X-ray Variability Research Articles

Quasi-simultaneous observations of radio and X-ray variability in three radio-quiet Seyfert galaxies

ABSTRACT Radio variability in some radio-quiet (RQ) active galactic nuclei suggests emission from regions close to the central engine, possibly the outer accretion disc corona. If the origins of the radio and the X-ray emission are physically related, their emission may be temporarily correlated, possibly with some time delays. We present the results of quasi-simultaneous radio and X-ray monitoring of three RQ Seyfert galaxies, Mrk 110, Mrk 766, and NGC 4593, carried out with the Very Large Array at 8.5 GHz over a period of about 300 d, and with the Rossi X-ray Timing Explorer at 2–10 keV over a period of about 2000 d. The radio core variability is likely detected in the highest resolution (A configuration) observations of Mrk 110 and NGC 4593, with a fractional variability amplitude of 6.3 per cent and 9.5 per cent, respectively. A cross-correlation analysis suggests an apparently strong (Pearson r = −0.89) and highly significant correlation (p = 1 × 10−6) in Mrk 110, with the radio lagging the X-ray by 56 d. However, a further analysis of the r values distribution for physically unrelated long time delays reveals that this correlation is not significant. This occurs since the Pearson correlation assumes white noise, while both the X-ray and the radio light curves follow red noise, which dramatically increases the chance, by a factor of ∼103, to get extremely high r values in uncorrelated data sets. A significantly longer radio monitoring with a higher sampling rate, preferably with a high-resolution fixed radio array, is required in order to reliably detect a delay.

Flux and spectral variability of Mrk 421 during its moderate activity state usingNuSTAR: Possible accretion disc contribution?

Context.The X-ray emission in BL Lac objects is believed to be dominated by synchrotron emission from their relativistic jets. However, when the jet emission is not strong, one could expect signatures of X-ray emission from inverse Compton scattering of accretion disc photons by hot and energetic electrons in the corona. Moreover, the observed X-ray variability can also originate in the disc, and get propagated and amplified by the jet.Aims.Here, we present results on the BL Lac object Mrk 421 using the Nuclear Spectroscopic Telescope Array data acquired during 2017 when the source was in a moderate X-ray brightness state. For comparison with high jet activity state, we also considered one epoch of data in April 2013 when the source was in a very high X-ray brightness state. Our aim is to explore the possibility of the signature of accretion disc emission in the overall X-ray emission from Mrk 421 and also examine changes in accretion parameters considering their contribution to spectral variations.Methods.We divided each epoch of data into different segments in order to find small-scale variability. Data for all segments were fitted using a simple power-law model. We also fitted the full epoch data using the two component advective flow (TCAF) model to extract the accretion flow parameters. Furthermore, we estimated the X-ray flux coming from the different components of the flow using the lowest normalisation method and analysed the relations between them. For consistency, we performed the spectral analysis using models available in the literature.Results.The simple power-law function does not fit the spectra well, and a cutoff needs to be added. The spectral fitting of the data using the TCAF model shows that the data can be explained with a model where (a) the size of the dynamic corona at the base of the jet is from ∼28 to 10 rs, (b) the disc mass accretion rate is from 0.021 to 0.051ṀEdd, (c) the halo mass accretion rate is from 0.22 to 0.35ṀEdd, and (d) the viscosity parameter of the Keplerian accretion disc from 0.18 to 0.25. In the assumed model, the total flux, disc and jet flux correlate with the radio flux observed during these epochs.Conclusions.From the spectral analysis, we conclude that the spectra of all the epochs of Mrk 421 in 2017 are well described by the accretion-disc-based TCAF model. The estimated disc and jet flux relations with radio flux show that accretion disc can contribute to the observed X-ray emission, when X-ray data (that cover a small portion of the broad band spectral energy distribution of Mrk 421) are considered in isolation. However, the present disc-based models are disfavoured with respect to the relativistic jet models when considering the X-ray data in conjunction with data at other wavelengths.

