A. K. H. Kong

University of California, Santa Barbara, Santa Barbara, California, United States

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Publications (211)702.79 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We report multi-wavelength observations of the unidentified Fermi object 2FGL J1653.6-0159. With the help of high-resolution X-ray observation, we have identified an X-ray and optical counterpart of 2FGL J1653.6-0159. The source exhibits a periodic modulation of 74.93 min in optical and possibly also in X-ray. We suggest that 2FGL J1653.6-0159 is a compact binary system with an orbital period of 74.93 min. Combining the gamma-ray and X-ray properties, 2FGL J1653.6-0159 is potentially a black widow/redback type gamma-ray millisecond pulsar (MSP). The optical and X-ray lightcurve profile shows that the companion is mildly heated by the high-energy emission and the X-rays are from intrabinary shock. Although no radio pulsation has been detected yet, we estimated that the spin period of the MSP is ~2 ms based on a theoretical model. If pulsation can be confirmed in the future, 2FGL J1653.6-0159 will become the first ultracompact rotation-powered MSP.
    08/2014;
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    ABSTRACT: We report the X-ray pulsation of ~173.3 ms for the "next Geminga", PSR J1836+5925, with recent XMM-Newton investigations. The X-ray periodicity is consistent wtih the gamma-ray ephemeris at the same epoch. The X-ray folded light curve has a sinusoidal structure which is different from the double-peaked gamma-ray pulse profile. We have also analysed the X-ray phase-averaged spectra which shows the X-ray emission from PSR J1836+5925 is thermal dominant. This suggests the X-ray pulsation mainly originates from the modulated hot spot on the stellar surface.
    The Astrophysical Journal Letters 08/2014; 793(1). · 6.35 Impact Factor
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    ABSTRACT: We report the optical identification of the companion to the {\it Fermi} black widow millisecond pulsar PSR J1544+4937. We find a highly variable source on Keck LRIS images at the nominal pulsar position, with 2 magnitude variations over orbital period in the B, g, R, and I bands. The nearly achromatic light curves are difficult to explain with a simply irradiated hemisphere model, and suggest that the optical emission is dominated by a nearly isothermal hot patch on the surface of the companion facing the pulsar. We roughly constrain the distance to PSR J1544+4937 to be between 2 and 5 kpc. A more reliable distance measurement is needed in order to constrain the composition of the companion.
    The Astrophysical Journal Letters 06/2014; 791(1). · 6.35 Impact Factor
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    ABSTRACT: We study mechanisms of multi-wavelength emissions (X-ray, GeV and TeV gamma-rays) from the gamma-ray binary LS~5039. This paper is composed of two parts. In the first part, we report on results of observational analysis using four year data of \fermi\ Large Area Telescope. Due to the improvement of instrumental response function and increase of the statistics, the observational uncertainties of the spectrum in $\sim$100-300 MeV bands and $>10$GeV bands are significantly improved. The present data analysis suggests that the 0.1-100GeV emissions from LS~5039 contain three different components; (i) the first component contributes to $<$1GeV emissions around superior conjunction, (ii) the second component dominates in 1-10GeV energy bands and (iii) the third component is compatible to lower energy tail of the TeV emissions. In the second part, we develop an emission model to explain the properties of the phase-resolved emissions in multi-wavelength observations. Assuming that LS~5039 includes a pulsar, we argue that both emissions from magnetospheric outer gap and inverse-Compton scattering process of cold-relativistic pulsar wind contribute to the observed GeV emissions. We assume that the pulsar is wrapped by two kinds of termination shock; Shock-I due to the interaction between the pulsar wind and the stellar wind and Shock-II due to the effect of the orbital motion. We propose that the X-rays are produced by the synchrotron radiation at Shock-I region and the TeV gamma-rays are produced by the inverse-Compton scattering process at Shock-II region.
