Browsing by Author "Webb, Natalie A."
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Item A ~3.8 hr Periodicity From an Ultrasoft Active Galactic Nucleus Candidate(2013-10-10) Lin, Dacheng; Irwin, Jimmy A.; Godet, Olivier; Webb, Natalie A.; Barret, Didier; University of Alabama TuscaloosaItem Discovery of a Highly Variable Dipping Ultraluminous X-Ray Source in M94(2013-12-20) Lin, Dacheng; Irwin, Jimmy A.; Webb, Natalie A.; Didier, Barret; Remillard, Ronald A.; University of Alabama TuscaloosaWe report the discovery of a new ultraluminous X-ray source (ULX) 2XMM J125048.6+410743 within the spiral galaxy M94. The source has been observed by ROSAT, Chandra, and XMM-Newton on several occasions, exhibiting as a highly variable persistent source or a recurrent transient with a flux variation factor of 100, a high duty cycle (at least ∼70%), and a peak luminosity of LX ∼ 2 × 1039 erg s−1 (0.2–10 keV, absorbed). In the brightest observation, the source is similar to typical low-luminosity ULXs, with the spectrum showing a high-energy cutoff but harder than that from a standard accretion disk. There are also sporadical short dips, accompanied by spectral softening. In a fainter observation with LX ∼ 3.6×1038 erg s−1, the source appears softer and is probably in the thermal state seen in Galactic black hole X-ray binaries (BHBs). In an even fainter observation (LX ∼ 9×1037 erg s−1), the spectrum is harder again, and the source might be in the steep-power-law state or the hard state of BHBs. In this observation, the light curve might exhibit ∼7 hr (quasi-)periodic large modulations over two cycles. The source also has a possible point-like optical counterpart from Hubble Space Telescope images. In terms of the colors and the luminosity, the counterpart is probably a G8 supergiant or a compact red globular cluster containing ∼2 × 105 K dwarfs, with some possible weak UV excess that might be ascribed to accretion activity. Thus, our source is a candidate stellar-mass BHB with a supergiant companion or with a dwarf companion residing in a globular cluster. Our study supports that some low-luminosity ULXs are supercritically accreting stellar-mass BHBs.Item DISCOVERY OF THE CANDIDATE OFF-NUCLEAR ULTRASOFT HYPER-LUMINOUS X-RAY SOURCE 3XMM J141711.1+522541(IOP Publishing, 2016-04-10) Lin, Dacheng; Carrasco, Eleazar R.; Webb, Natalie A.; Irwin, Jimmy A.; Dupke, Renato; Romanowsky, Aaron J.; Ramirez-Ruiz, Enrico; Strader, Jay; Homan, Jeroen; Barret, Didier; Godet, Olivier; University System Of New Hampshire; University of New Hampshire; Centre National de la Recherche Scientifique (CNRS); Universite de Toulouse; Universite Toulouse III - Paul Sabatier; University of Alabama Tuscaloosa; University of Michigan System; University of Michigan; Eureka Scientific; California State University System; San Jose State University; University of California System; University of California Santa Cruz; Michigan State University; Massachusetts Institute of Technology (MIT)We report the discovery of an off-nuclear ultrasoft hyper-luminous X-ray source candidate 3XMM J141711.1 + 522541 in the inactive S0 galaxy SDSS. J141711.07+522540.8 (z = 0.41827, dL = 2.3 Gpc) in the Extended Groth Strip. It is located at a projected offset of similar to 1.'' 0 (5.2 kpc) from the nucleus of the galaxy and was serendipitously detected in five XMM-Newton observations in 2000 July. Two observations have enough counts and can be fitted with a standard thermal disk with an apparent inner disk temperature kT(MCD) similar to 0.13 keV and a 0.28-14.2 keV unabsorbed luminosity LX similar to 4 x 10(43) erg s(-1) in the source rest frame. The source was still detected in three Chandra observations in 2002 August, with similarly ultrasoft but fainter spectra (kT(MCD) similar to 0.17 keV, LX similar to 0.5 x 10(43) erg s(-1)). It was not detected in later observations, including two by Chandra in 2005 October, one by XMM-Newton in 2014 January, and two by Chandra in 2014 September-October, implying a long-term flux variation factor of > 14. Therefore the source could be a transient with an outburst in 2000-2002. It has a faint optical counterpart candidate, with apparent magnitudes of m(F606W) = 26.3 AB mag and m(F814W) = 25.5 AB mag in 2004 December (implying an absolute V-band magnitude of similar to-15.9 AB mag). We discuss various explanations for the source and find that it is best explained as a massive black hole (BH) embedded in the nucleus of a possibly stripped satellite galaxy, with the X-ray outburst due to tidal disruption of a surrounding star by the BH. The BH mass is similar to 10(5)M circle dot, assuming the peak X-ray luminosity at around the Eddington limit.Item Large decay of X-ray flux in 2XMM J123103.2+110648: evidence for a tidal disruption event(Oxford University Press, 2017-02-24) Lin, Dacheng; Godet, Olivier; Ho, Luis C.; Barret, Didier; Webb, Natalie A.; Irwin, Jimmy A.; University System Of New Hampshire; University of New Hampshire; Centre National de la Recherche Scientifique (CNRS); Universite de Toulouse; Universite Toulouse III - Paul