DSpace Repository :: Browsing by Author "Irwin, Jimmy A."

Browsing by Author "Irwin, Jimmy A."

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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 Tuscaloosa
The Birthplace of Low-Mass X-Ray Binaries: Field Versus Globular Cluster Populations
(2005-09-20) Irwin, Jimmy A.; University of Alabama Tuscaloosa
Recent Chandra studies of low-mass X-ray binaries (LMXBs) within early-type galaxies have found that LMXBs are commonly located within globular clusters of the galaxies. However, whether all LMXBs are formed within globular clusters has remained an open question. If all LMXBs formed within globular clusters, the summed X-ray luminosity of the LMXBs in a galaxy should be directly proportional to the number of globular clusters in the galaxy regardless of where the LMXBs currently reside. We have compared these two quantities over the same angular area for a sample of 12 elliptical and S0 galaxies observed with Chandra and found that the correlation between the two quantities is weaker than expected if all LMXBs formed within globular clusters. This indicates that a significant number of the LMXBs were formed in the field and naturally accounts for the spread in field-tocluster fractions of LMXBs from galaxy to galaxy. We also find that the ‘‘pollution’’ of globular cluster LMXBs into the field has been minimal within elliptical galaxies, but there is evidence that roughly half of the LMXBs originally in the globular clusters of S0 galaxies in our sample have escaped into the field. This is likely due to higher globular cluster disruption rates in S0s, resulting from stronger gravitational shocks caused by the passage of globular clusters through the disks of S0 galaxies that are absent in elliptical galaxies.
Buoyant AGN Bubbles in the Quasi-isothermal Potential of NGC 1399
(IOP Publishing, 2017-10-01) Su, Yuanyuan; Nulsen, Paul E. J.; Kraft, Ralph P.; Forman, William R.; Jones, Christine; Irwin, Jimmy A.; Randall, Scott W.; Churazov, Eugene; Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; University of Alabama Tuscaloosa; Max Planck Society
The Fornax Cluster is a low-mass cool-core galaxy cluster. We present a deep Chandra study of NGC. 1399, the central dominant elliptical galaxy of Fornax. The cluster center harbors two symmetric X-ray cavities coincident with a pair of radio lobes fed by two collimated jets along a north-south axis. A temperature map reveals that the active galactic nucleus (AGN) outburst has created a channel filled with cooler gas out to a radius of 10 kpc. The cavities are surrounded by cool bright rims and filaments that may have been lifted from smaller radii by the buoyant bubbles. X-ray imaging suggests a potential ghost bubble of. 5 kpc diameter to the northwest. We find that the amount of gas lifted by AGN bubbles is comparable to that which would otherwise cool, demonstrating that AGN-driven outflow is effective in offsetting cooling in low-mass clusters. The cluster cooling timescale is > 30 times longer than the dynamical timescale, which is consistent with the lack of cold molecular gas at the cluster center. The X-ray hydrostatic mass is consistent within 10%, with the total mass derived from the optical data. The observed entropy profile rises linearly, following a steeper slope than that observed at the centers of massive clusters; gas shed by stars in NGC. 1399 may be incorporated in the hot phase. However, it is far-fetched for supernova-driven outflow to produce and maintain the thermal distribution in NGC. 1399, and it is in tension with the metal content in the hot gas.