    06/2014;
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    ABSTRACT: In re-analyzing the archival Chandra data of the globular cluster 47 Tucanae, we have detected a new diffuse X-ray emission feature within the half-mass radius of the cluster. The spectrum of the diffuse emission can be described by a power-law model plus a plasma component with photon index Γ ~ 1.0 and plasma temperature kT ~ 0.2 keV. While the thermal component is apparently uniform, the non-thermal contribution falls off exponentially from the core. The observed properties could possibly be explained in the context of multiple shocks resulting from the collisions among the stellar wind in the cluster and the inverse Compton scattering between the pulsar wind and the relic photons.
    The Astrophysical Journal Letters 06/2014; 788(2):L40. · 6.35 Impact Factor
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    ABSTRACT: In re-analyzing the archival Chandra data of the globular cluster 47 Tucanae, we have detected a new diffuse X-ray emission feature within the half-mass radius of the cluster. The spectrum of the diffuse emission can be described by a power-law model plus a plasma component with photon index $\Gamma\sim1.0$ and plasma temperature $kT\sim0.2$ keV. While the thermal component is apparently uniform, the non-thermal contribution falls off exponentially from the core. The observed properties could possibly be explained in the context of multiple shocks resulted from the collisions among the stellar wind in the cluster and the inverse Compton scattering between the pulsar wind and the relic photons.
    05/2014;
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    ABSTRACT: In re-analyzing the archival Chandra data of the globular cluster 47 Tucanae, we have detected a new diffuse X-ray emission feature within the half-mass radius of the cluster. The spectrum of the diffuse emission can be described by a power-law model plus a plasma component with photon index $\Gamma\sim1.0$ and plasma temperature $kT\sim0.2$ keV. While the thermal component is apparently uniform, the non-thermal contribution falls off exponentially from the core. The observed properties could possibly be explained in the context of multiple shocks resulted from the collisions among the stellar wind in the cluster and the inverse Compton scattering between the pulsar wind and the relic photons.
    04/2014;
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    ABSTRACT: We report on XMM-Newton and Chandra observations of the Galactic supernova remnant candidate G38.7-1.4, together with complementary radio, infrared, and gamma-ray data. An approximately elliptical X-ray structure is found to be well correlated with radio shell as seen by the Very Large Array. The X-ray spectrum of G38.7-1.4 can be well-described by an absorbed collisional ionization equilibrium plasma model, which suggests the plasma is shock heated. Based on the morphology and the spectral behaviour, we suggest that G38.7-1.4 is indeed a supernova remnant belongs to a mix-morphology category.
    03/2014; 785(2).
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    ABSTRACT: 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$21 and triggered optical photometry, spectroscopy and soft X-ray monitoring of the outburst. Peaking at an absolute magnitude of M_R$\approx$-6.6 mag, and with a decay time of 1 mag per day, it is a faint and very fast novae. It shows optical emission lines of He/N and expansion velocities of 1900 to 2600 km/s 1-4 days after the optical peak. The Swift monitoring of the X-ray evolution revealed a supersoft source (SSS) with $kT_{eff}$ $\approx$ 90-110 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. Both the duration and effective temperature of the SS phase imply M>1.32 $M_\odot$, while MESA models of accreting WDs allow us to constrain the accretion rate to $\dot{M}>1.7\times10^{-7}$ $M_\odot$/yr and WD mass >1.32 $M_\odot$. 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.
    The Astrophysical Journal 01/2014; 786(1). · 6.73 Impact Factor
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    ABSTRACT: Recent observations strongly suggest that the millisecond pulsar binary PSR J1023+0038 has developed an accretion disk since 2013 June. We present the multi-wavelength analysis of PSR J1023+0038, which reveals that 1) its gamma-rays suddenly brightened within a few days in June/July 2013 and has remained at a high gamma-ray state for several months; 2) both UV and X-ray fluxes have increased by roughly an order of magnitude, and 3) the spectral energy distribution has changed significantly after the gamma-ray sudden flux change. Time variabilities associated with UV and X-rays are on the order of 100-500 seconds and 50-100 seconds respectively. Our model suggests that a newly formed accretion disk due to the sudden increase of the stellar wind could explain the changes of all these observed features. The increase of UV is emitted from the disk, and a new component in gamma-rays is produced by inverse Compton scattering between the new UV component and pulsar wind. The increase of X-rays results from the enhancement of injection pulsar wind energy into the intra-binary shock due to the increase of the stellar wind. We also predict that the radio pulses may be blocked by the evaporated winds from the disk and the pulsar is still powered by rotation.
