arXiv subjects

Wenda Zhang

Publications and source records attributed to Wenda Zhang.

At least 37 records · Page 2Linked to original sources

arXiv2024

Detection of an Orphan X-ray Flare from a Blazar Candidate EP240709a with Einstein Probe

Blazars are often observed to flare across multiple wavelengths. Orphan flares from blazars have been only detected a few times, providing an opportunity to understand the structure of the jet in the accreting system. We report a remarkable orphan X-ray flare from a blazar candidate EP240709a, detected by Einstein Probe (EP) in July 2024. The multi-band spectral properties and variability support EP240709a as a high-energy peaked BL Lacertae-type object. The flux in 0.5-10 keV increases by at least 28 times to the value of low state in 2020, with non-detection of remarkable flaring in other bands during the same period. EP240709a exhibits the harder-when-brighter tendency in the X-ray band during the orphan flare, while its infrared-optical spectra are featureless. We employ one-zone and two-zone leptonic synchrotron self-Compton models to perform the spectral energy distribution fitting. Detecting this rare orphan flare shows the potential of EP in discovering peculiar activities from AGN in high-cadence X-ray sky surveys.

astro-ph.HE↗

arXiv2024

Investigating the consistency of the shape and flux of X-ray reflection spectra in the hard state with an accretion disk reaching close to the black hole

The observed spectra from black hole (BH) X-ray binaries (XRBs) typically consist of two primary components: multitemperature blackbody (BB) originating from the accretion disk in soft X-ray, and a power-law like component in hard X-ray due to Comptonization of soft photons by the hot corona. Illumination of the disk by the corona gives rise to another key component known as reflection. A fraction of the incident hard X-ray radiation is naturally absorbed and re-emitted as a BB at lower energies, referred to as reprocessed BB. For densities relevant to XRBs and typical ionization values, the reprocessed BB may become significant in the soft X-ray region and should be noticeable in the observed spectra as a consequence of reflection. The absence of any BB component in the low/hard state of BH XRB may not be consistent with reflection of high irradiating flux observed as power-law from appropriately dense disk of XRB. We focus on the low/hard state of the BH XRB MAXI J1820+070. We simultaneously fit the shape and flux of the reflection spectra, allowing us to estimate the correct density and ionization of the slab and, correspondingly, the reprocessed BB. Our fitting suggests that the disk in principle may extend close to the BH and still the reprocessed BB due to disk illumination remains small enough to be consistent with the data as opposed to earlier study. The inner reflection component is highly ionized and its fit is primarily driven by its contribution to the continuum. The reprocessed BB cannot resolve whether the disk is extended close to the BH or not in the hard state. For this specific observation, the flux in inner reflection component turns out to be quite low with respect to outer reflection or power-law. Outflowing slab corona covering the inner region of the disk could be the possible geometry of the source with the underlying disk reaching close to the BH. (shortened)

astro-ph.HE↗

arXiv2024

Einstein Probe discovery of EP240408a: a peculiar X-ray transient with an intermediate timescale

We report the discovery of a peculiar X-ray transient, EP240408a, by Einstein Probe (EP) and follow-up studies made with EP, Swift, NICER, GROND, ATCA and other ground-based multi-wavelength telescopes. The new transient was first detected with Wide-field X-ray Telescope (WXT) on board EP on April 8th, 2024, manifested in an intense yet brief X-ray flare lasting for 12 seconds. The flare reached a peak flux of 3.9x10^(-9) erg/cm2/s in 0.5-4 keV, about 300 times brighter than the underlying X-ray emission detected throughout the observation. Rapid and more precise follow-up observations by EP/FXT, Swift and NICER confirmed the finding of this new transient. Its X-ray spectrum is non-thermal in 0.5-10 keV, with a power-law photon index varying within 1.8-2.5. The X-ray light curve shows a plateau lasting for about 4 days, followed by a steep decay till becoming undetectable about 10 days after the initial detection. Based on its temporal property and constraints from previous EP observations, an unusual timescale in the range of 7-23 days is found for EP240408a, which is intermediate between the commonly found fast and long-term transients. No counterparts have been found in optical and near-infrared, with the earliest observation at 17 hours after the initial X-ray detection, suggestive of intrinsically weak emission in these bands. We demonstrate that the remarkable properties of EP240408a are inconsistent with any of the transient types known so far, by comparison with, in particular, jetted tidal disruption events, gamma-ray bursts, X-ray binaries and fast blue optical transients. The nature of EP240408a thus remains an enigma. We suggest that EP240408a may represent a new type of transients with intermediate timescales of the order of about 10 days. The detection and follow-ups of more of such objects are essential for revealing their origin.

