keV Flux Research Articles

Intervening nuclear obscuration changing the X-ray look of the z≈6 quasi-stellar object CFHQS J164121+375520

X-ray observations of the optically selected z=6.025 quasi-stellar object (QSO) CFHQS J164121+375520 (hereafter J1641) revealed that its flux dropped by a factor of ≳7 between 2018, when it was a bright and soft X-ray source, and 2021. Such a strong variability amplitude has not been observed before among z>6 QSOs, and the underlying physical mechanism was unclear. We carried out a new X-ray and rest-frame UV monitoring campaign of J1641 over 2022--2024. We detected J1641 with in the 2--7 keV band, while no significant emission is detected at softer X-ray energies, making J1641 an X-ray changing-look QSO at z>6. Compared with the 2018 epoch, the 0.5--2 keV flux dropped by a factor of $>20$. We ascribe this behavior to intervening, and still ongoing, obscuration by Compton-thick gas intercepting our line of sight between 2018 and 2021. The screening material could be an inner disk or a failed nuclear wind whose thickness increased. Another possibility is that we have witnessed an occultation event due to dust-free clouds located at parsec or subparsec scales, similar to those recently invoked to explain the remarkable X-ray weakness of active galactic nuclei discovered by JWST. These interpretations are also consistent with the lack of strong variations in the QSO rest-frame UV light curve over the same period. Future monitoring of J1641 and the possible discovery of other X-ray changing look QSOs at z>6 will return precious information about the physics of rapid supermassive black hole growth at high redshifts.

NuSTAR Observations of a Varying-flux Quasar in the Epoch of Reionization

Abstract With enough X-ray flux to be detected in a 160 s scan by SRG/eROSITA, the z = 6.19 quasar CFHQS J142952+544717 is, by far, the most luminous X-ray source known at z > 6. We present deep (245 ks) NuSTAR observations of this source; with ∼180 net counts in the combined observations, CFHQS J142952+544717 is the most distant object ever observed by the observatory. Fortuitously, this source was independently observed by Chandra ∼110 days earlier, enabling the identification of two nearby (30″ and 45″ away), fainter X-ray sources. We jointly fit both Chandra and NuSTAR observations—self-consistently including interloper sources—and find that, to greater than 90% confidence, the observed 3–7 keV flux varied by a factor of ∼2.6 during that period, corresponding to approximately two weeks in the quasar rest frame. This brightening is one of the most extreme instances of statistically significant X-ray variability seen in the Epoch of Reionization. We discuss possible scenarios that could produce such rapid change, including X-ray emission from jets too faint at radio frequencies to be observed.

Rising Near-ultraviolet Spectra in Stellar Megaflares

Flares from M dwarf stars can attain energies up to 104 times larger than solar flares but are generally thought to result from similar processes of magnetic energy release and particle acceleration. Larger heating rates in the low atmosphere are needed to reproduce the shape and strength of the observed continua in stellar flares, which are often simplified to a blackbody model from the optical to the far-ultraviolet (FUV). The near-ultraviolet (NUV) has been woefully undersampled in spectral observations despite this being where the blackbody radiation should peak. We present Hubble Space Telescope NUV spectra in the impulsive phase of a flare with E TESS ≈ 7.5 × 1033 erg and a flare with E TESS ≈ 1035 erg and the largest NUV flare luminosity observed to date from an M star. The composite NUV spectra are not well represented by a single blackbody that is commonly assumed in the literature. Rather, continuum flux rises toward shorter wavelengths into the FUV, and we calculate that an optical T = 104 K blackbody underestimates the short-wavelength NUV flux by a factor of ≈6. We show that rising NUV continuum spectra can be reproduced by collisionally heating the lower atmosphere with beams of E ≳ 10 MeV protons or E ≳ 500 keV electrons and flux densities of 1013 erg cm−2 s−1. These are much larger than the canonical values describing accelerated particles in solar flares.