Radiative shock oscillation model for the long-term flares of Sgr A*

ABSTRACT We examine time-dependent 2D relativistic radiation magnetohydrodynamic (MHD) flows to develop the shock oscillation model for the long-term flares of Sagittarius A* (Sgr A*). Adopting modified flow parameters in addition to the previous studies, we confirm quasi-periodic flares with periods of ∼5 and 10 d that are compatible with observations by Chandra, Swift, and XMM–Newton monitoring of Sgr A*. Using a simplified two-temperature model of ions and electrons, we find that the flare due to synchrotron emission lags that of bremsstrahlung emission by 1–2 h that are qualitatively comparable to the time lags of 1–5 h reported in several simultaneous observations of radio and X-ray variability in Sgr A*. The synchrotron emission is confined in a core region of 3Rg size with the strong magnetic field, while the bremsstrahlung emission mainly originates in a distant region of 10–20Rg behind the oscillating shock, where Rg is the Schwarzschild radius. The time lag is estimated as the transit time of the acoustic wave between the above two regions. The time-averaged distribution of radiation shows a strong anisotropic nature along the rotational axis but isotropic distribution in the radial direction. A high-velocity jet with ∼0.6c along the rotational axis is intermittently found in a narrow funnel region with a collimation angle ∼15°. The shock oscillating model explains well the flaring rate and the time lag between radio and X-ray emissions for the long-term flares of Sgr A*.

Hard X-Ray Emission in Centaurus A

We use 13 yr of Swift/BAT observations to probe the nature and origin of the hard X-ray (14–195 KeV) emission in Centaurus A. Since the beginning of the Swift operation in 2004, significant X-ray variability in the 14–195 KeV band has been detected, with mild changes in the source spectrum. Spectral variations became more eminent after 2013, following a softer-when-brighter trend. Using the power spectral density (PSD) method, we find that the observed hard X-ray photon flux variations are consistent with a red-noise process of slope, −1.3, with no evidence for a break in the PSD. We find a significant correlation between the hard X-ray and 230 GHz radio flux variations, with no time delay longer than 30 days. The temporal and spectral analysis confirms that the X-ray emission generated by the accretion in the ADAF model is sub-dominant as compared with the emission arising from that produced by the inner regions of the radio jet.

First eROSITA study of nearby M dwarfs and the rotation-activity relation in combination with TESS

We present the first results with the ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission, and we combine the new X-ray data with observations with the Transiting Exoplanet Survey Satellite (TESS). We used theSUPERBLINKproper motion catalog of nearby M dwarfs as input sample to search for eROSITA and TESS data. We extractedGaiaDR2 data for the full M dwarf catalog, which comprises ~9000 stars, and we calculated the stellar parameters from empirical relations with optical/IR colors. Then we cross-matched this catalog with the eROSITA Final Equatorial Depth Survey (eFEDS) and the first eROSITA all-sky survey (eRASS1). After a meticulous source identification in which we associated the closestGaiasource with the eROSITA X-ray detections, our sample of M dwarfs is defined by 687 stars with SpT = K5..M7 (673 from eRASS1 and 14 from eFEDS). While for eRASSl we used the data from the source catalog provided by the eROSITA_DE consortium, for the much smaller eFEDS sample, we performed the data extraction, and we analyzed the X-ray spectra and light curves. This unprecedented data base for X-ray emitting M dwarfs allowed us to place a quantitative constraint on the mass dependence of the X-ray luminosity, and to determine the change in the activity level with respect to pre-main-sequence stars. TESS observations are available for 489 of 687 X-ray detected M dwarfs. By applying standard period search methods, we were able to determine the rotation period for 180 X-ray detected M dwarfs. This is about one-forth of the X-ray sample. With the joint eROSITA and TESS sample, and combining it with our compilation of historical X-ray and rotation data for M dwarfs, we examined the mass dependence of the saturated regime of the rotation-activity relation. A first comparison of eROSITA hardness ratios and spectra shows that 65% of the X-ray detected M dwarfs have coronal temperatures of ~0.5 keV. We performed a statistical investigation of the long-term X-ray variability of M dwarfs by comparing the eROSITA measurements to those obtained ~30 yr earlier during the ROSAT all-sky survey (RASS). Evidence for X-ray flares is found in various parts of our analysis: directly from an inspection of the eFEDS light curves, in the relation between RASS and eRASSl X-ray luminosities, and in a subset of stars that displays hotter X-ray emission than the bulk of the sample according to the hardness ratios. Finally, we point out the need to obtain X-ray spectroscopy for more M dwarfs to study the coronal temperature-luminosity relation, which is not well constrained by our eFEDS results.