Sabatier; Peking University; University of Alabama TuscaloosaThe X-ray source 2XMM J123103.2+110648 was previously found to show pure thermal X-ray spectra and an similar to 3.8 h periodicity in three XMM-Newton X-ray observations in 20032005, and the optical spectrum of the host galaxy suggested it as a type 2 active galactic nucleus candidate. We have obtained new X-ray observations of the source, with Swift and Chandra in 2013-2016, in order to shed new light on its nature based on its long-term evolution property. We found that the source could be in an X-ray outburst, with the X-ray flux decreasing by an order of magnitude in the Swift and Chandra observations, compared with the XMM-Newton observations 10 yr ago. There seemed to be significant spectral softening associated with the drop of X-ray flux (disc temperature kT similar to 0.16-0.2 keV in XMM-Newton observations versus kT similar to 0.09 +/- 0.02 keV in the Chandra observation). Therefore, the Swift and Chandra followup observations support our previous suggestion that the source could be a tidal disruption event (TDE), though it seems to evolve slower than most of the other TDE candidates. The apparent long duration of this event could be due to the presence of a long super-Eddington accretion phase and/or slow circularization.Item A likely decade-long sustained tidal disruption event(Nature Portfolio, 2017-02-02) Lin, Dacheng; Guillochon, James; Komossa, S.; Ramirez-Ruiz, Enrico; Irwin, Jimmy A.; Maksym, W. Peter; Grupe, Dirk; Godet, Olivier; Webb, Natalie A.; Barret, Didier; Zauderer, B. Ashley; Duc, Pierre-Alain; Carrasco, Eleazar R.; Gwyn, Stephen D. J.; University System Of New Hampshire; University of New Hampshire; Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; University of California System; University of California Santa Cruz; University of Alabama Tuscaloosa; Seoul National University (SNU); Morehead State University; Centre National de la Recherche Scientifique (CNRS); Universite de Toulouse; Universite Toulouse III - Paul Sabatier; New York University; CEA; UDICE-French Research Universities; Universite Paris Cite; Universite Paris Saclay; National Research Council CanadaMultiwavelength flares from tidal disruption and accretion of stars can be used to find and study otherwise dormant massive black holes in galactic nuclei(1). Previous well-monitored candidate flares were short-lived, with most emission confined to within similar to 1 year(2-5). Here we report the discovery of a well-observed super-long (> 11 years) luminous X-ray flare from the nuclear region of a dwarf starburst galaxy. After an apparently fast rise within similar to 4 months a decade ago, the X-ray luminosity, though showing a weak trend of decay, has been persistently high at around the Eddington limit (when the radiation pressure balances the gravitational force). The X-ray spectra are soft - steeply declining towards higher energies and can be described with Comptonized emission from an optically thick low-temperature corona, a super-Eddington accretion signature often observed in accreting stellar-mass black holes(6). Dramatic spectral softening was also caught in one recent observation, implying either a temporary transition from the super-Eddington accretion state to the standard thermal state, or the presence of a transient highly blueshifted (similar to 0.36c) warm absorber. All these properties in concert suggest a tidal disruption event with an unusually long super-Eddington accretion phase that has never before been observed.Item A luminous X-ray outburst from an intermediate-mass black hole in an off-centre star cluster(Nature Portfolio, 2018-06-14) Lin, Dacheng; Strader, Jay; Carrasco, Eleazar R.; Page, Dany; Romanowsky, Aaron J.; Homan, Jeroen; Irwin, Jimmy A.; Remillard, Ronald A.; Godet, Olivier; Webb, Natalie A.; Baumgardt, Holger; Wijnands, Rudy; Barret, Didier; Duc, Pierre-Alain; Brodie, Jean P.; Gwyn, Stephen D. J.; University System Of New Hampshire; University of New Hampshire; Michigan State University; Universidad Nacional Autonoma de Mexico; California State University System; San Jose State University; University of California System; University of California Santa Cruz; Eureka Scientific; University of Alabama Tuscaloosa; Massachusetts Institute of Technology (MIT); Universite de Toulouse; Universite Toulouse III - Paul Sabatier; Centre National de la Recherche Scientifique (CNRS); University of Queensland; University of Amsterdam; CNRS - National Institute for Earth Sciences & Astronomy (INSU); UDICE-French Research Universities; Universites de Strasbourg Etablissements Associes; Universite de Strasbourg; National Research Council CanadaA unique signature for the presence of massive black holes in very dense stellar regions is occasional giant-amplitude out-bursts of multi-wavelength radiation from tidal disruption and subsequent accretion of stars that make a close approach to the black holes(1). Previous strong tidal disruption event (TDE) candidates were all associated with the centres of largely isolated galaxies(2-6). Here, we report