Cocoon shock, X-ray cavities, and extended inverse Compton emission in Hercules A: Clues from Chandra observations

We present a detailed analysis of jet activity in the radio galaxy 3C 348 at the center of the galaxy cluster Hercules A. We aim to investigate the jet-driven shock fronts, the radio-faint X-ray cavities, the eastern jet, and the presence of extended inverse Compton (IC) X-ray emission from the radio lobes. We used archival Chandra observations to investigate surface brightness profiles extracted in several directions and to measure the spectral properties of the hot gas and of the nonthermal emission from the radio jet and lobes. We detect two pairs of shock fronts: one in the north-south direction at 150 kpc from the center, and another in the east-west direction at 280 kpc. These shocks have Mach numbers of $ M and $ M respectively. Together, they form a complete cocoon surrounding the large radio lobes. Based on the distance of the shocks from the center, we estimate that the corresponding jet outburst is 90 -- 150 Myr old. We confirm the presence of two radio-faint cavities within the cocoon, which are misaligned from the main lobes and each approximately 100 kpc wide and 40–60 Myr old. A backflow from the radio lobes might explain why the cavities appear to be dynamically younger than the surrounding cocoon shock front. We also detect nonthermal X-ray emission from the eastern jet and from the large radio lobes. The X-ray emission from the jet is visible at 80 kpc from the active galactic nucleus and can be accounted for by an IC model with mild Doppler boosting ($ A synchrotron model could explain the observed radio-to-X-ray spectrum only for very high Lorentz factors $ $ of the electrons in the jet. For the large radio lobes, we argue that the X-ray emission has an IC origin, with a 1 keV flux density of $21.7 (statistical) (systematic) $ nJy. A thermal model is unlikely, as it would require an unrealistically high temperature, density, and pressure for the gas in the lobes, along with strong depolarization of the radio lobes, which are instead highly polarized. The IC detection, combined with the synchrotron flux density, suggests a magnetic field of $12 mu G in the lobes.

The properties of the Galactic hard X-ray and soft γ-ray background based on 20 years of INTEGRAL/IBIS observations

We present results of a study of the Galactic hard X-ray and soft γ-ray background emission performed with the IBIS telescope aboard the INTEGRAL observatory using data obtained over more than 20 years of operations. The study of the Galactic background at energies between 10 keV and a few MeV is problematic due to the contribution of point sources, high instrumental background and large-scale extent of the emission, which leads to the need of utilizing complex model-dependent methods. Using the unique properties of the IBIS coded-mask telescope, we developed a model-independent approach to study diffuse continuum emission near the Galactic plane in the 25−60, 60−80, and 80−200 keV bands. The comparison of the 25−60 keV longitude profile with the near infrared intensity shows excellent agreement, confirming the stellar origin of the Galactic Ridge X-ray Emission (GRXE). The Galactic X-ray background is significantly detected from the direction of the Galactic bulge up to 200 keV. We built broad-band spectra of the Galactic background in three large regions, the Galactic bulge and two spiral arms at l≈±20∘. The spectral analysis reveals two distinct components with a minimum at about 80 keV. The low-energy (≲60 keV) component, associated with the GRXE, is consistent with a one-dimensional accretion flow model of intermediate polars with an average white dwarf mass of about 0.7 M⊙. The high-energy part of the spectrum, dominating above ∼60 keV and attributed to the γ-ray background, is consistent with a power-law model with photon index Γ=1.55. The total 30−80 keV flux budget of 1.5×10−9 erg s−1 cm−2 observed within the effective IBIS field of view (≈286 deg2) in the Galactic bulge region, consists of 2/3 of GRXE and 1/3 of γ-ray background. Finally, we provide the Python code of the IBIS/ISGRI background model, which can be used to measure the X-ray intensity of the Galactic background in different parts of the Milky Way.