astro-ph.HE↗

arXiv2024

Constraining the geometry of the dipping atoll 4U 1624-49 with X-ray spectroscopy and polarimetry

We present the spectro-polarimetric results obtained from simultaneous X-ray observations with IXPE, NuSTAR and NICER of the dipping neutron star X-ray binary 4U 1624-49. This source is the most polarized Atoll source so far observed with IXPE, with a polarization degree of 2.7% $\pm$ 0.9% in the 2-8 keV band during the non-dip phase and marginal evidence of an increasing trend with energy. The higher polarization degree compared to other Atolls can be explained by the high inclination of the system ($i \approx 60$°). The spectra are well described by the combination of a soft thermal emission, a Comptonized component, plus reflection of soft photons off the accretion disk. During the dips, the hydrogen column density of the highly-ionized absorber increases while the ionization state decreases. The Comptonized radiation seems to be the dominant contribution to the polarized signal, with additional reflected photons which significantly contribute even if their fraction in the total flux is not high.

astro-ph.HE↗

arXiv2024

X-ray Sources Classification Using Machine Learning: A Study with EP-WXT Pathfinder LEIA

X-ray observations play a crucial role in time-domain astronomy. The Einstein Probe (EP), a recently launched X-ray astronomical satellite, emerges as a forefront player in the field of time-domain astronomy and high-energy astrophysics. With a focus on systematic surveys in the soft X-ray band, EP aims to discover high-energy transients and monitor variable sources in the universe. To achieve these objectives, a quick and reliable classification of observed sources is essential. In this study, we developed a machine learning classifier for autonomous source classification using data from the EP-WXT Pathfinder Lobster Eye Imager for Astronomy (LEIA) and EP-WXT simulations. The proposed Random Forest classifier, built on selected features derived from light curves, energy spectra, and location information, achieves an accuracy of approximately 95% on EP simulation data and 98% on LEIA observational data. The classifier is integrated into the LEIA data processing pipeline, serving as a tool for manual validation and rapid classification during observations. This paper presents an efficient method for the classification of X-ray sources based on single observations, along with implications of most effective features for the task. This work facilitates rapid source classification for the EP mission and also provides valuable insights into feature selection and classification techniques for enhancing the efficiency and accuracy of X-ray source classification that can be adapted to other X-ray telescope data.

astro-ph.IM↗

arXiv2024

Inferring the iron K emissivity profiles of accretion discs irradiated by extended coronae

One of the most promising methods to measure the spin of an accreting black hole is fitting the broad iron K$α$ line in the X-ray spectrum. The line profile also depends on the geometry of the hard X-ray emitting corona. To put constraints on the black hole spin and corona geometry, it is essential to understand how do they affect the iron K$α$ line emissivity profile. In this work, we present calculations of the illumination and the iron K$α$ emissivity profiles performed with the Monte-Carlo GR radiative transfer code Monk. We focus on distinction between the illumination and emissivity profiles, which is in most previous studies neglected. We show that especially for the case of black hole X-ray binaries (BHXRBs), the difference is very large. For active galactic nuclei (AGNs), the emissivity profile has a more similar shape as the illumination profile, but it is notably steeper in the innermost region within a few gravitational radii. We find out that the different behavior between AGN and black hole X-ray binary discs is due to the different energy spectra of the illuminating radiation. This suggests that the emissivity profile of the iron K$α$ line cannot be determined by black hole spin and corona geometry alone and the energy spectrum of the illuminating radiation has to be taken into account. We also examined the effect of including the self-irradiation, and find it to be more important than the corona emission in BHXRBs.

astro-ph.HE↗

arXiv2024

An IXPE-Led X-ray Spectro-Polarimetric Campaign on the Soft State of Cygnus X-1: X-ray Polarimetric Evidence for Strong Gravitational Lensing

