arXiv subjects

Jifeng Liu

Publications and source records attributed to Jifeng Liu.

At least 235 records · Page 13Linked to original sources

arXiv2015

Two Optical Counterpart Candidates of M82-X1 from HST Observations

Optical counterparts can provide significant constraints on the physical nature of ultraluminous X-ray sources (ULXs). In this letter, we identify six point sources in the error circle of a ULX in M82, namely M82 X-1, by registering Chandra positions onto Hubble Space Telescope images. Two objects are considered as optical counterpart candidates of M82 X-1, which show F658N flux excess compared to the optical continuum that may suggest the existence of an accretion disk. The spectral energy distributions of the two candidates match well with the spectra for supergiants, with stellar types as F5-G0 and B5-G0, respectively. Deep spatially resolved spectroscopic follow-up and detailed studies are needed to identify the true companion and confirm the properties of this BH system.

astro-ph.SR↗

arXiv2015

A likely Micro-quasar in the Shadow of M82 X-1

The ultra-luminous X-ray source (ULX) M82 X-1 is one of the most promising intermediate mass black hole candidates in the local universe based on its high X-ray luminosities ($10^{40}-10^{41} {\rm erg s^{-1}}$) and quasi-periodic oscillations, and is possibly associated with a radio flare source. In this work, applying the sub-pixel technique to the 120 ks Chandra observation (ID: 10543) of M82 X-1, we split M82 X-1 into two sources separated by 1.1$\arcsec$. The secondary source is not detected in other M82 observations. The radio flare source is found to associate not with M82 X-1, but with the nearby transient source S1 with an outburst luminosity of $\sim 10^{39} {\rm erg s^{-1}}$. With X-ray outburst and radio flare activities analogous to the recently discovered micro-quasar in M31, S1 is likely to be a micro-quasar hidden in the shadow of M82 X-1.

astro-ph.HE↗

arXiv2013

Puzzling accretion onto a black hole in the ultraluminous X-ray source M101 ULX-1

There are two proposed explanations for ultraluminous X-ray sources (ULXs) with luminosities in excess of $10^{39}$ erg s$^{-1}$. They could be intermediate-mass black holes (more than 100-1,000, solar masses, $M_\odot$) radiating at sub-maximal (sub-Eddington) rates, as in Galactic black-hole X-ray binaries but with larger, cooler accretion disks. Alternatively, they could be stellar-mass black holes radiating at Eddington or super-Eddington rates. On its discovery, M101 ULX-1 had a luminosity of $3\times10^{39}$ erg s$^{-1}$ and a supersoft thermal disk spectrum with an exceptionally low temperature -- uncomplicated by photons energized by a corona of hot electrons -- more consistent with the expected appearance of an accreting intermediate-mass black hole. Here we report optical spectroscopic monitoring of M101 ULX-1. We confirm the previous suggestion that the system contains a Wolf-Rayet star, and reveal that the orbital period is 8.2 days. The black hole has a minimum mass of $5M_\odot$, and more probably a mass of $20-30M_\odot$, but we argue that it is very unlikely to be an intermediate-mass black hole. Therefore its exceptionally soft spectra at high Eddinton ratios violate the expectations for accretion onto stellar-mass black holes. Accretion must occur from captured stellar wind, which has hitherto been thought to be so inefficient that it could not power an ultraluminous source.

astro-ph.HE↗

arXiv2013

Breaking the energy-bandwidth limit of electro-optic modulators: theory and a device proposal

In this paper, we quantitatively analyzed the trade-off between energy per bit for switching and modulation bandwidth of classical electro-optic modulators. A formally simple energy-bandwidth limit (Eq. 10) is derived for electro-optic modulators based on intra-cavity index modulation. To overcome this limit, we propose a dual cavity modulator device which uses a coupling modulation scheme operating at high bandwidth (> 200 GHz) not limited by cavity photon lifetime and simultaneously features an ultra-low switching energy of 0.26 aJ, representing over three orders of magnitude energy consumption reduction compared to state-of-the-art electro-optic modulators.

