Extreme X-ray Variability Research Articles

Fragments of harmony amid apparent chaos: A closer look at the X-ray quasi-periodic eruptions of the galaxy RX J1301.9+2747

Quasi-periodic eruptions (QPEs) are an extreme X-ray variability phenomenon associated with low-mass ($M_ BH < 10^7 M_ odot $) supermassive black holes (SMBHs). First discovered in the nucleus of the galaxy GSN 069, they have been so far securely detected in five other galaxies, including RX J1301.9+2747. When detected, the out-of-QPE emission (quiescence) is consistent with the high-energy tail of thermal emission from an accretion disk. In this article we present the X-ray properties of RX J1301.9+2747, both in quiescence and during QPEs, and complement this information with radio observations. We analyze X-ray data taken during five XMM-Newton observations between 2000 and 2022. The last three observations were taken in coordination with radio observations with the Karl G. Jansky Very Large Array. We also made use of EXOSAT, ROSAT, and Chandra archival observations taken between 1983 and 2009. XMM-Newton detected 34 QPEs of which eight have significantly lower amplitudes than the others. No correlated radio/X-ray variability was observed during QPEs. In terms of timing properties, the QPEs in RX J1301.9+2747 do not exhibit the striking regularity observed in the discovery source GSN 069. In fact there is no clear repetition pattern between QPEs: the average time separation between their peaks is about four hours, but it can be as short as one, and as long as six hours. The QPE spectral properties of RX J1301.9+2747 as a function of energy are, however, very similar to those of GSN 069 and of other QPE sources. During their evolution, X-ray QPEs follow a hysteresis pattern in the temperature-luminosity plane, with a hotter rise than decay. The quiescent emission of RX J1301.9+2747 is more complex than that of GSN 069, as it requires a soft X-ray excess-like component in addition to the thermal emission from the accretion disk. Its long-term X-ray quiescent flux variations are of a low amplitude and not strictly monotonic, with a general decay over the course of $ years. We discuss our observational results in terms of some of the ideas and models that have been proposed so far for the physical origin of QPEs.

Quasi-periodic eruptions from impacts between the secondary and a rigidly precessing accretion disc in an extreme mass-ratio inspiral system

X-ray quasi-periodic eruptions (QPEs) represent a recently discovered example of extreme X-ray variability associated with supermassive black holes. These are high-amplitude bursts recurring every few hours that are detected in the soft X-ray band from the nuclei of nearby galaxies whose optical spectra lack the broad emission lines typically observed in unobscured active galaxies. The physical origin of this new X-ray variability phenomenon is still unknown and several theoretical models have been presented. However, no attempt has been made so far to account for the varying QPE recurrence time and luminosity in individual sources, nor for the diversity of the QPE phenomenology in the different known erupters. We present a semi-analytical model based on an extreme mass-ratio inspiral (EMRI) system where the secondary intersects, along its orbit, a rigidly precessing accretion disc surrounding the primary. We assume that QPEs result from emission from an adiabatically expanding, initially optically thick gas cloud expelled from the disc plane at each impact. We produced synthetic X-ray light curves, which we then compared with X-ray data from four QPE sources: GSN 069, eRO-QPE1, eRO-QPE2, and RX J1301.9+2747. Our model aptly reproduces the diversity of QPE properties between the considered objects and it is also able to naturally account for the varying QPE amplitudes and recurrence times in individual sources. Future implementations will enable us to refine the match with the data and to estimate the system parameters precisely, making additional use of multi-epoch QPE data. We briefly discuss the nature of the secondary object, as well as the possible implications of our findings for the EMRI population at large.

X-Ray Unveiling Events in a z ≈ 1.6 Active Galactic Nucleus in the 7 Ms Chandra Deep Field-South

We investigate the extreme X-ray variability of a z = 1.608 active galactic nucleus in the 7 Ms Chandra Deep Field-South (XID 403), which showed two significant X-ray brightening events. In the first event, XID 403 brightened by a factor of >2.5 in ≲6.1 rest-frame days in the observed-frame 0.5–5 keV band. The event lasted for ≈5.0–7.3 days, and then XID 403 dimmed by a factor of >6.0 in ≲6.1 days. After ≈1.1–2.5 yr in the rest frame (including long observational gaps), it brightened again, with the 0.5–5 keV flux increasing by a factor of >12.6. The second event lasted over 251 days, and the source remained bright until the end of the 7 Ms exposure. The spectrum is a steep power law (photon index Γ = 2.8 ± 0.3) without obscuration during the second outburst, and the rest-frame 2–10 keV luminosity reaches ; there is no significant spectral evolution within this epoch. The infrared-to-UV spectral energy distribution of XID 403 is dominated by the host galaxy. There is no significant optical/UV variability and R-band (rest-frame ≈2500 Å) brightening contemporaneous with the X-ray brightening. The extreme X-ray variability is likely due to two X-ray unveiling events, where the line of sight to the corona is no longer shielded by high-density gas clumps in a small-scale dust-free absorber. XID 403 is probably a high-redshift analog of local narrow-line Seyfert 1 galaxies, and the X-ray absorber is a powerful accretion disk wind. On the other hand, we cannot exclude the possibility that XID 403 is an unusual candidate for tidal disruption events.