The eROSITA Final Equatorial-Depth Survey (eFEDS)

The 140-square-degrees Final Equatorial-Depth Survey (eFEDS) field, observed with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma mission, provides a first look at the variable eROSITA sky. We analysed the intrinsic X-ray variability of the eFEDS sources and provide X-ray light curves and tables with variability test results in the 0.2–2.3 keV (soft) and 2.3–5.0 keV (hard) bands. We performed variability tests using the traditional normalised excess variance and maximum amplitude variability methods (as performed for the 2RXS catalogue), and we present results from the Bayesian excess variance and Bayesian block methods. We identified 65 sources as being significantly variable in the soft band. In the hard band, only one source is found to vary significantly. For the most variable sources, the light curves are well fit by an empirical stellar flare model and reveal extreme flare properties. A few highly variable active galactic nuclei have also been detected. About half of the variable eFEDS sources are detected in the X-rays for the first time with eROSITA. Comparison with 2RXS andXMM-Newtonobservations provides variability information on timescales of years to decades.

A Rapid and Large-amplitude X-Ray Dimming Event in a z ≈ 2.6 Radio-quiet Quasar

We report a dramatic, fast X-ray dimming event in a z = 2.627 radio-quiet type 1 quasar, which has an estimated supermassive black hole (SMBH) mass of 6.3 × 109 M ⊙. In the high X-ray state, it showed a typical level of X-ray emission relative to its UV/optical emission. Then its 0.5–2 keV (rest-frame 1.8–7.3 keV) flux dropped by a factor of ≈7.6 within two rest-frame days. The dimming is associated with spectral hardening, as the 2–7 keV (rest-frame 7.3–25.4 keV) flux dropped by only 17%, and the effective power-law photon index of the X-ray spectrum changed from ≈2.3 to ≈0.9. The quasar has an infrared (IR)-to-UV spectral energy distribution and a rest-frame UV spectrum similar to those of typical quasars, and it does not show any significant long-term variability in the IR and UV/optical bands. Such an extremely fast and large-amplitude X-ray variability event has not been reported before in luminous quasars with such massive SMBHs. The X-ray dimming is best explained by a fast-moving absorber crossing the line of sight and fully covering the X-ray emitting corona. Adopting a conservatively small size of 5 GM BH/c 2 for the X-ray corona, the transverse velocity of the absorber is estimated to be ≈0.9c. The quasar is likely accreting with a high or even super-Eddington accretion rate, and the high-velocity X-ray absorber is probably related to a powerful accretion-disk wind. Such an energetic wind may eventually evolve into a massive galactic-scale outflow, providing efficient feedback to the host galaxy.

Rapid X-ray variability in Mkn 421 during a multiwavelength campaign

ABSTRACT The study of short-term variability properties in AGN jets has the potential to shed light on their particle acceleration and emission mechanisms. We report results from a 4-d coordinated multiwavelength campaign on the highly peaked blazar (HBL) Mkn 421 in 2019 January. We obtained X-ray data from AstroSAT, BVRI photometry with the Whole Earth Blazar Telescope (WEBT), and TeV data from First G-APD Cherenkov Telescope to explore short-term multiwavelength variability in this HBL. The X-ray continuum is rapidly variable on time-scales of tens of ks. Fractional variability amplitude increases with energy across the synchrotron hump, consistent with previous studies; we interpret this observation in the context of a model with multiple cells whose emission spectra contain cutoffs that follow a power-law distribution. We also performed time-averaged and time-resolved (time-scales of 6 ks) spectral fits; a broken power-law model fits all spectra well; time-resolved spectral fitting reveals the usual hardening when brightening behaviour. Intra-X-ray cross-correlations yield evidence for the 0.6–0.8 keV band to likely lead the other bands by an average of 4.6 ± 2.6 ks, but only during the first half of the observation. The source displayed minimal night-to-night variability at all wavebands thus precluding significant interband correlations during our campaign. The broad-band SED is modelled well with a standard one-zone leptonic model, yielding jet parameters consistent with those obtained from previous SEDs of this source.