The Remarkable X-Ray Spectra and Variability of the Ultraluminous Weak-line Quasar SDSS J1521+5202

We present a focused X-ray and multiwavelength study of the ultraluminous weak-line quasar (WLQ) SDSS J1521+5202, one of the few X-ray weak WLQs that is amenable to basic X-ray spectral and variability investigations. J1521+5202 shows striking X-ray variability during 2006–2023, by up to a factor of ≈32 in 0.5–2 keV flux, and our new 2023 Chandra observation caught it in its brightest X-ray flux state to date. Concurrent infrared/optical observations show only mild variability. The 2023 Chandra spectrum can be acceptably described by a power law with intrinsic X-ray absorption, and it reveals a nominal intrinsic level of X-ray emission relative to its optical/ultraviolet emission. In contrast, an earlier Chandra spectrum from 2013 shows apparent spectral complexity that is not well fit by a variety of models, including ionized absorption or standard Compton-reflection models. Overall, the observations are consistent with the thick-disk plus outflow model previously advanced for WLQs, where a nominal level of underlying X-ray emission plus variable absorption leads to the remarkable observed X-ray variability. In the case of J1521+5202, it appears likely that the outflow, and not the thick disk itself, lies along our line of sight and causes the X-ray absorption.

Constraining Wind-driven Accretion onto Gaia BH3 with Chandra

Gaia BH3 is the most massive known stellar-origin black hole in the Milky Way, with a mass M • ≈ 33 M ⊙. Detected from Gaia’s astrometry, this black hole is in the mass range of those observed via gravitational waves, whose nature is still highly debated. Hosted in a binary system with a companion giant star that is too far away for Roche-lobe mass transfer, this black hole could nonetheless accrete at low levels due to wind-driven mass loss from its companion star, thus accreting in advection-dominated accretion flow, or ADAF, mode. Using stellar wind models, we constrain its Eddington ratio in the range 10−9 < f Edd < 10−7, corresponding to radiative efficiencies 5 × 10−5 < ϵ < 10−3, compatible with radiatively inefficient accretion modes. Chandra ACIS-S observed this object and obtained the most sensitive upper bound of its [2–10] keV flux: F X < 3.25 × 10−15 erg s−1 cm−2 at 90% confidence level corresponding to L [2–10] < 2.10 × 1029 erg s−1. Using ADAF emission models, we constrained its accretion rate to f Edd < 4.91 × 10−7 at the apastron, in agreement with our theoretical estimate. At the periastron, we expect fluxes ∼50 times larger. Because of the inferred low rates, accretion did not significantly contribute to black hole growth over the system’s lifetime. Detecting the electromagnetic emission from Gaia BH3 will be fundamental to informing stellar wind and accretion disk models.

XMM-Newton – NuSTAR monitoring campaign of the Seyfert 1 galaxy IC 4329A

We present the results of a joint XMM-Newton and NuSTAR campaign on the active galactic nucleus (AGN) IC 4329A, consisting of 9 × 20 ks XMM-Newton observations, and 5 × 20 ks NuSTAR observations within nine days, performed in August 2021. Within each observation, the AGN is not very variable, and the fractional variability never exceeds 5%. Flux variations are observed between the different observations on timescales of days, with a ratio of 30% of the minimum and maximum 2–10 keV flux. These variations follow the softer-when-brighter behavior typically observed in AGN. In all observations, a soft excess is clearly present. Consistently with previous observations, the X-ray spectra of the source exhibit a cutoff energy between 140 and 250 keV that is constant within the error in the different observations. We detected a prominent component of the 6.4 keV Fe Kα line consistent with being constant during the monitoring, consisting of an unresolved narrow core and a broader component likely originating in the inner accredion disk. We find that the reflection component is weak (Rmax = 0.009 ± 0.002) and most likely originates in distant neutral medium. We also found a warm absorber component together with an ultrafast outflow. Their energetics show that these outflows have enough mechanical power for significant feedback on the environment of the AGN.

An enormous increase in atmospheric positron flux during a summer thunderstorm on Mount Aragats

On July 11, 2023, we observed a remarkable 500% increase in positron flux, coinciding with a significant Thunderstorm Ground Enhancement (TGE). The enhanced flux of electrons and gamma rays was attributed to relativistic runaway electron avalanches (RREAs) generated within the dipole formed between the main negatively charged layer in the middle of the thundercloud and the Lower Positively Charged Region (LPCR) at the bottom of the thundercloud. Concurrently, a substantial enhancement in the 511 keV gamma-ray flux resulting from electron-positron annihilation was recorded. This surge is intricately linked to the LPCR within the thundercloud. The emergence of the LPCR induces a polarity change in the atmospheric electric field (AEF) below the LPCR (fourth dipole), leading to the deceleration of electrons and the acceleration of positrons.Particle flux measurements were conducted using scintillation and NaI(TL) spectrometers. To mitigate the contamination of natural gamma radiation and refine the 511 keV flux measurements, the ORTEC spectrometer was shielded with a 4 cm thick lead filter. CORSIKA simulations corroborate the observed positron flux enhancement.Highlighting the synergy between high-energy physics in the atmosphere and astroparticle physics, we introduce a new scenario to elucidate the enigmatic large flux of galactic positrons measured by the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station (ISS).