We present the first X-ray spectropolarimetric results for Cygnus X-1 in its soft state from a campaign of five IXPE observations conducted during 2023 May-June. Companion multiwavelength data during the campaign are likewise shown. The 2-8 keV X-rays exhibit a net polarization degree PD=1.99%+/-0.13% (68% confidence). The polarization signal is found to increase with energy across IXPE's 2-8 keV bandpass. The polarized X-rays exhibit an energy-independent polarization angle of PA=-25.7+/-1.8 deg. East of North (68% confidence). This is consistent with being aligned to Cyg X-1's AU-scale compact radio jet and its pc-scale radio lobes. In comparison to earlier hard-state observations, the soft state exhibits a factor of 2 lower polarization degree, but a similar trend with energy and a similar (also energy-independent) position angle. When scaling by the natural unit of the disk temperature, we find the appearance of a consistent trendline in the polarization degree between soft and hard states. Our favored polarimetric model indicates Cyg X-1's spin is likely high (a* above ~0.96). The substantial X-ray polarization in Cyg X-1's soft state is most readily explained as resulting from a large portion of X-rays emitted from the disk returning and reflecting off the disk surface, generating a high polarization degree and a polarization direction parallel to the black hole spin axis and radio jet. In IXPE's bandpass, the polarization signal is dominated by the returning reflection emission. This constitutes polarimetric evidence for strong gravitational lensing of X-rays close to the black hole.

astro-ph.HE↗

arXiv2024

Tracking the X-ray Polarization of the Black Hole Transient Swift J1727.8-1613 during a State Transition

We report on an observational campaign on the bright black hole X-ray binary Swift J1727.8$-$1613 centered around five observations by the Imaging X-ray Polarimetry Explorer (IXPE). These observations track for the first time the evolution of the X-ray polarization of a black hole X-ray binary across a hard to soft state transition. The 2--8 keV polarization degree decreased from $\sim$4\% to $\sim$3\% across the five observations, but the polarization angle remained oriented in the North-South direction throughout. Based on observations with the Australia Telescope Compact Array (ATCA), we find that the intrinsic 7.25 GHz radio polarization aligns with the X-ray polarization. Assuming the radio polarization aligns with the jet direction (which can be tested in the future with higher spatial resolution images of the jet), our results imply that the X-ray corona is extended in the disk plane, rather than along the jet axis, for the entire hard intermediate state. This in turn implies that the long ($\gtrsim$10 ms) soft lags that we measure with the Neutron star Interior Composition ExploreR (NICER) are dominated by processes other than pure light-crossing delays. Moreover, we find that the evolution of the soft lag amplitude with spectral state does not follow the trend seen for other sources, implying that Swift J1727.8$-$1613 is a member of a hitherto under-sampled sub-population.

astro-ph.HE↗

arXiv2023

Rapid dimming followed by a state transition: a study of the highly variable nuclear transient AT 2019avd over 1000+ days

The tidal disruption of a star around a supermassive black hole (SMBH) offers a unique opportunity to study accretion onto a SMBH on a human-timescale. We present results from our 1000+ days NICER, Swift and Chandra monitoring campaign of AT 2019avd, a nuclear transient with TDE-like properties. Our primary finding is that approximately 225 days following the peak of X-ray emission, there is a rapid drop in luminosity exceeding two orders of magnitude. This X-ray drop-off is accompanied by X-ray spectral hardening, followed by a 740-day plateau phase. During this phase, the spectral index decreases from 6.2+-1.1 to 2.3+-0.4, while the disk temperature remains constant. Additionally, we detect pronounced X-ray variability, with an average fractional root mean squared amplitude of 47%, manifesting over timescales of a few dozen minutes. We propose that this phenomenon may be attributed to intervening clumpy outflows. The overall properties of AT 2019avd suggest that the accretion disk evolves from a super-Eddington to a sub-Eddington luminosity state, possibly associated with a compact jet. This evolution follows a pattern in the hardness-intensity diagram similar to that observed in stellar-mass black holes, supporting the mass invariance of accretion-ejection processes around black holes.

astro-ph.HE↗

arXiv2023

Constraining the X-ray radiation origin of 3C 273 in the low state by polarization