physics.optics↗

arXiv2013

Analysis of a State Changing Supersoft X-ray Source in M31

We report on observations of a luminous supersoft X-ray source (SSS) in M31, r1-25, that has exhibited spectral changes to harder X-ray states. We document these spectral changes. In addition, we show that they have important implications for modeling the source. Quasisoft states in a source that has been observed as an SSS represent a newly- discovered phenomenon. We show how such state changers could prove to be examples of unusual black hole or neutron star accretors. Future observations of this and other state changers can provide the information needed to determine the nature(s) of these intriguing new sources.

astro-ph.HE↗

arXiv2012

Ultraviolet Spectra of ULX Systems

To further understand the nature of the optical counterparts associated with Ultraluminous X-Ray sources (ULXs), we obtained far ultraviolet spectra of the reported counterparts to the ULXs in NGC 1313, Holmberg II, NGC 5204, and M81. The spectral resolution of the ACS prism spectra degrades from 300 at 1300 A to 40 at 1850 A, so longer wavelength features have lower S/N. The spectra of the ULXs in NGC 1313, Ho II, and NGC 5204 are quite similar, showing the N V 1240 A line at about the same equivalent width strength. The presence of this and other emission lines confirms the presence of an accretion disk, probably diluted by the light of an early B star companion. The spectra differ strongly from high mass X-ray binaries dominated by O star winds (e.g., Cyg X-1) and are most similar to intermediate mass X-ray binary systems in the Milky Way and LMC. This indicates that the mass transfer is due to Roche Lobe overflow. The spectrum of the ULX in M81 is quite weak but suggestive of a late-type Wolf-Rayet star.

astro-ph.HE↗

arXiv2011

Dynamical mass constraints on the ultraluminous X-ray Source NGC 1313 X-2

Dynamical mass measurements hold the key to answering whether ultraluminous X-ray sources (ULXs) are intermediate mass black holes (IMBHs) or stellar mass black holes with special radiation mechanisms. NGC 1313 X-2 is so far the only ULX with HST light curves, the orbital period, and the black hole's radial velocity amplitude based on the He II $\lambda4686$Å disk emission line shift of $\sim200$ km/s. We constrain its black hole mass and other parameters by fitting observations to a binary light curve code with accommodations for X-ray heating of the accretion disk and the secondary. Given the dynamical constraints from the observed light curves and the black hole radial motion and the observed stellar environment age, the only acceptable models are those with 40-50 Myrs old intermediate mass secondaries in their helium core and hydrogen shell burning phase filling 40%-80% of their Roche lobes. The black hole can be a massive black hole of a few tens of $M_\odot$ that can be produced from stellar evolution of low metalicity stars, or an IMBH of a few hundred to above 1000$M_\odot$ if its true radial velocity $2K^\prime<40$ km/s. Further observations are required to better measure the black hole radial motion and the light curves in order to determine whether NGC 1313 X-2 is a stellar black hole or an IMBH.

astro-ph.HE↗

arXiv2011

On the Nature of the Progenitor of the Type Ia SN2011fe in M101

The explosion of a Type Ia supernova, SN 2011ef, in the nearby Pinwheel galaxy (M101 at 6.4 Mpc) provides an opportunity to study pre-explosion images and search for the progenitor, which should consist of a white dwarf (WD), possibly surrounded by an accretion disk, in orbit with another star. We report on our use of deep Chandra observations to limit the luminosity and temperature of the pre-explosion white dwarf (WD). It is found that if the spectrum was a blackbody, then WDs of highest possible temperatures and luminosities are excluded but, even if the WD was emitting at the Eddington luminosity, values of kT less than roughly 60 eV are permitted. This allows the progenitor to be an accreting nuclear-burning WD with an expanded photosphere. Pre-SN HST observations were used to derive a lower limit of about 10 eV for the expanded photosphere. Li et al.\, (2011) have already ruled out the possibility of a giant donor. We consider the combined emission from the WD, disk, and donor, and find that even the combined emission from a bright subgiant, WD and disk would not likely have been observed prior to explosion, and neither would some local candidates for the nuclear-burning WD model.