The high-energy tail of energetic electron precipitation: solar wind drivers and geomagnetic responses

Compositional NOx changes caused by energetic electron precipitation (EEP) at a specific altitude and those co-dependent on vertical transport are referred to as the EEP direct and indirect effect, respectively. The direct effect of EEP at lower mesospheric and upper stratospheric altitudes is linked to the high-energy tail of EEP (≳ 300 keV). The relative importance of this direct effect on NOx, ozone, and atmospheric dynamics remains unresolved due to inadequate particle measurements and scarcity of polar mesospheric NOx observations. An accurate parameterization of the high-energy tail of EEP is, therefore, crucial. This study utilizes EEP flux data from MEPED aboard the POES/Metop satellites from 2004–2014. Data from both hemispheres (55–70° N/S) are combined in daily flux estimates. 164 peaks above the 90th percentile of the ≳ 30 keV flux are identified. These peaks are categorized into absolute E1 and E3 events representing weak and strong ≳ 300 keV responses, respectively. A subset of absolute E1 and E3 events with similar ≳ 30 keV responses is termed overlapping events. Additionally, relative E1 and E3 events are determined by the relative strength of the ≳ 300 keV response, scaled by the initial ≳ 30 keV flux. A comparison between E1 and E3 events aims to identify solar wind and geomagnetic conditions leading to high-energy EEP responses and to gain insight into the conditions that generate a high-energy tail, independent of the initial ≳ 30 keV flux level. Superposed epoch analysis of mesospheric NO density from SOFIE confirms an observable direct impact on lower mesospheric chemistry associated with the absolute E3 events. A probability assessment based on absolute events identifies specific thresholds in the solar wind-magnetosphere coupling function (epsilon) and the geomagnetic indices Kp*10 and Dst, capable of determining the occurrence or exclusion of absolute E1 and E3 events. Elevated solar wind speeds persisting in the recovery phase of a deep Dst trough appear characteristic of overlapping and relative E3 events. This study provides insight into which parameters are important for accurately modeling the high-energy tail of EEP.

A targeted search for FRB counterparts with Konus-Wind

ABSTRACT We present results of the search for hard X-ray/soft gamma-ray emission in coincidence with publicly reported (via Transient Name Server, TNS1) fast radio bursts (FRBs). The search was carried out using continuous Konus-Wind data with 2.944 s time resolution. We perform a targeted search for each individual burst from 581 FRBs, along with a stacking analysis of the bursts from eight repeating sources in our sample and a separate stacking analysis of the bursts from the non-repeating FRBs. We find no significant associations in either case. We report upper bounds on the hard X-ray (20–1500 keV) flux assuming four spectral models, which generally describe spectra of short and long gamma-ray bursts (GRBs), magnetar giant flares, and the short burst, coincident with FRB 200428 from a Galactic magnetar. Depending on the spectral model, our upper bounds are in the range of (0.1–2) × 10−6 erg cm−2. For 18 FRBs with known distances, we present upper bounds on the isotropic equivalent energy release and peak luminosity. For the nearest FRB 200120E, we derive the most stringent upper bounds of Eiso ≤ 2.0 × 1044 erg and Liso ≤ 1.2 × 1044 erg s−1. Furthermore, we report lower bounds on radio-to-gamma-ray fluence ratio Eradio/Eiso ≥ 10−11–10−9 and compare our results with previously reported searches and theoretical predictions for high-energy counterparts to FRBs.