Multiband Emission Properties of 1ES 1959+650

We have systemically studied the properties of 1ES 1959+650, specifically its multiband variability and emission mechanisms. We carefully analyze multiband variability properties by four different kinds of trusted approaches. In contrast with previous research and thanks to our more detailed data, we find that an unprecedented significant quasi-periodic component with timescale in the optical R band indeed exists. We suggest that this light curve may result from accretion flow instabilities. Further, among the multiband spectra, we find no significant correlation between different bands, which implies that the physical origin of the dominant radiation for different bands is not the same. Motivated by these results, we go on further to study the radiation mechanisms based on quasi-simultaneous spectra. Here, a model that self-consistently involves protons, electrons, and photons is used to study the light curve and interpret the spectral energy distribution of 1ES 1959+650. Considering the variability properties and model calculation, we suggest that the variation of the cutoff energy of electrons and protons may dominate the variability of X-ray and γ-ray bands in 1ES 1959+650.

Discovery of late-time X-ray flare and anomalous emission line enhancement after the nuclear optical outburst in a narrow-line Seyfert 1 Galaxy

CSS J102913+404220 is an atypical narrow-line Seyfert 1 galaxy with an energetic optical outburst occurring co-spatially with its nucleus. We present a detailed analysis of multi-wavelength photometric and spectroscopic observations of this object covering a period of a decade since outburst. We detect mid-infrared (MIR) flares delayed by about two months relative to the optical outburst and with an extremely high peak luminosity of L4.6 μm > 1044 erg s−1. The MIR peak luminosity is at least an order of magnitude higher than any known supernovae explosions, suggesting the optical outburst might be due to a stellar tidal disruption event (TDE). We find late-time X-ray brightening by a factor of ≳30 with respect to what is observed about 100 days after the optical outburst peak, followed by a flux fading by a factor of ∼4 within two weeks, making it one of the active galactic nuclei (AGNs) with extreme variability. Despite the dramatic X-ray variability, there are no coincident strong flux variations in optical, UV, and MIR bands. This unusual variability behavior has been seen in other highly accreting AGNs and could be attributed to absorption variability. In this scenario, the decrease in the covering factor of the absorber with accretion rate could cause the X-ray brightening, possibly induced by the TDE. Most strikingly, while the UV/optical continuum remains almost unchanged with time, an evident enhancement in the flux of the Hα broad emission line is observed about a decade after the nuclear optical outburst, which is an anomalous behavior never seen in any other AGN. Such an Hα anomaly could be explained by the replenishment of gas clouds and excitation within the broad line region (BLR) that perhaps originates from its interaction with outflowing stellar debris. Our results highlight the importance of the late-time evolution of a TDE, which can affect the accreting properties of the AGN, as suggested by recent simulations.

X-ray variability of HD 189733 across eight years of XMM-Newton observations

The characterization of exoplanets, their formation, evolution, and chemical changes is tightly linked to our knowledge of their host stars. In particular, stellar X-rays and UV emission have a strong impact on the dynamical and chemical evolution of planetary atmospheres. We analyzed 25 XMM-Newton observations encompassing about eight years and totaling about 958 ks in order to study the X-ray emission of HD 189733 A. We find that the corona of HD 189733 A has an average temperature of 0.4 keV and it is only during flares that the mean temperature increases to 0.9 keV. Apart from the flares, there is no significant change in the flux and hardness of the coronal emission on a timescale of several months to years. Thus, we conclude that there is no detectable activity cycle on such timescales. We identified the flares and built their energy distribution. The number of flares observed around the phases of the planetary eclipses is not statistically different from the number of flares during transit phases. However, we do find a hint of a difference in the flare-energy distributions, as the flares observed around the planetary eclipses tend to be more energetic than the flares observed around the primary transits of the planet. We modeled the distribution of the number of flares per day with a power law, showing that it is steeper than the one observed in the Sun and in other Main Sequence stars. The steepness hints at a significant fraction of undetected micro-flares. Altogether, the plasma temperatures below 1 keV observed during the flares, along with the slightly larger fraction of energetic flares seen at the secondary transits highlight the peculiarity of the corona of HD 189733 A and points to star-planet interaction as the plausible origin of part of its X-ray emission. However, more observational and modeling efforts are required to confirm or disprove this scenario.