The Cheshire Cat Gravitational Lens: The Formation of a Massive Fossil Group
(2015-06-20) Irwin, Jimmy A.; Dupke, Renato; Carrasco, Eleazar R.; Maksym, W. Peter; Johnson, Lucas; White, Raymond E. III; University of Alabama Tuscaloosa
The Cheshire Cat is a relatively poor group of galaxies dominated by two luminous elliptical galaxies surrounded by at least four arcs from gravitationally lensed background galaxies that give the system a humorous appearance. Our combined optical/X-ray study of this system reveals that it is experiencing a line of sight merger between two groups with a roughly equal mass ratio with a relative velocity of ∼1350 km s−1 . One group was most likely a lowmass fossil group, while the other group would have almost fit the classical definition of a fossil group. The collision manifests itself in a bimodal galaxy velocity distribution, an elevated central X-ray temperature and luminosity indicative of a shock, and gravitational arc centers that do not coincide with either large elliptical galaxy. One of the luminous elliptical galaxies has a double nucleus embedded off-center in the stellar halo. The luminous ellipticals should merge in less than a Gyr, after which observers will see a massive 1.2–1.5 × 1014 M⊙ fossil group with an Mr = −24.0 brightest group galaxy at its center. Thus, the Cheshire Cat offers us the first opportunity to study a fossil group progenitor. We discuss the limitations of the classical definition of a fossil group in terms of magnitude gaps between the member galaxies. We also suggest that if the merging of fossil (or nearfossil) groups is a common avenue for creating present-day fossil groups, the time lag between the final galactic merging of the system and the onset of cooling in the shock-heated core could account for the observed lack of well-developed cool cores in some fossil groups.
Computational Methods in Studies of Galaxies: Automated Gravitational Lens Searches and Chemical Evolution of the Milky Way
(University of Alabama Libraries, 2024) Parul, Hanna; Bailin, Jeremy
In the LambdaCDM framework, dark matter forms the backbone of cosmic structure. While dark matter itself remains undetected, it governs the properties and evolution of the luminous parts of galaxies, which are extensively studied using various computational methods, including cosmological simulations and machine learning techniques. In the first part of this dissertation, I use cosmological simulations of Milky Way-mass galaxies from the FIRE-2 suite to examine the relationship between galactic history and present-day properties in chemical abundance space. I demonstrate that early bursty star formation leaves a characteristic pattern in the stellar distribution on the [Fe/H]-[alpha/Fe] plane, allowing estimation of the transition time between different star formation modes. I assess the observability of this signal and compare results between the Milky Way and simulated galaxies. I continue with inspecting the effect of gas accretion history on the presence of alpha-bimodality in the stellar distribution on the [Fe/H]-[alpha/Fe] plane. I show that the late-time infall of metal-poor gas promotes the build-up of a metal-poor low-alpha tail in the outer disks of Milky Way-mass galaxies. In the second part, I address challenges in automated gravitational lens detection for large-scale surveys. Most modern lens-finding algorithms employing neural networks are trained on simulated lenses, potentially leading to poor performance on observational data. I evaluate how well domain adaptation methods can mitigate the discrepancy between simulated and real data and improve the quality of identified lens candidates.
The Detection of a Cooling Flow Elliptical Galaxy from O VI Emission
(2001-06-01) Bregman, Joel N.; Miller, Eric D.; Irwin, Jimmy A.; University of Alabama Tuscaloosa
Diffuse Gas and Low-Mass X-Ray Binaries in the Chandra Observation of the S0 Galaxy NGC 1553
(2001-05-01) Blanton, Elizabeth L.; Sarazin, Craig L.; Irwin, Jimmy A.; University of Alabama Tuscaloosa
Diffuse ionized regions in the vicinity of active galaxies
(University of Alabama Libraries, 2011) Darnell, Erin Kay; Keel, W. C.; University of Alabama Tuscaloosa
Using a sample of 39 Active Galactic Nuclei (AGN), we investigated the incidence of giant ionized clouds in the vicinity of active galaxies. We carried out remote observations of the sample using the Southeastern Association for Research in Astronomy (SARA) North telescope at Kitt Peak and the SARA South telescope on Cerro Tololo. Frames were taken in continuum V and using a narrowband filter that transmits the redshifted [O III] 5007 Å line. We formed an emission line image by subtracting the combined and scaled V frames from the combined [O III] frames. To reduce uncorrelated noise, the emission line images are median filtered with a 1".9 x 1".9 box size. To bring out large diffuse regions, we convolve the emission line image with a circular Gaussian function of 3".42 FWHM. Emergent structures are determined to be starlight or ionized gas. 21 members of our sample were recently shown to be tidally disrupted in an atomic Hydrogen (HI) study. No extended [O III]-line emission clouds were seen in the vicinity of any from this group. We found one new instance of extended ionized emission clouds near the Seyfert 1 galaxy RX J1103.2-0654.
