, and thus the stellar mass of the cluster, may be higher (abridged).

astro-ph.HE↗

arXiv2013

WISE J233237.05-505643.5: a Double-Peaked Broad-Lined AGN with Spiral-Shaped Radio Morphology

We present radio continuum mapping, optical imaging and spectroscopy of the newly discovered double-peaked broad-lined AGN WISE J233237.05-505643.5 at redshift z = 0.3447. This source exhibits an FR-I and FR-II hybrid-morphology, characterized by bright core, jet, and Doppler-boosted lobe structures in ATCA continuum maps at 1.5, 5.6, and 9 GHz. Unlike most FR-II objects, W2332-5056 is hosted by a disk-like galaxy. The core has a projected 5" linear radio feature that is perpendicular to the curved primary jet, hinting at unusual and complex activity within the inner 25 kpc. The multi-epoch optical-near-IR photometric measurements indicate significant variability over a 3-20 year baseline from the AGN component. Gemini-South optical data shows an unusual double-peaked emission-line features: the centroids of the broad-lined components of H-alpha and H-beta are blueshifted with respect to the narrow lines and host galaxy by ~ 3800 km/s. We examine possible cases which involve single or double supermassive black holes in the system, and discuss required future investigations to disentangle the mystery nature of this system.

astro-ph.CO↗

arXiv2012

Optical emission of the black hole X-ray transient MAXI J1659-152 during quiescence

We report on the optical detection of the black hole X-ray transient MAXI J1659-152 during its quiescent state. By using the Canada France Hawaii Telescope (CFHT), we observed MAXI J1659-152 about 7 months after the end of an X-ray outburst. The optical counterpart of MAXI J1659-152 is clearly detected with a r'-band magnitude of 23.6-23.8. The detection confirms that the optical emission of MAXI J1659-152 during quiescence is relatively bright comparing to other black hole X-ray transients. This implies that the distance to MAXI J1659-152 is 4.6-7.5 kpc for a M2 dwarf companion star, or 2.3-3.8 kpc for a M5 dwarf companion star. By comparing with other measurements, a M2 dwarf companion is more likely.

astro-ph.HE↗

arXiv2012

Quiescent X-Ray/Optical Counterparts of the Black Hole Transient H 1705-250

We report the result of a new Chandra observation of the black hole X-ray transient H 1705-250 in quiescence. H 1705-250 was barely detected in the new 50 ks Chandra observation. With 5 detected counts, we estimate the source quiescent luminosity to be Lx~9.1e30 erg/s in the 0.5-10 keV band (adopting a distance of 8.6 kpc). This value is in line with the quiescent luminosities found among other black hole X-ray binaries with similar orbital periods. By using images taken with the Faulkes Telescope North, we derive a refined position of H 1705-250. We also present the long-term lightcurve of the optical counterpart from 2006 to 2012, and show evidence for variability in quiescence.

astro-ph.HE↗

arXiv2012

The Chandra Local Volume Survey: The X-ray Point Source Catalog of NGC 300

We present the source catalog of a new Chandra ACIS-I observation of NGC 300 obtained as part of the Chandra Local Volume Survey. Our 63 ks exposure covers ~88% of the D25 isophote (R~6.3 kpc) and yields a catalog of 95 X-ray point sources detected at high significance to a limiting unabsorbed 0.35-8 keV luminosity of ~10^36 erg s^-1. Sources were cross-correlated with a previous XMM-Newton catalog, and we find 75 "X-ray transient candidate" sources that were detected by one observatory, but not the other. We derive an X-ray scale length of 1.7+/-0.2 kpc and a recent star formation rate of 0.12 Msun yr^-1, in excellent agreement with optical observations. Deep, multi-color imaging from the Hubble Space Telescope, covering ~32% of our Chandra field, was used to search for optical counterparts to the X-ray sources, and we have developed a new source classification scheme to determine which sources are likely X-ray binaries, supernova remnants, and background AGN candidates. Finally, we present the X-ray luminosity functions (XLFs) at different X-ray energies, and we find the total NGC 300 X-ray point source population to be consistent with other late-type galaxies hosting young stellar populations (<50 Myr). We find XLF of sources associated with older stellar populations has a steeper slope than the XLF of X-ray sources coinciding with young stellar populations, consistent with theoretical predictions.