astro-ph.HE↗

arXiv2010

Formation of the black-hole binary M33 X-7 via mass-exchange in a tight massive system

M33 X-7 is among the most massive X-Ray binary stellar systems known, hosting a rapidly spinning 15.65 Msun black hole orbiting an underluminous 70 Msun Main Sequence companion in a slightly eccentric 3.45 day orbit. Although post-main-sequence mass transfer explains the masses and tight orbit, it leaves unexplained the observed X-Ray luminosity, star's underluminosity, black hole's spin, and eccentricity. A common envelope phase, or rotational mixing, could explain the orbit, but the former would lead to a merger and the latter to an overluminous companion. A merger would also ensue if mass transfer to the black hole were invoked for its spin-up. Here we report that, if M33 X-7 started as a primary of 85-99 Msun and a secondary of 28-32 Msun, in a 2.8-3.1 day orbit, its observed properties can be consistently explained. In this model, the Main Sequence primary transferred part of its envelope to the secondary and lost the rest in a wind; it ended its life as a ~16 Msun He star with a Fe-Ni core which collapsed to a black hole (with or without an accompanying supernova). The release of binding energy and, possibly, collapse asymmetries "kicked" the nascent black hole into an eccentric orbit. Wind accretion explains the X-Ray luminosity, while the black hole spin can be natal.

astro-ph.SR↗

arXiv2010

The intriguing evolutionary history of the massive black hole X-ray binary M33 X-7

Black hole (BH) X-ray binaries (XRBs) are X-ray luminous binary systems comprising a BH accreting matter from a companion star. Understanding their origins sheds light on the still not well understood physics of BH formation. M33 X-7 hosts one of the most massive stellar-mass BH among all XRBs known to date, a 15.65 Msun BH orbiting a 70 Msun companion star in a 3.45 day orbit. The high masses of the two components and the tight orbit relative to the large H-rich stellar component challenge our understanding of the typically invoked BH-XRBs formation channels. The measured underluminosity of the optical component further complicates the picture. A solution to the evolutionary history of this system that can account for all its observed properties has yet to be presented, and here we propose the first scenario that is consistent with the complete set of current observational constraints. In our model, M33 X-7 started its life hosting a 85-99 Msun primary and a 28-32 Msun companion in a Keplerian orbit of 2.8-3.1 days. In order to form a BH of 15.65 Msun, the initially most massive component transferred part of its envelope to the companion star and lost the rest in a strong stellar wind. During this dynamically stable mass transfer phase the companion accreted matter, to become the presently underluminous 70 Msun star.

astro-ph.SR↗

arXiv2010

Precise Measurement of the Spin Parameter of the Stellar-Mass Black Hole M33 X-7

In prior work, {\it Chandra} and Gemini-North observations of the eclipsing X-ray binary M33 X-7 have yielded measurements of the mass of its black hole primary and the system's orbital inclination angle of unprecedented accuracy. Likewise, the distance to the binary is known to a few percent. In an analysis based on these precise results, fifteen {\it Chandra} and {\it XMM-Newton} X-ray spectra, and our fully relativistic accretion disk model, we find that the dimensionless spin parameter of the black hole primary is $a_* = 0.77 \pm 0.05$. The quoted 1-$σ$ error includes all sources of observational uncertainty. Four {\it Chandra} spectra of the highest quality, which were obtained over a span of several years, all lead to the same estimate of spin to within statistical errors (2%), and this estimate is confirmed by 11 spectra of lower quality. There are two remaining uncertainties: (1) the validity of the relativistic model used to analyze the observations, which is being addressed in ongoing theoretical work; and (2) our assumption that the black hole spin is approximately aligned with the angular momentum vector of the binary, which can be addressed by a future X-ray polarimetry mission.

astro-ph↗

arXiv2009

Populations of Supersoft X-ray Sources: Novae, tidal disruption, Type Ia supernovae, accretion-induced collapse, ionization, and intermediate-mass black holes?