Simple Analytic Formula Relating the Mass and Spin of Accreting Compact Objects to Their Rapid X-Ray Variability

Abstract Following the previous research on epicyclic oscillations of accretion disks around black holes (BHs) and neutron stars (NSs), a new model of high-frequency quasiperiodic oscillations (QPOs) has been proposed, so-called cusp torus (CT) model, which deals with oscillations of fluid in marginally overflowing accretion tori (i.e., tori terminated by cusps). According to preliminary investigations, the model provides better fits of the NS QPO data compared to the relativistic precession (RP) model. It also implies a significantly higher upper limit on the Galactic microquasar BH spins. A short analytic formula has been noticed to well reproduce the model’s predictions on the QPO frequencies in Schwarzschild spacetimes. Here we derive an extended version of this formula that applies to rotating compact objects. We start with the consideration of Kerr spacetimes and derive a formula that is not restricted to a particular specific angular momentum distribution of the inner accretion flow, such as a Keplerian or constant one. Finally, we consider Hartle–Thorne spacetimes and include corrections implied by the NS oblateness. For a particular choice of a single parameter, our relation provides frequencies predicted by the CT model. For another value, it provides frequencies predicted by the RP model. We conclude that the formula is well applicable to rotating oblate NSs and both models. We briefly illustrate the application of our simple formula on several NS sources and confirm the expectation that the CT model is compatible with realistic values of the NS mass and provides better fits of data than the RP model.

Central Black Hole Mass in the Distant Tidal Disruption Event Candidate of Swift J2058.4+0516

Abstract In this article, the central black hole (BH) mass in the distant tidal disruption event (TDE) of Swift J2058.4+0516, the second candidate of relativistic jet birth related to a TDE, is estimated. Swift J2058.4+0516 has quite different BH masses estimated through different indirect methods in the literature. Therefore, it is necessary and interesting to determine the central BH mass in Swift J2058.4+0516 by another independent method. Here, based on the theoretical TDE model applied to describe the long-term time-dependent X-ray variabilities of Swift J2058.4+0516, the central BH mass can be well determined to be around 1.05 − 0.29 + 0.39 × 10 5 M ⊙ , after considering the suggested intrinsic beaming effects from such relativistic jets tightly related to TDEs. Moreover, Swift J2058.4+0516 is a unique object in the space of BH masses versus energy transfer efficiencies of the reported TDE candidates, providing interesting clues to detect and/or anticipate candidates of relativistic TDEs that cause the birth of relativistic jets.

X-ray variability of transitional millisecond pulsars: a faint, stable, and fluctuating disc

ABSTRACT Transitional millisecond pulsars (tMSPs) have emerged in the last decade as a unique class of neutron stars at the crossroads between accretion- and rotation-powered phenomena. In their (sub-luminous) accretion disc state, with X-ray luminosities of order 1033–1034 erg s−1, they switch rapidly between two distinct X-ray modes: the disc-high (DH) and disc-low (DL) states. We present a systematic XMM–Newton and Chandra analysis of the aperiodic X-ray variability of all three currently known tMSPs, with a main focus on their disc state and separating DH and DL modes. We report the discovery of flat-topped broad-band noise in the DH state of two of them, with break frequencies of 2.8 mHz (PSR J1023 + 0038) and 0.86 mHz (M28-I). We argue that the lowest frequency variability is similar to that seen in disc-accreting X-ray binaries in the hard state, at typical luminosities at least two orders of magnitude higher than tMSPs. We find strong variability in the DH state around 1 Hz, not typical of hard state X-ray binaries, with fractional rms amplitudes close to 30 per cent. We discuss our results and use them to constrain the properties of the accretion disc, assuming that the X-ray variability is produced by fluctuations in mass accretion rate, and that the break frequency corresponds to the viscous time-scale at the inner edge of the disc. In this context, we find that the newly found break frequencies are broadly consistent with a disc truncated close to the light cylinder with $\dot{M}\simeq 10^{13}-5\times 10^{14}$ g s−1 and a viscosity parameter α ≳ 0.2.