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arXiv2012

Investigation of CTA 1 with Suzaku Observation

We report on an 105 ks Suzaku observation of the supernova remnant CTA 1 (G119.5+10.2). The Suzaku soft X-ray observation was carried out with both timing mode and imaging mode. A ~ 10' extended feature, which is interpreted as a bow-shock component of the pulsar wind nebula (PWN), is revealed in this deep observation for the first time. The nebular spectrum can be modelled by a power-law with a photon index of ~ 1.8 which suggests a slow synchrotron cooling scenario. The photon index is approximately constant across this extended feature. We compare and discuss our observations of this complex nebula with previous X-ray investigations. We do not obtain any significant pulsation from the central pulsar in the soft (0.2-12 keV) and hard (10-60 keV) X-ray data. The non-detection is mainly due to the loss of the precise imaging ability to accurately determine the source contribution. The spectra of XIS and HXD can be directly connected without a significant spectral break according to our analysis. Future observations of NuSTAR and Astro-H would be able to resolve the contamination and provide an accurate hard X-ray measurement of CTA 1.

astro-ph.HE↗

arXiv2012

Fermi Large Area Telescope observations of GRB 110625A

Gamma-ray bursts (GRBs) that emit photons at GeV energies form a small but significant population of GRBs. However, the number of GRBs whose GeV-emitting period is simultaneously observed in X-rays remains small. We report gamma-ray observations of GRB 110625A using Fermi's Large Area Telescope (LAT) in the energy range 100 MeV to 20 GeV. Gamma-ray emission at these energies was clearly detected using data taken between 180s and 580s after the burst, an epoch after the prompt emission phase. The GeV light curve differs from a simple power-law decay, and probably consists of two emission periods. Simultaneous Swift/XRT observations did not show flaring behaviors as in the case of GRB 100728A. We discuss the possibility that the GeV emission is the synchrotron self-Compton radiation of underlying ultraviolet flares.

astro-ph.HE↗

arXiv2012

First Detection of A Sub-kpc Scale Molecular Outflow in the Starburst Galaxy NGC 3628

We successfully detected a molecular outflow with a scale of 370-450 pc in the central region of the starburst galaxy NGC 3628 through deep CO(1-0) observations by using the Nobeyama Millimeter Array (NMA). The mass of the outflowing molecular gas is ~2.8x10^7 M_sun, and the outflow velocity is ~90(+/-10) km s^{-1}. The expansion timescale of the outflow is 3.3-6.8 Myr, and the molecular gas mass flow rate is 4.1-8.5 M_sun yr^{-1}. It requires mechanical energy of (1.8-2.8)x10^{54} erg to create this sub-kpc scale molecular outflow. In order to understand the evolution of the molecular outflow, we compare the physical properties between the molecular outflow observed from our NMA CO(1-0) data and the plasma gas from the soft X-ray emission of the Chandra X-ray Observatory (CXO) archival data. We found that the distribution between the molecular outflow and the strong plasma outflow seems to be in a similar region. In this region, the ram pressure and the thermal pressure of the plasma outflow are 10^{-(8-10)} dyne cm^{-2}, and the thermal pressure of molecular outflow is 10^{-(11-13)} dyne cm^{-2}. This implies the molecular outflow is still expanding outward. The molecular gas consumption timescale is estimated as 17-27 Myr, and the total starburst timescale is 20-34 Myr. The evolutionary parameter is 0.11-0.25, suggesting that the starburst activity in NGC 3628 is still in a young stage.

astro-ph.GA↗

arXiv2011

Discovery of GeV gamma-ray emission from PSR B1259-63/LS 2883

The binary system PSR B1259-63/LS 2883 consists of a 47.8 ms radio pulsar that orbits the companion Be star with a period of 3.4 years in a highly eccentric orbit. The system is well sampled in radio, X-rays, and TeV gamma-rays, and shows orbital-phase-dependent variability in all observed frequencies. Here we report on the discovery of >100 MeV gamma-rays from PSR B1259-63/LS 2883 through the 2010 pariastron passage. Using data collected with the Large Area Telescope aboard Fermi from 33 days before pariastron to 75 days after pariastron, PSR B1259-63/LS 2883 is detected at a significance of 13.6 standard deviations. The gamma-ray light curve is highly variable over the above period, with changing photon index that correlates with gamma-ray flux. In particular, two major flares that occur after the pariastron passage were observed. The onset of gamma-ray emission occurs close to, but not at the same orbital phases as, the two disk passages that occur ~1 month before and ~1 month after the pariastron passage. The fact that the GeV orbital light curve is different from that of the X-ray and TeV light curves strongly suggests that GeV gamma-ray emission originates from a different component. We speculate that the observed GeV flares may be resulting from Doppler boosting effects.

astro-ph.HE↗