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 Tuscaloosa
We 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.
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.
Elemental Abundancies in the X-ray Gas of Early-type Galaxies with XMM-Newton and Chandra Observations
(2009-05-10) Ji, Jun; Irwin, Jimmy A.; Athey, Alex; Bregman, Joel N.; Lloyd-Davies, Edward J.; University of Alabama Tuscaloosa
The source of hot gas in elliptical galaxies is thought to be due to stellar mass loss, with contributions from supernova (SN) events and possibly from infall from a surrounding environment. This picture predicts supersolar values for the metallicity of the gas toward the inner part of the galaxy, which can be tested by measuring the gas phase abundances. We use high-quality data for 10 nearby early-type galaxy from XMM-Newton, featuring both the European Photon Imaging Camera and the Reflection Grating Spectrometer, where the strongest emission lines are detected with little blending; some Chandra data are also used. We find excellent consistency in the elemental abundances between the different XMM-Newton instruments and good consistency with Chandra. Differences in abundances with aperture size and model complexity are examined, but large differences rarely occur. For a twotemperature thermal model plus a point source contribution, the median Fe and O abundances are 0.86 and 0.44 of the solar value, while Si and Mg abundances are similar to that for Fe. This is similar to stellar abundances for these galaxies but SNe were expected to enhance the gas phase abundances considerably, which is not observed.
Evidence for a Stellar Disruption by an Intermediate-Mass Black Hole in an Extragalactic Globular Cluster
(2010-03-20) Irwin, Jimmy A.; Brink, Thomas G.; Bregman, Joel N.; Roberts, Timothy P.; University of Alabama Tuscaloosa
Finding fossil galaxy system progenitors using strong gravitational lensing
(University of Alabama Libraries, 2018) Johnson, Lucas Edward; Irwin, Jimmy A.; University of Alabama Tuscaloosa
Fossil galaxy systems have been described as some of the oldest systems in the universe, where the central galaxy has cannibalized most nearby member galaxies over cosmic time. While the progenitors to fossil systems have been predicted to exist in numerical simulations, little effort has gone into locating them until now. The discovery of fossil progenitors in the CASSOWARY catalog of strong gravitational lensing demonstrates that not all fossils are old, and their formation histories are more complex than originally thought. These progenitors have optical characteristics consistent with them being the transition phase between non-fossils and fossils, as we are observing the central galaxies in mid-assembly. We also identify a bias where systems acting as strong gravitational lenses are ~5 times more likely to be seen as fossils than non-lensing systems. Chandra X-ray images of eight CASSOWARY fossil progenitors show them being significantly over-luminous and hotter than comparable non-fossils which could be due to the strong lensing bias in our data, or fossils have characteristically deeper potential wells than non-fossils. Two progenitors were luminous enough to see a rise in gas temperature toward their cores which suggests these may be undergoing group mergers akin to the previously studied progenitor CSWA 2 verifying this as a viable fossil formation mechanism. Refinements to our original CASSOWARY data using the Hubble Space Telescope allowed us to disentangle complex merging environments at the centers of these eight progenitors, which further solidified the notion that progenitors are indeed transitioning toward fossil systems.