Populations of highly variable X-ray sources in theXMM–Newtonslew survey

ABSTRACTWe present the identifications of a flux-limited sample of highly variable X-ray sources on long time-scales from the second catalogue of the XMM–Newton SLew survey (XMMSL2). The carefully constructed sample, comprising 265 sources (2.5 per cent) selected from the XMMSL2 clean catalogue, displayed X-ray variability of a factor of more than 10 in 0.2–2 keV compared to the ROSAT All Sky Survey. Of the sample sources, 94.3 per cent are identified. The identification procedure follows a series of cross-matches with astronomical data bases and multiwavelength catalogues to refine the source position and identify counterparts to the X-ray sources. Assignment of source type utilizes a combination of indicators including counterparts offset, parallax measurement, spectral colours, X-ray luminosity, and light-curve behaviour. We identified 40 per cent of the variables with stars, 10 per cent with accreting binaries, and at least 30.4 per cent with active galactic nuclei. The rest of the variables are identified as galaxies. It is found that the mean effective temperatures of the highly variable stars are lower than those of less variable stars. Our sample of highly variable AGN tend to have lower black hole masses, redshifts, and marginally lower soft X-ray luminosities compared to the less variable ones, while no difference was found in the Eddington ratio distributions. Five flaring events are tidal disruption events published previously. This study has significantly increased the number of variable sources in XMMSL2 with identifications and provides greater insight on the nature of many o f the sources, enabling further studies of highly variable X-ray sources.

Multiwavelength campaign on the Super-Eddington NLS1 RX J0134.2-4258 – I. Peculiar X-ray spectra and variability

ABSTRACTWe have conducted a new long-term multiwavelength campaign on one of the most super-Eddington narrow-line Seyfert 1s (NLS1s) known, namely RX J0134.2-4258. In this first paper, we report deep simultaneous X-ray observations performed by XMM–Newton and NuSTAR on 2019 December 19, during which RX J0134.2-4258 was fortuitously at one of its lowest X-ray flux states. However, there is a clear rise above 4 keV which implies that the intrinsic source flux may be higher. The X-ray spectra observed between 1996 and 2019 show drastic variability, probably due to complex, variable absorption along the line of sight. Unusually, the soft X-ray excess appears extremely weak in all these spectra, even when the hard X-ray spectrum has a steep slope of Γ ≃ 2.2. We explore the spectral-timing properties of the new (low X-ray flux) and archival (high X-ray flux) XMM–Newton data, fitting their time-average, rms, and lag spectra simultaneously. The variability spectra indicate the presence of a very weak soft X-ray Comptonization component, whose shape is similar to the soft excess in normal super-Eddington NLS1s, but with flux relative to the power law which is lower by more than one order of magnitude. Above 4 keV the low-flux data are dominated by a different component, which lags with respect to the lower energy emission. This is consistent with an origin of reflection or partial covering absorption from low ionization material located within 100 Rg. We interpret this as further indication of the presence of a clumpy disc wind.

The relation between X-ray and ultraviolet variability of quasars

ABSTRACT The relation between X-ray and UV/optical variability in AGNs has been explored in many individual sources, however a large sample study is yet absent. Through matching the XMM-Newton serendipitous X-ray and UV source catalogs with SDSS quasars, we build a sample of 802 epoch-pairs of 525 quasars showing clear variability in logFX − logFUV space. After correcting for the effect of photometric noise, we find 35.6 ± 2.1 per cent of the epoch-pairs show asynchronous variability between X-ray and UV (brightening in one band but dimming in the other). This indicates only in 28.8 ± 4.2 per cent of the epoch-pairs the X-ray and UV variability are intrinsically coordinated. The variability synchronicity exhibits no dependence on physical parameters of quasars or the time lag of the epoch-pairs, except for stronger variability tends to have stronger synchronicity. Switches between synchronous and asynchronous variability are also seen in individual sources. The poor coordination clearly contradicts both the X-ray reprocessing model and the accretion rate variation model for AGN variability. The ratios of the observed X-ray variability amplitude to that in UV span a broad range and peak at ∼2. The dominant fraction of the ratios appear too small to be attributed to X-ray reprocessing, and too large for accretion rate variation. The inhomogeneous disc model which incorporates both X-ray and UV/optical variability in AGNs is favoured by the observed stochastic relation between X-ray and UV variations.