X-Ray Properties of PSR J1811-1925 by NuSTAR

We analyzed the spectral properties and pulse profile of PSR J1811–1925, a pulsar located in the center of composite supernova remnant (SNR) G11.2–0.3, by using high timing resolution archival data from the Nuclear Spectroscopic Telescope Array Mission (NuSTAR). Analysis of archival Chandra data over different regions rules out the SNR shell as the site of the hard X-ray emission while spectral analysis indicates that the NuSTAR photons originate in the pulsar and its nebula. The pulse profile exhibits a broad single peak up to 35 keV. The jointed spectrum by combining NuSTAR and Chandra can be well fitted by a power-law model with a photon index of Γ = 1.58 ± 0.04. The integrated flux of jointed spectrum over 1–10 keV is 3.36 × 10−12 erg cm−2 s−1. The spectrum of pulsar having photon index Γ = 1.33 ± 0.06 and a 1–10 keV flux of 0.91 × 10−12 erg cm−2 s−1. We also performed the phase-resolved spectral analysis by splitting the whole pulse-on phase into five phase bins. The photon indices of the bins are all around 1.4, indicating that the photon index does not evolve with the phase.

Simultaneous millimetric and X-ray intraday variability in the radio-quiet AGN MCG+08-11-11

Most of the active galactic nuclei (AGN) are radio quiet (RQ) and, differently from radio-loud (RL) AGN, they do not show a signature of large-scale or powerful jets. The physical origin of their radio emission thus remains broadly unclear. The observation of flat and inverted radio spectra at gigahertz frequencies seems to support, however, the presence of an unresolved synchrotron self-absorbed region in the close environment of the supermassive black hole. Its size could be as small as that of the X-ray corona. Since synchrotron self-absorption decreases strongly with frequency, these sources need to be observed in the millimetric (mm) domain. We report here a 12 h simultaneous mm-X-ray observation of the RQ AGN MCG+08-11-11 by NOEMA and NuSTAR, respectively. The mm flux shows a weak but clear increase along the pointing with a fractional variability of 2.0 ± 0.1%. The 3–10 keV flux of NuSTAR also increases and shows a fractional variability of 7.0 ± 1.5%. A structure function analysis shows a local maximum in the mm light curve corresponding to 2–3% of variability on a timescale of ∼2 × 104 s (100–300 Rg light crossing time). Assuming an optically thick mm emitting medium, this translates into an upper limit of its size of ∼1300 Rg. The observation of fast variability in radio-mm and X-ray wavelengths, as well as a similar variability trend, thoroughly support the idea that the mm emission is emitted by a region close to, and potentially related to, the X-ray corona such as an outflow or weak jet.

A Reemerging Bright Soft X-Ray State of the Changing-look Active Galactic Nucleus 1ES 1927+654: A Multiwavelength View

1ES1927+654 is a nearby active galactic nucleus (AGN) that has shown an enigmatic outburst in optical/UV followed by X-rays, exhibiting strange variability patterns at timescales of months to years. Here we report the unusual X-ray, UV, and radio variability of the source in its postflare state (2022 January–2023 May). First, we detect an increase in the soft X-ray (0.3–2 keV) flux from 2022 May to 2023 May by almost a factor of 5, which we call the bright soft state. The hard X-ray 2–10 keV flux increased by a factor of 2, while the UV flux density did not show any significant changes (≤30%) in the same period. The integrated energy pumped into the soft and hard X-rays during this period of 11 months is ∼3.57 × 1050 erg and 5.9 × 1049 erg, respectively. From the energetics, it is evident that whatever is producing the soft excess (SE) is pumping out more energy than either the UV or hard X-ray source. Since the energy source presumably is ultimately the accretion of matter onto the supermassive black hole, the SE-emitting region must be receiving the majority of this energy. In addition, the source does not follow the typical disk–corona relation found in AGNs, neither in the initial flare (from 2017 to 2019) nor in the current bright soft state (2022–2023). We found that the core (<1 pc) radio emission at 5 GHz gradually increased until 2022 March, but showed a dip in 2022 August. The Güdel–Benz relation (L radio/L X-ray ∼ 10−5), however, is still within the expected range for radio-quiet AGNs, and further follow-up radio observations are currently being undertaken.