Gas Sloshing Regulates and Records the Evolution of the Fornax Cluster
(IOP Publishing, 2017-12-10) Su, Yuanyuan; Nulsen, Paul E. J.; Kraft, Ralph P.; Roediger, Elke; ZuHone, John A.; Jones, Christine; Forman, William R.; Sheardown, Alex; Irwin, Jimmy A.; Randall, Scott W.; Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; University of Hull; University of Alabama Tuscaloosa
We present results of a joint Chandra and XMM-Newton analysis of the Fornax Cluster, the nearest galaxy cluster in the southern sky. Signatures of merger-induced gas sloshing can be seen in the X-ray image. We identify four sloshing cold fronts in the intracluster medium, residing at radii of 3 kpc (west), 10 kpc (northeast), 30 kpc (southwest), and 200 kpc (east). Despite spanning over two orders of magnitude in radius, all four cold fronts fall onto the same spiral pattern that wraps around the BCG NGC. 1399, likely all initiated by the infall of NGC. 1404. The most evident front is to the northeast, 10 kpc from the cluster center, which separates low-entropy high-metallicity gas and high-entropy low-metallicity gas. The metallicity map suggests that gas sloshing, rather than an AGN outburst, is the driving force behind the redistribution of the enriched gas in this cluster. The innermost cold front resides within the radius of the strong cool core. The sloshing timescale within the cooling radius, calculated from the Brunt-Vasala frequency, is an order of magnitude shorter than the cooling time. It is plausible that gas sloshing is contributing to the heating of the cool core, provided that gas of different entropies can be mixed effectively via Kelvin-Helmholtz instability. The estimated age of the outermost front suggests that this is not the first infall of NGC. 1404.
Hard X-Ray Emission from the M87 AGN Detected with NuSTAR
(IOP Publishing, 2017-11-01) Wong, Ka-Wah; Nemmen, Rodrigo S.; Irwin, Jimmy A.; Lin, Dacheng; Eureka Scientific; Minnesota State Colleges & Universities; Minnesota State University Mankato; Universidade de Sao Paulo; University of Alabama Tuscaloosa; University System Of New Hampshire; University of New Hampshire
M87 hosts a 3-6 billion solar mass black hole with a remarkable relativistic jet that has been regularly monitored in radio to TeV bands. However, hard X-ray emission. greater than or similar to 10 keV, which would be expected to primarily come from the jet or the accretion flow, had never been detected from its unresolved X-ray core. We report NuSTAR detection up to 40 keV from the the central regions of M87. Together with simultaneous Chandra observations, we have constrained the dominant hard X-ray emission to be from its unresolved X-ray core, presumably in its quiescent state. The core spectrum is well fitted by a power law with photon index Gamma = 2.11(-0.11)(+0.15) . The measured flux density at 40 keV is consistent with a jet origin, although emission from the advection-dominated accretion flow cannot be completely ruled out. The detected hard X-ray emission is significantly lower than that predicted by synchrotron self-Compton models introduced to explain emission above a GeV.
Hot Diffuse Emission in the Nuclear Starburst Region of NGC 2903
(2012-10-20) Yukita, Mihoko; Swartz, Douglas A.; Tennant, Allyn F.; Soria, Roberto; Irwin, Jimmy A.; University of Alabama Tuscaloosa
We present a deep Chandra observation of the central regions of the late-type barred spiral galaxy NGC 2903. The Chandra data reveal soft (kTe ∼ 0.2–0.5 keV) diffuse emission in the nuclear starburst region and extending ∼2 (∼5 kpc) to the north and west of the nucleus. Much of this soft hot gas is likely to be from local active star-forming regions; however, besides the nuclear region, the morphology of hot gas does not strongly correlate with the bar or other known sites of active star formation. The central ∼650 pc radius starburst zone exhibits much higher surface brightness diffuse emission than the surrounding regions and a harder spectral component in addition to a soft component similar to the surrounding zones. We interpret the hard component as also being of thermal origin with kTe ∼ 3.6 keV and to be directly associated with a wind fluid produced by supernovae and massive star winds similar to the hard diffuse emission seen in the starburst galaxy M82. The inferred terminal velocity for this hard component, ∼1100 km s−1, exceeds the local galaxy escape velocity suggesting a potential outflow into the halo and possibly escape from the galaxy gravitational potential. Morphologically, the softer extended emission from nearby regions does not display an obvious outflow geometry. However, the column density through which the X-rays are transmitted is lower in the zone to the west of the nucleus compared to that from the east and the surface brightness is relatively higher suggesting some of the soft hot gas originates from above the disk: viewed directly from the western zone but through the intervening disk of the host galaxy along sight lines from the eastern zone. There are several point-like sources embedded in the strong diffuse nuclear emission zone. Their X-ray spectra show them to likely be compact binaries. None of these detected point sources are coincident with the mass center of the galaxy and we place an upper limit luminosity from any point-like nuclear source to be <2 × 1038 erg s−1 in the 0.5–8.0 keV band, which indicates that NGC 2903 lacks an active galactic nucleus. Heating from the nuclear starburst and a galactic wind may be responsible for preventing cold gas from accreting onto the galactic center.