Observations of hundreds of supersoft x-ray sources (SSSs) in external galaxies have shed light on the diversity of the class and on the natures of the sources. SSSs are linked to the physics of Type Ia supernovae and accretion-induced collapse, ultraluminous x-ray sources and black holes, the ionization of the interstellar medium, and tidal disruption by supermassive black holes. The class of SSSs has an extension to higher luminosities: ultraluminous SSSs have luminosities above 10^39 erg/s. There is also an extension to higher energies: quasisoft x-ray sources (QSSs) emit photons with energies above 1 eV, but few or none with energies above 2 keV. Finally, a significant fraction of the SSSs found in external galaxies switch states between observations, becoming either quasisoft or hard. For many systems ``supersoft'' refers to a temporary state; SSSs are sources, possibly including a variety of fundamentally different system types, that pass through such a state. We review those results derived from extragalactic data and related theoretical work that are most surprising and that suggest directions for future research.

astro-ph.CO↗

arXiv2009

A Determination of the Spin of the Black Hole Primary in LMC X-1

The first extragalactic X-ray binary, LMC X-1, was discovered in 1969. In the 1980s, its compact primary was established as the fourth dynamical black-hole candidate. Recently, we published accurate values for the mass of the black hole and the orbital inclination angle of the binary system. Building on these results, we have analyzed 53 X-ray spectra obtained by RXTE and, using a selected sample of 18 of these spectra, we have determined the dimensionless spin parameter of the black hole to be a* = 0.92(-0.07,+0.05). This result takes into account all sources of observational and model-parameter uncertainties. The standard deviation around the mean value of a* for these 18 X-ray spectra, which were obtained over a span of several years, is only 0.02. When we consider our complete sample of 53 RXTE spectra, we find a somewhat higher value of the spin parameter and a larger standard deviation. Finally, we show that our results based on RXTE data are confirmed by our analyses of selected X-ray spectra obtained by the XMM-Newton, BeppoSAX and Ginga missions.

astro-ph.HE↗

arXiv2009

The Dynamic X-ray Sky of the Local Universe

Over the next decade, we can expect time domain astronomy to flourish at optical and radio wavelengths. In parallel with these efforts, a dedicated transient "machine" operating at higher energies (X-ray band through soft gamma-rays) is required to reveal the unique subset of events with variable emission predominantly visible above 100 eV. Here we focus on the transient phase space never yet sampled due to the lack of a sensitive, wide-field and triggering facility dedicated exclusively to catching high energy transients and enabling rapid coordinated multi-wavelength follow-up. We first describe the advancements in our understanding of known X-ray transients that can only be enabled through such a facility and then focus on the classes of transients theoretically predicted to be out of reach of current detection capabilities. Finally there is the exciting opportunity of revealing new classes of X-ray transients and unveiling their nature through coordinated follow-up observations at longer wavelengths.

astro-ph.HE↗

arXiv2008

Chandra Acis Survey of X-Ray Point Sources in 383 Nearby Galaxies I. the Source Catalog

The Chandra data archive after eight years' accumulation is a treasure for various studies, and in this paper we exploit this valuable resource to study the X-ray point source populations in nearby galaxies. By December 14, 2007, 383 galaxies within 40 Mpc with isophotal major axis above 1 arcminute have been observed by 626 public ACIS observations, 60% of which are for the first time analyzed by this survey to study the X-ray point sources. Uniform data analysis procedures are applied to the 626 ACIS observations and lead to the detection of 28099 point sources, which belong to 17599 independent sources. These include 8700 sources observed twice or more and 1000 sources observed 10 times or more, providing us a wealth of data to study the long term variability of these X-ray sources. Cross correlation of these sources with galaxy isophotes led to 8519 sources within the D25 isophotes of 351 galaxies, 3305 sources between the D25 and 2D25 isophotes of 309 galaxies, and additionally 5735 sources outside 2D25 isophotes of galaxies. This survey has produced a uniform catalog, by far the largest, of 11824 X-ray point sources within 2D25 isophotes of 380 galaxies with 71% of them reported for the first time. With a uniform dataset and good statistics, this survey enables future works on various topics, such as X-ray luminosity functions, and multiwavelength identification and classification.