Parametrization of Energetic Ion and Electron Fluxes in the Near‐Earth Magnetotail

AbstractThe near‐Earth plasma sheet region is the main source of energetic (tens to hundreds keV) ion and electron populations transported by convection and injections into the inner magnetosphere. Energetic ions from the plasma sheet contribute to the ring current, whereas energetic electrons contribute to the radiation belt seed population for further acceleration to relativistic energies. Near‐Earth plasma sheet energetic fluxes have been traditionally used to set boundary conditions for radiation belt and ring current models. This study provides an empirical parametrization for ∼75 keV flux intensity as a function of the geomagnetic activity index auroral electrojet and the equatorial magnetic field Bz. Such parametrization includes the dynamic magnetic field configuration in the near‐Earth plasma sheet and may be merged with empirical magnetic field models. We also provide models extending this parametrization to the [20, 300] keV of electron energy range and [75, 300] keV of ion energy range. The parametrization is developed based on THEMIS and Geostationary Operational Environmental Satellite measurements, and verified by comparison with MMS measurements in the near‐Earth plasma sheet. This parametrization incorporates meso‐scale transient flux variations associated with Bz perturbations into ring current and radiation belt simulations.

XMM-Newton Observations of Two Archival X-Ray Weak Type 1 Quasars: Obscuration Induced X-Ray Weakness and Variability

We report XMM-Newton observations of two examples of an unclassified type of X-ray weak quasars from the Pu et al. survey of X-ray weak quasars in the Chandra archive, SDSS J083116.62+321329.6 at z = 1.797 and SDSS J142339.87+042041.1 at z = 1.702. They do not belong to the known populations of X-ray weak quasars that show broad absorption lines, weak ultraviolet (UV) broad emission lines, or red optical/UV continua. Instead, they display typical quasar UV spectra and spectral energy distributions. In the XMM-Newton observations, both quasars show nominal levels of X-ray emission with typical quasar X-ray spectral shapes (power-law photon indices of and ), displaying strong X-ray variability compared to the archival Chandra data (variability factors of and in terms of the 2 keV flux density). Simultaneous optical (rest-frame UV) spectra indicate no strong variability compared to the archival spectra. Long-term optical/UV and infrared light curves do not show any substantial variability either. We consider that the X-ray weakness observed in the Chandra data is due to X-ray obscuration from a small-scale dust-free absorber, likely related to accretion-disk winds. Such X-ray weak/absorbed states are probably rare in typical quasars, and thus both targets recovered to X-ray nominal-strength states in the XMM-Newton observations.

Long-term X-Ray Outburst in the TeV-detected Blazar Mrk 501 in 2021–2022: Further Clues for the Emission and Unstable Processes

This paper presents the results of a detailed X-ray timing and spectral analysis of Mrk 501, which are based mainly on the Swift data obtained during 2021 February–2022 December. The source showed strongly enhanced X-ray activity, characterized by a long-term increase in the baseline 0.3–10 keV flux level superimposed by shorter-term flares on timescales of a few weeks to about 2 months. During some time intervals, Mrk 501 was the brightest blazar in the X-ray sky and, moreover, showed an intense intraday variability, which was sometimes detected within the exposures lasting a few hundred seconds. These instances were characterized by fractional variability amplitudes of 4%–18% and were mostly observed during short-term X-ray flares. The source exhibited extreme spectral properties with dominance of the spectral curvature, frequent occurrence of hard photon indices in the 0.3–10 keV and 0.3–300 GeV bands, and the peak of a synchrotron spectral energy distribution in the hard X-ray range. These properties demonstrate the importance of relativistic magnetic reconnection, first-order Fermi mechanism within the magnetic field of different confinement efficiencies, stochastic acceleration, and hadronic processes. The X-ray and γ-ray fluxes showed a log-normal distribution, which hints at the imprint of accretion disk instabilities on the blazar jet, as well as the possible presence of hadronic cascades and random fluctuations in the particle acceleration rate. The optical–UV and γ-ray variabilities showed a weak or absent correlation with the X-ray flaring activity, which is not consistent with simple leptonic models and requires more complex particle acceleration, emission, and variability scenarios.