Investigating the effects of progenitor metallicity on observable features of chandrasekhar mass type ia supernovae and improving detonation models of sub-chandrasekhar mass type ia supernovae
(University of Alabama Libraries, 2018) Miles, Broxton Jake; Townsley, Dean M.; University of Alabama Tuscaloosa
Type Ia Supernovae are generally considered to be the result of the thermonuclear disruption of carbon oxygen white dwarfs. However, the exact mechanism behind the explosion remains uncertain. The pre-explosion progenitor of a white dwarf has never been observed, so all conclusions must be drawn from comparisons between observed events and computational models. Here, work is presented on identifying spectral features indicative of progenitor metallicity. Metallicity affects the production of alpha-chain elements, which leaves imprints in the spectra. Two features are found that may be signals of progenitor metallicity, a Ti feature at 4200 \AA\ and an Fe feature at 5500 \AA . The second portion of this work focuses on the accurate modeling of detonations in sub-Chandrasekhar mass type Ia supernovae. The scales of the burning processes involved, compared to the size of the white dwarf, make fully resolving the detonation computationally impossible in full-star simulations. To mitigate this problem, past studies have used sub-grid scale models that attempt to capture the energetics of the explosion and post-process the results to calculate their models' nucleosynthetic products. If sub-grid models are to be believed, they must have accurate treatments of detonation physics such as curvature and shock strengthening. In low-density regions of the white dwarf, the curvature of the detonation front slows its propagation, affecting the production of intermediate mass elements. We find that the sharp density gradient in the outer radii of the white dwarf counteracts the weakening effect of curvature, resulting in more complete burning than expected in this low density region.
Investigating the Potential Dilution of the Metal Content of Hot Gas in Early-Type Galaxies by Accreted Cold Gas
(2013-03-20) Su, Yuanyuan; Irwin, Jimmy A.; University of Alabama Tuscaloosa
Investigation of magnetic relaxation mechanisms and dynamic magnetic properties in thin films using ferromagnetic resonance (fmr) technique
(University of Alabama Libraries, 2016) Khodadadi, Behrouz; Mewes, Tim; University of Alabama Tuscaloosa
Investigating the damping processes and the behavior of dynamic magnetic properties in ferromagnetic thin films has been an important key towards design and fabrication of different microwave and magnetic recording devices. This thesis discusses the dynamic magnetic properties and also the physics behind different relaxation mechanisms in ferromagnetic thin films using comprehensive experimental investigations by means of broadband ferromagnetic resonance (FMR) technique. In chapter one the basics of ferromagnetic resonance technique and the experimental features of the FMR setup used in this study are discussed, also the FMR data analysis is explained. Chapter two is devoted to the study of the interfacial perpendicular magnetic anisotropy (PMA) and damping parameter in Co2FeAl thin films. In chapter three broadband temperature dependent FMR measurements were carried out on Ni80Fe20/Gd thin films to investigate the behavior of ferromagnetic relaxation, and gyromagnetic ratio as the system goes through the Curie temperature of Gd. In chapter four, the ferromagnetic relaxation mechanisms in ferrites are discussed. The low loss Nikel Ferrite and Lithium ferrite single crystal, thin and ultra-thin films were characterized by detailed FMR measurements to investigate the effect of microstructural defects on the magnetization relaxation. A comprehensive study on the interlayer exchange coupling strength in Co90Fe10/Ru/ Co90Fe10 multilayers is the subject of chapter five, in which the mutual spin pumping is discussed as a recently discovered channel for relaxation in exchange coupled multilayers.
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 Tuscaloosa
The 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.

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