    12/2013; 785(2).
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    ABSTRACT: We present radio continuum mapping, optical imaging, and spectroscopy of the newly discovered double-peaked, broad-lined active galactic nucleus (AGN) WISE J233237.05–505643.5 at redshift z = 0.3447. This source exhibits an FR-I and FR-II hybrid morphology, characterized by a bright core, jet, and Doppler-boosted lobe structures in Australian Telescope Compact Array 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 yr baseline from the AGN component. Gemini South optical data show unusual double-peaked emission-line features: the centroids of the broad-lined components of Hα and Hβ are blueshifted with respect to the narrow lines and host galaxy by ~3800 km s–1. We examine possible cases that involve single or double supermassive black holes in the system and discuss the required future investigations to disentangle the mysterious nature of this system.
    The Astrophysical Journal 12/2013; 779(1):41-. · 6.73 Impact Factor
  • P. H. T. Tam, A. K. H. Kong, K. L. Li
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    ABSTRACT: Following an earlier report of decreased brightness in optical and X-rays, and possibly gamma-rays, of XSS J12270-4859 (ATel #5647), we provide further information based on Fermi/LAT and Swift/XRT observations of this gamma-ray emitting low-mass X-ray binary.
    The Astronomer's Telegram. 12/2013;
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    ABSTRACT: We have investigated the X-ray and $\gamma$-ray properties of the redback millisecond pulsar PSR J1723-2837 with XMM-Newton, Chandra and Fermi. We have discovered the X-ray orbital modulation of this binary system with the minimum that coincides with the phases of radio eclipse. The X-ray emission is clearly non-thermal in nature which can be well described by a simple power-law with a photon index of $\sim1.2$. The phase-averaged luminosity is $\sim9\times10^{31}$ erg/s in 0.3-10 keV which consumes $\sim0.2\%$ of the spin-down power. We have detected the $\gamma-$ray emission in $0.1-300$ GeV from this system at a significance of $\sim6\sigma$ for the first time. The $\gamma-$rays in this energy range consumes $\sim2\%$ of the spin-down power and can be modeled by a power-law with a photon index of $\sim2.6$. We discuss the high energy properties of the new redback in the context of a intrabinary shock model.
    The Astrophysical Journal 11/2013; 781(1). · 6.73 Impact Factor
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    ABSTRACT: 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).
    Monthly Notices of the Royal Astronomical Society 10/2013; 437(2). · 5.52 Impact Factor
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    ABSTRACT: 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.
    10/2013;
  • A. K. H. Kong, K. L. Li, E. Ofek
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    ABSTRACT: We here report a possible detection of X-ray emission from the supernova iPTF13bvn (ATel #5137). A series of Swift ToO observations have been performed since 2013 June 17. From June 17 to July 13, there are 9 observations with a total exposure of 31.5 ks. From the combined X-Ray Telescope (XRT) image, there is an excess of X-ray photons at the supernova position. In the 0.3-10 keV band, we searched a 9 arcsec radius circular region centered at the supernova position and detected 8 photons.
    The Astronomer's Telegram. 07/2013;
  • R. H. H. Huang, C. Y. Hui, L. Trepl, A. K. H. Kong
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    ABSTRACT: We report on XMM-Newton observations of the Galactic supernova remnant G296.7-0.9. A detailed spectro-imaging X-ray study of G296.7-0.9 was performed. We detected an incomplete shell-like X-ray structure which is located near the boundary of the radio emission at a frequency of 843 MHz. The X-ray spectrum can be best described by an absorbed ionization plasma model accompanied with metallic emission lines, which suggests the plasma is shock heated. No promising compact stellar remnant associated with G296.7-0.9 was found. No Gamma-ray emission of G296.7-0.9 from Fermi-LAT telescope was detected in our study.