Swift Follow-up of Reported Radio Pulsars at Fermi 4FGL Unassociated Sources

Following the discovery of radio pulsars at the position of Fermi-LAT unassociated sources by the TRAPUM group, we conduct Swift-XRT observations of six of those 4FGL sources to determine if any pulsar-like X-ray sources are present and to confirm the reported detection of an X-ray counterpart via eROSITA at 4FGL J1803.1−6708. At two of the six targets, we detect no X-ray sources at the TRAPUM radio position, placing an upper limit on the 0.3–10.0 keV flux. At 4FGL J1803.1−6708 we find an X-ray source at the TRAPUM and eROSITA position. At 4FGL J1858.3−5424 we find a new X-ray counterpart at the TRAPUM position with signal-to-noise ratio S/N = 4.17, but also detect a distinct and separate X-ray source. At 4FGL J1823.8−3544 and 4FGL J1906.4−1757 we detect no X-ray flux at the TRAPUM positions, but we do detect separate X-ray sources elsewhere in the Fermi error ellipse. At these last two targets, our newly detected Swift sources are possible alternatives to the radio pulsar associations proposed by TRAPUM. Our findings confirm several of the discoveries reported by the TRAPUM group but suggest that further observations and investigations are necessary to confirm the low-energy counterpart of several unassociated sources.

Extragalactic fast X-ray transient candidates discovered by Chandra (2014–2022)

Context. Extragalactic fast X-ray transients (FXTs) are short flashes of X-ray photons of unknown origin that last a few minutes to hours. Aims. We extend the previous search for extragalactic FXTs (based on sources in the Chandra Source Catalog 2.0, CSC2) to further Chandra archival data between 2014 and 2022. Methods. We extracted X-ray data using a method similar to that employed by CSC2 and applied identical search criteria as in previous work. Results. We report the detection of eight FXT candidates, with peak 0.3–10 keV fluxes between 1 × 10−13 to 1 × 10−11 erg cm−2 s−1 and T90 values from 0.3 to 12.1 ks. This sample of FXTs likely has redshifts between 0.7 and 1.8. Three FXT candidates exhibit light curves with a plateau (≈1−3 ks duration) followed by a power-law decay and X-ray spectral softening, similar to what was observed for a few before-reported FXTs. In light of the new, expanded source lists (eight FXTs with known redshifts from a previous paper and this work), we have updated the event sky rates derived previously, finding 36.9−8.3+9.7 deg−2 yr−1 for the extragalactic samples for a limiting flux of ≳1 × 10−13 erg cm−2 s−1, calculated the first FXT X-ray luminosity function, and compared the volumetric density rate between FXTs and other transient classes. Conclusions. Our latest Chandra-detected extragalactic FXT candidates boost the total Chandra sample by ∼50%, and appear to have a similar diversity of possible progenitors.

Strong and Rapid X-Ray Variability of the Super-Eddington Accreting Quasar SDSS J081456.10+532533.5

We report strong and rapid X-ray variability found from the super-Eddington accreting quasar SDSS J081456.10+532533.5 at z = 0.1197. It has a black hole mass of 2.7 × 107 M ⊙ and a dimensionless accretion rate of ≈4 measured from reverberation-mapping observations. It showed weak X-ray emission in the 2021 February Chandra observation, with the 2 keV flux density being times lower compared to an archival Swift observation. The 2 keV flux density is also times weaker compared to the expectation from its optical/UV emission. In a follow-up XMM-Newton observation 32 days later, the 2 keV flux density increased by a factor of , and the spectra are best described by a power law modified with partial-covering absorption; the absorption-corrected intrinsic continuum is at a nominal flux level. Nearly simultaneous optical spectra reveal no variability, and there is only mild long-term optical/infrared variability from archival data (with a maximum variability amplitude of ≈50%). We interpret the X-ray variability with an obscuration scenario, where the intrinsic X-ray continuum does not vary but the absorber has a variable column density and covering factor along the line of sight. The absorber is likely the small-scale clumpy accretion wind that has been proposed to be responsible for similar X-ray variability in other super-Eddington accreting quasars.