3C 273 is one of the nearest high-luminosity quasars. Although classified as a blazar, 3C 273 also has some features in Seyferts, whose X-ray may originate from the corona. Since both jet and corona produce power-law spectra in X-ray, the spectrum cannot completely distinguish their contributions to 3C 273 in the low state. X-ray polarimetric observations provide the chance to constrain the X-ray radiation origin of 3C 273 in the low state. We perform general relativistic radiative transfer simulations with the code MONK to compute the X-ray polarization in 2-10 keV from the jets, sphere coronae, and slab coronae for 3C 273. We find that the radiation from the jet in 2-10 keV has a larger polarization degree than that of the corona: the polarization degree in 2-10 keV from the corona is unpolarized, while these are 4.1%-15.8% for the jet with a vertical or radial magnetic field and $\leq$5.0% for the jet with toroidal magnetic field. The X-ray polarization of the corona and jet is sensitive to optical depth and geometry, and the main driver for this dependence is the number of scatterings. These results show that X-ray polarization can effectively constrain the X-ray radiation origin of 3C 273 in the low state.

astro-ph.HE↗

arXiv2023

Discovery of X-ray Polarization from the Black Hole Transient Swift J1727.8-1613

We report the first detection of the X-ray polarization of the bright transient Swift J1727.8-1613 with the Imaging X-ray Polarimetry Explorer. The observation was performed at the beginning of the 2023 discovery outburst, when the source resided in the bright hard state. We find a time- and energy-averaged polarization degree of 4.1%+/-0.2% and a polarization angle of 2.2+/-1.3 degrees (errors at 68% confidence level; this translates to about 20-sigma significance of the polarization detection). This finding suggests that the hot corona emitting the bulk of the detected X-rays is elongated, rather than spherical. The X-ray polarization angle is consistent with that found in sub-mm wavelengths. Since the sub-mm polarization was found to be aligned with the jet direction in other X-ray binaries, this indicates that the corona is elongated orthogonal to the jet.

astro-ph.HE↗

arXiv2023

An apparent positive relation between spin and orbital angular momentum in X-ray binaries

The origin of current angular momentum (AM) of the black hole (BH) in X-ray binary (XRB) is still unclear, which is related with the birth and/or the growth of the BH. Here we collect the spin parameters $a_{*}$ measured in BH XRBs and find an apparent bimodal distribution centered at $\sim$ 0.17 and 0.83. We find a positive relation between the spin parameter and the orbital period/orbital separation through combining distinct XRB categories, including neutron star (NS) low-mass X-ray binaries (LMXBs), Roche-lobe overflow (RLOF) BH XRBs and wind-fed BH XRBs. It seems that the AM of the compact star and the binary orbit correlates by combining the different XRB systems. These positive relations imply that accretion process is a common mechanism for spinning up the compact star in these diverse XRB systems. We infer that the low and high spin BH XRBs may experience different evolution and accretion history, which corresponds to the bimodal distribution of the BH spin parameters. The low spin BHs ($a_{*}<0.3$) are similar to the NS LMXBs, the compact star of which is spun-up by the low-level accretion, and the high spin BHs ($a_{*}>0.5$) had experienced a short hypercritical accretion ($\gg \dot{M}_\mathrm{Edd}$) period, during which, the BH spin dramatically increased.

astro-ph.HE↗

arXiv2023

Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1

A black hole x-ray binary (XRB) system forms when gas is stripped from a normal star and accretes onto a black hole, which heats the gas sufficiently to emit x-rays. We report a polarimetric observation of the XRB Cygnus X-1 using the Imaging x-ray Polarimetry Explorer. The electric field position angle aligns with the outflowing jet, indicating that the jet is launched from the inner x-ray emitting region. The polarization degree is (4.01+-0.20)% at 2 to 8 kiloelectronvolts, implying that the accretion disk is viewed closer to edge-on than the binary orbit. The observations reveal that hot x-ray emitting plasma is spatially extended in a plane perpendicular to the jet axis, not parallel to the jet.

astro-ph.HE↗

arXiv2023

X-ray Polarimetry as a Tool to Measure the Black Hole Spin in Microquasars: Simulations of IXPE Capabilities