    Proceedings of the International Astronomical Union 03/2013; 8(S291):402-404.
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    ABSTRACT: Using the data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope, we have searched for the gamma-ray pulsations from the direction of globular cluster M28 (NGC 6626). We report the discovery of a signal with the frequency consistent with that of the energetic millisecond pulsar (MSP) PSR B1821-24 in M28. A weighted H-test test statisic (TS) of 28.8 is attained which corresponds to a chance probability of ~1e-5 (4.3-sigma detection). With a phase-resolved analysis, the pulsed component is found to contribute ~25% of the total observed gamma-ray emission from the cluster. On the other hand, the unpulsed level provides a constraint for the underlying MSP population and the fundamental plane relations for the scenario of inverse Compton scattering. Follow-up timing observations in radio/X-ray are encouraged for further investigating this periodic signal candidate.
    The Astrophysical Journal Letters 02/2013; 765(2). · 6.35 Impact Factor
  • K. L. Li, A. K. H. Kong
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    ABSTRACT: Following the CRTS detection of a new optical transient near the Galactic plane (b~30 deg), SSS130101:122222 (ATel #4699), we searched in the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010 AJ, 140, 1868) catalog and found a possible WISE pre-burst counterpart, WISE J122221.66-311524.6, with an offset of 0.5 arcsec.
    The Astronomer's Telegram. 01/2013;
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    ABSTRACT: The Chandra Local Volume Survey is a deep, volume-limited X-ray survey of five nearby galaxies (NGC 55, NGC 300, NGC 404, NGC 2403, and NGC 4214) with matched Hubble observations down to M_ 0, spanning a range stellar masses, metallicities, morphologies, and star formation histories. The X-ray emission detected in normal, non-active spiral galaxies such as these is dominated by X-ray binaries (XRBs). While studies of XRBs in the Milky Way often suffer from significant distance uncertainties and considerable extinction along Galactic lines of sight, these difficulties are minimized when studying X-ray sources in nearby galaxies: extinction effects are less problematic when observing galaxies away from the Galactic disk, and distance uncertainties are reduced since all X-ray sources are essentially equidistant from the observer. We present preliminary results correlating the X-ray source population properties of these five galaxies - such as the X-ray luminosity functions (XLFs) and radial X-ray source distributions down to ~10^36 erg/s - with the morphologies and recent star formation histories of the host galaxies.
    01/2013;

Publication Stats

1k Citations
702.79 Total Impact Points

Institutions

  • 2014
    • University of California, Santa Barbara
      • Department of Physics
      Santa Barbara, California, United States
  • 2008–2014
    • National Tsing Hua University
      • Department of Physics
      Hsin-chu-hsien, Taiwan, Taiwan
  • 2012
    • National Taiwan Normal University
      T’ai-pei, Taipei, Taiwan
  • 2010–2012
    • The University of Hong Kong
      • Department of Physics
      Hong Kong, Hong Kong
  • 2005–2008
    • Massachusetts Institute of Technology
      • • Department of Physics
      • • Kavli Institute for Astrophysics and Space Research
      Cambridge, Massachusetts, United States
  • 2001–2008
    • Harvard-Smithsonian Center for Astrophysics
      • • Smithsonian Astrophysical Observatory
      • • Institute for Theory and Computation
      Cambridge, Massachusetts, United States
  • 2005–2006
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 2001–2004
    • Tufts University
      • Department of Physics and Astronomy
      Georgia, United States
  • 2000
    • Universiteit Utrecht
      • Astronomical Institute
      Utrecht, Provincie Utrecht, Netherlands