the discovery of a luminous X-ray outburst from a massive star cluster at a projected distance of 12.5 kpc from the centre of a large lenticular galaxy. The luminosity peaked at similar to 10(43) erg s(-1) and decayed systematically over 10 years, approximately following a trend that supports the identification of the event as a TDE. The X-ray spectra were all very soft, with emission confined to be less than or similar to 3.0 keV, and could be described with a standard thermal disk. The disk cooled significantly as the luminosity decreased-a key thermal-state signature often observed in accreting stellar-mass black holes. This thermal-state signature, coupled with very high luminosities, ultrasoft X-ray spectra and the characteristic power-law evolution of the light curve, provides strong evidence that the source contains an intermediate-mass black hole with a mass tens of thousand times that of the solar mass. This event demonstrates that one of the most effective means of detecting intermediate-mass black holes is through X-ray flares from TDEs in star clusters.Item Multiwavelength follow-up observations of the tidal disruption event candidate 2XMMi J184725.1-631724(Oxford University Press, 2017-11-16) Lin, Dacheng; Strader, Jay; Carrasco, Eleazar R.; Godet, Olivier; Grupe, Dirk; Webb, Natalie A.; Barret, Didier; Irwin, Jimmy A.; University System Of New Hampshire; University of New Hampshire; Michigan State University; Centre National de la Recherche Scientifique (CNRS); Universite de Toulouse; Universite Toulouse III - Paul Sabatier; Morehead State University; University of Alabama Tuscaloosa; Seoul National University (SNU)The ultrasoft X-ray flare 2XMMi J184725.1-631724 was serendipitously detected in two XMM-Newton observations in 2006 and 2007, with a peak luminosity of 6 x 10(43) erg s(-1). It was suggested to be a tidal disruption event (TDE) because its position is consistent with the centre of an inactive galaxy. It is the only known X-ray TDE candidate whose X-ray spectra showed evidence of a weak steep power-law component besides a dominant supersoft thermal disc. We have carried out multiwavelength follow-up observations of the event. Multiple X-ray monitorings show that the X-ray luminosity has decayed significantly after 2011. Especially, in our deep Chandra observation in 2013, we detected a very faint counterpart that supports the nuclear origin of 2XMMi J184725.1-631724 but had an X-ray flux a factor of similar to 1000 lower than in the peak of the event. Compared with follow-up ultraviolet (UV) observations, we found that there might be some enhanced UV emission associated with the TDE in the first XMM-Newton observation. We also obtained a high-quality UV-optical spectrum with the Southern Astrophysical Research (SOAR) Telescope and put a very tight constraint on the persistent nuclear activity, with a persistent X-ray luminosity expected to be lower than the peak of the flare by a factor of > 2700. Therefore, our multiwavelength follow-up observations strongly support the TDE explanation of the event.Item AN ULTRASOFT X-RAY FLARE FROM 3XMM J152130.7+074916: A TIDAL DISRUPTION EVENT CANDIDATE(IOP Publishing, 2015-09-20) Lin, Dacheng; Maksym, Peter W.; Irwin, Jimmy A.; Komossa, S.; Webb, Natalie A.; Godet, Olivier; Barret, Didier; Grupe, Dirk; Gwyn, Stephen D. J.; University System Of New Hampshire; University of New Hampshire; University of Alabama Tuscaloosa; Max Planck Society; Universite de Toulouse; Universite Toulouse III - Paul Sabatier; Centre National de la Recherche Scientifique (CNRS); Morehead State University; National Research Council CanadaWe report on the discovery of an ultrasoft X-ray transient source, 3XMM J152130.7+074916. It was serendipitously detected in an XMM-Newton observation on 2000 August 23, and its location is consistent with the center of the galaxy SDSS J152130.72+074916.5 (z = 0.17901 and d(L) = 866 Mpc). The high-quality X-ray spectrum can be fitted with a thermal disk with an apparent inner disk temperature of 0.17 keV and a rest-frame 0.24-11.8 keV unabsorbed luminosity of similar to 5 x 10(43) erg s(-1), subject to a fast-moving warm absorber. Short-term variability was also clearly observed, with the spectrum being softer at lower flux. The source was covered but not detected in a Chandra observation on 2000 April 3, a Swift observation on 2005 September 10, and a second XMM-Newton observation on 2014 January 19, implying a large variability (>260) of the X-ray flux. The optical spectrum of the candidate host galaxy, taken similar to 11 years after the XMM-Newton detection, shows no sign of nuclear activity. This, combined with its transient and ultrasoft properties, leads us to explain the source as tidal disruption of a star by the supermassive black hole in the galactic center. We attribute the fast-moving warm absorber detected in the first XMM-Newton observation to the super-Eddington outflow associated with the event and the short-term variability to a disk instability that caused fast change of the inner disk radius at a constant mass accretion rate.