Measurements of the angular momentum (spin) of astrophysical black holes are extremely important, as they provide information on the black hole formation and evolution. We present simulated observations of a X-ray binary system with the Imaging X-ray Polarimetry Explorer (IXPE), with the aim to study the robustness of black hole spin and geometry measurements using X-ray polarimetry. As a representative example, we used the parameters of GRS 1915+105 in its former unobscured, soft state. In order to simulate the polarization properties, we modeled the source emission with a multicolor blackbody accounting for thermal radiation from the accretion disk, including returning radiation. Our analysis shows that the polarimetric observations in the X-ray waveband will be able to estimate both spin and inclination of the system with a good precision (without returning radiation we obtained for the lowest spin $Δa \leq 0.4$ (0.4/0.998 $\sim$ 40%) for spin and $Δi \leq 30^\circ$ (30$^\circ$/70$^\circ$ $\sim$ 43%) for inclination, while for the higher spin values we obtained $Δa \leq 0.12$ ($\sim$ 12%) for spin and $Δi \leq 20^\circ$ ($ \sim $ 29%) for inclination, within 1$ σ$ errors). When focusing on the case of returning radiation and treating inclination as a known parameter, we were able to successfully reconstruct spin and disk albedo in $Δa \leq 0.15$ ($\sim$ 15%) interval and $Δ$ albedo $\leq 0.45$ (45%) intervals within 1$ σ$ errors. We conclude that X-ray polarimetry will be a useful tool to constrain black hole spins, in addition to timing and spectral-fitting methods.

astro-ph.HE↗

arXiv2022

Investigating the X-ray polarization of lamp-post coronae in BHXRB

High-sensitivity X-ray polarimetric observations of black hole X-ray binaries, which will soon become available with the launches of space-borne X-ray observatories with sensitive X-ray polarimeters, will be able to put independent constraints on the black hole as well as the accretion flow, and possibly break degeneracies that cannot be resolved by spectral/timing observations alone. In this work we perform a series of general relativistic Monte-Carlo radiative transfer simulations to study the expected polarization properties of X-ray radiation emerging from lamp-post coronae in black hole X-ray binaries. We find that the polarization degree of the coronal emission of black hole X-ray binaries is sensitive to the spin of the black hole, the height of the corona, and the dynamics of the corona.

astro-ph.HE↗

arXiv2022

Polarization properties of weakly magnetized Neutron Stars in Low Mass X-ray Binaries

X-ray polarimetry missions like IXPE will be able to measure for the first time the polarization properties of accreting, weakly magnetized neutron stars in Low Mass X-ray Binaries. In this work we present simulations of the expected X-ray polarized signal including the coronal emission for different geometries of the corona itself, i.e. a slab above the accretion disc and a spherical shell around the neutron star. The simulations are performed with the fully relativistic Monte Carlo code monk capable of computing the X-ray polarization degree and angle for various physical input parameters of the neutron star, disc and corona. Different coronal geometries result in significantly different X-ray polarization properties, which can therefore be used to constrain the geometry of the systems.

astro-ph.HE↗

arXiv2022

Sub-additive unstable topological pressure on saturated subsets

In this paper, we continue our investigation on sub-additive pressures for $C^1$-smooth partially hyperbolic diffeomorphisms. Under the assumption of unstable almost product property, we show that the unstable Bowen topological pressure on the saturated set of a given non-empty compact connected set of invariant measures equals the infimum of the summation of unstable metric entropy and the corresponding \emph{Lyapunov exponent}, where the infimum is taken over all invariant measures inside the compact connected set above. Moreover, we also show that the unstable topological capacity pressure on the saturated sets above coincide with the unstable topological pressure of the whole system.

math.DS↗

arXiv2021

XMM-Newton detection of soft time lags in the TDE candidate AT 2018fyk

In this letter we report a tentative detection of soft time lags (i.e. variability of softer photons lags behind the variability of harder photons) in one $XMM-Newton$ observation of the tidal disruption event (TDE) candidate AT 2018fyk while the source was in the hard spectral state. The lags are detected at $6.51\times10^{-5}~\rm Hz$. The amplitude of the lags with respect to 0.5$-$1 keV monotonically decreases with the photon energy, from $\sim 1200~\rm s$ at 0.3$-$0.5 keV to $\sim -4200~\rm s$ at 3$-$5 keV (in our convention a positive lag means lagging behind the reference band). We find that the amplitude is proportional to the logarithm of the energy separation between the examined band and the reference band. The energy-dependent covariance spectrum indicates that the correlated variability is more likely to be associated with the non-thermal radiation. The soft lags are difficult to reconcile with the reverberation scenario that are used to explain the soft lags in active galactic nuclei. On the other hand, the observed soft lags are consistent with the picture that the soft X-rays are down-scattered hard X-rays by the outflow as predicted by ``unification'' models of TDEs.

astro-ph.HE↗