AT 2019avd: A Tidal Disruption Event with a Two-phase Evolution

Abstract Tidal disruption events (TDEs) can uncover the quiescent supermassive black holes (SMBHs) at the center of galaxies and also offer a promising method to study them. After the disruption of a star by an SMBH, the highly elliptical orbit of the debris stream will be gradually circularized due to the self-crossing, and then the circularized debris will form an accretion disk. The recent TDE candidate AT 2019avd has double peaks in its optical light curve, and the X-ray emerges near the second peak. The durations of the peaks are ∼400 and 600 days, respectively, and the separation between them is ∼700 days. We fit its spectral energy distribution and analyze its light curves in the optical/UV, mid-infrared, and X-ray bands. We find that this source can be interpreted as a two-phase scenario in which the first phase is dominated by the stream circularization, and the second phase is the delayed accretion. We use the succession of the self-crossing model and delayed accretion model to fit the first and second peaks, respectively. The fitting result implies that AT 2019avd can be interpreted by the partial disruption of a 0.9 M ⊙ star by a 7 × 106 M ⊙ SMBH, but this result is sensitive to the stellar model. Furthermore, we find that the large-amplitude (by factors up to ∼5) X-ray variability in AT 2019avd can be interpreted as the rigid-body precession of the misaligned disk due to the Lense–Thirring effect of a spinning SMBH, with a precession period of 10−25 days.

FRAMEx. II. Simultaneous X-Ray and Radio Variability in Active Galactic Nuclei—The Case of NGC 2992

Abstract Using simultaneous Very Long Baseline Array and Neil Gehrels Swift Observatory X-ray Telescope observations of the active galactic nucleus (AGN) in NGC 2992 over a six-month observing campaign, we observed a large drop in core 5 cm radio luminosity, by a factor of >3, in tandem with a factor of >5 increase in 2–10 keV X-ray luminosity. While NGC 2992 has long been an important object for studies of X-ray variability, our study is the first simultaneous X-ray and radio variability campaign for this object. We observe that the X-ray spectral index does not change over the course of the flare, consistent with a change in the bulk amount of Comptonizing plasma, potentially due to a magnetic reconnection event in the accretion disk. The drop in apparent radio luminosity can be explained by a change in free–free absorption, which we calculate to correspond to an ionized region with a physical extent and electron density consistent with the broad-line region (BLR). Our results are consistent with magnetic reconnection events in the dynamic accretion disk creating outbursts of ionizing material, increasing Compton up-scattering of UV accretion disk photons and feeding material into the BLR. These findings present an important physical picture for the dynamical relationship between X-ray and radio emission in AGNs.

On the single and multiple associations of COVID-19 post-acute sequelae: 6-month prospective cohort study

Medical research is progressing to clarify the full spectrum of sub-acute and long-term effects of the post-COVID-19 syndrome. However, most manuscripts published to date only analyze the effects of post-COVID-19 in patients discharged from hospital, which may induce significant bias. Here, we propose a pioneering study to analyze the single and multiple associations between post-COVID-19 characteristics with up to 6-months of follow-up in hospitalized and non-hospitalized COVID-19 patients. The cohort study was conducted from May to October 2020 at the University Hospital Virgen de la Nieves, the leading hospital assigned for patients with COVID-19 in Granada, Spain. A total of 372 and 217 patients—with 217 and 207 included in the first and second follow-up visits—were referred 2 and 6 months after diagnosing COVID-19, respectively. We find out that post-COVID-19 clinical and mental health impairment symptoms are correlated with patient gender. Logistic adjustments showed strong statistically robust single and multiple associations of demographic, clinical, mental health, X-ray, laboratory indices, and pulmonary function variables. The functional lung tests are good predictors of chest CT imaging abnormalities in elderly patients. Bilateral lung involvement, subpleural reticulum, ground-glass opacity, peripheral lung lesions, and bronchiectasis were the most common findings of the high-resolution computed tomography images. Non-hospitalized patients suffer more severe thromboembolic events and fatigue than those hospitalized.