astro-ph↗

arXiv2008

No periodicity revealed for an "eclipsing" ultraluminous supersoft X-ray source in M81

Luminous supersoft X-ray sources found in the Milky Way and Magellanic Clouds are likely white dwarfs that steadily or cyclically burn accreted matter on their surface, which are promising type Ia supernova progenitors. Observations of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources that are generally hotter and more luminous, including some ultraluminous supersoft sources (ULSs) that are possibly intermediate mass black holes of a few thousand solar masses. In this paper we report our X-ray spectral and timing analysis for M81-ULS1, an ultraluminous supersoft source in the nearby spiral galaxy M81. M81-ULS1 has been persistently supersoft in 17 Chandra ACIS observations spanning six years, and its spectrum can be described by either a $kT_{bb}\approx70$ eV blackbody for a $\sim1.2M_\odot$ white dwarf, or a $kT_{in} \approx 80$ eV multicolor accretion disk for a $\gtrsim10^3M_\odot$ intermediate mass black hole. In two observations, the light curves exhibited dramatic flux drop/rise on time scales of $10^3$ seconds, reminiscent of eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive search for periodicity in the reasonable range of 50 ksec to 50 days failed to reveal an orbital period. The failure to reveal any periodicity is consistent with the long period ($\ge30$ yrs) predicted for this system given the optical identification of the secondary with an asymptotic giant star. Also, the eclipse-like dramatic flux changes in hours are hard to explain under the white dwarf model, but can in principle be explained by disk temperature changes induced by accretion rate variations under the intermediate mass black hole model.

astro-ph↗

arXiv2008

An Ultraluminous Supersoft X-ray Source in M81: An Intermediate-Mass Black Hole?

Ultraluminous supersoft X-ray sources (ULSSS) exhibit supersoft spectra with blackbody temperatures of 50-100 eV and bolometric luminosities above $10^{39}$ erg s$^{-1}$, and are possibly intermediate mass black holes (IMBHs) of $\ge10^3 M_\odot$ or massive white dwarfs that are progenitors of type Ia supernovae. In this letter we report our optical studies of such a source in M81, M81-ULS1, with HST archive observations. M81-ULS1 is identified with a point-like object, the spectral energy distribution of which reveals a blue component in addition to the companion of an AGB star. The blue component is consistent with the power-law as expected from the geometrically-thin accretion disk around an IMBH accretor, but inconsistent with the power-law as expected from the X-ray irradiated flared accretion disk around a white dwarf accretor. This result is strong evidence that M81-ULS1 is an IMBH instead of a white dwarf.

astro-ph↗

arXiv2007

Optical studies of the ultraluminous X-ray source NGC1313 X-2

NGC1313 X-2 was among the first ultraluminous X-ray sources discovered, and has been a frequent target of X-ray and optical observations. Using the HST/ACS multi-band observations, this source is identified with a unique counterpart within an error circle of $0\farcs2$. The counterpart is a blue star on the edge of a young cluster of $\le10^7$ years amid a dominant old stellar population. Its spectral energy distribution is consistent with that for a Z=0.004 star with 8.5 $M_\odot$ about $5\times10^6$ years old, or for an O7 V star at solar metallicity. The counterpart exhibited significant variability of $Δm = 0.153\pm0.033$ mag between two F555W observations separated by three months, reminiscent of the ellipsoidal variability due to the orbital motion of this ULX binary.

astro-ph↗