ERASSt J074426.3 + 291606: prompt accretion disc formation in a ‘faint and slow’ tidal disruption event

ABSTRACTWe report on multiwavelength observations of the tidal disruption event (TDE) candidate eRASSt J074426.3 + 291606 (J0744), located in the nucleus of a previously quiescent galaxy at z = 0.0396. J0744 was first detected as a new, ultra-soft X-ray source (photon index ∼4) during the second SRG/eROSITA All-Sky Survey (eRASS2), where it had brightened in the 0.3–2 keV band by a factor of more than ∼160 relative to an archival 3σ upper limit inferred from a serendipitous Chandra pointing in 2011. The transient was also independently found in the optical by the Zwicky Transient Facility (ZTF), with the eRASS2 detection occurring only ∼20 d after the peak optical brightness, suggesting that the accretion disc formed promptly in this TDE. Continued X-ray monitoring over the following ∼400 d by eROSITA, NICER XTI and Swift XRT showed a net decline by a factor of ∼100, albeit with large amplitude X-ray variability where the system fades, and then rebrightens, in the 0.3–2 keV band by a factor ∼50 during an 80-d period. Contemporaneous Swift UVOT observations during this extreme X-ray variability reveal a relatively smooth decline, which persists over ∼400 d post-optical peak. The peak observed optical luminosity (absolute g-band magnitude ∼−16.8 mag) from this transient makes J0744 the faintest optically detected TDE observed to date. However, contrasting the known set of ‘faint and fast’ TDEs, the optical emission from J0744 decays slowly (exponential decay time-scale ∼120 d), making J0744 the first member of a potential new class of ‘faint and slow’ TDEs.

Deciphering the extreme X-ray variability of the nuclear transient eRASSt J045650.3−203750

Context. During its all-sky survey, the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) observatory has uncovered a growing number of X-ray transients associated with the nuclei of quiescent galaxies. Benefitting from its large field of view and excellent sensitivity, the eROSITA window into time-domain X-ray astrophysics yields a valuable sample of X-ray selected nuclear transients. Multi-wavelength follow-up enables us to gain new insights into understanding the nature and emission mechanism of these phenomena. Aims. We present the results of a detailed multi-wavelength analysis of an exceptional repeating X-ray nuclear transient, eRASSt J045650.3−203750 (hereafter J0456−20), uncovered by SRG/eROSITA in a quiescent galaxy at a redshift of z ∼ 0.077. We aim to understand the radiation mechanism at different luminosity states of J0456−20, and provide further evidence that similar accretion processes are at work for black hole accretion systems at different black hole mass scales. Methods. We describe our temporal analysis, which addressed both the long- and short-term variability of J0456−20. A detailed X-ray spectral analysis was performed to investigate the X-ray emission mechanism. Results. Our main findings are that (1) J0456−20 cycles through four distinctive phases defined based on its X-ray variability: an X-ray rising phase leading to an X-ray plateau phase that lasts for abouttwo months. This is terminated by a rapid X-ray flux drop phase during which the X-ray flux can drop drastically by more than a factor of 100 within one week, followed by an X-ray faint state for about two months before the X-ray rising phase starts again. (2) The X-ray spectra are generally soft in the rising phase, with a photon index ≳3.0, and they become harder as the X-ray flux increases. There is evidence of a multi-colour disk with a temperature of Tin ∼ 70 eV in the inner region at the beginning of the X-ray rising phase. The high-quality XMM-Newton data suggest that a warm and hot corona might cause the X-ray emission through inverse Comptonisation of soft disk seed photons during the plateau phase and at the bright end of the rising phase. (3) J0456−20 shows only moderate UV variability and no significant optical variability above the host galaxy level. Optical spectra taken at different X-ray phases are constant in time and consistent with a typical quiescent galaxy with no indication of emission lines. (4) Radio emission is (as yet) only detected in the X-ray plateau phase and rapidly declines on a timescale of two weeks. Conclusions. J0456−20 is likely a repeating nuclear transient with a tentative recurrence time of ∼223 days. It is a new member of this rare class. We discuss several possibilities to explain the observational properties of J0456−20. We currently favour a repeating partial tidal disruption event as the most likely scenario. The long-term X-ray evolution is explained as a transition between a thermal disk-dominated soft state and a steep power-law state. This implies that the corona can be formed within a few months and is destroyed within a few weeks.

Sensitive Chandra coverage of a representative sample of weak-line quasars: revealing the full range of X-ray properties

ABSTRACT We present deeper Chandra observations for weak-line quasars (WLQs) in a representative sample that previously had limited X-ray constraints, and perform X-ray photometric analyses to reveal the full range of X-ray properties of WLQs. Only 5 of the 32 WLQs included in this representative sample remain X-ray undetected after these observations, and a stacking analysis shows that these 5 have an average X-ray weakness factor of >85. One of the WLQs in the sample that was known to have extreme X-ray variability, SDSS J1539+3954, exhibited dramatic X-ray variability again: it changed from an X-ray normal state to an X-ray weak state within ≈3 months in the rest frame. This short time-scale for an X-ray flux variation by a factor of ≳9 further supports the thick disc and outflow (TDO) model proposed to explain the X-ray and multiwavelength properties of WLQs. The overall distribution of the X-ray-to-optical properties of WLQs suggests that the TDO has an average covering factor of the X-ray emitting region of ∼0.5, and the column density of the TDO can range from $N_{\rm H}\, \sim 10^{23-24}~{\rm cm}^{-2}$ to $N_{\rm H}\, \gtrsim 10^{24}~{\rm cm}^{-2}$, which leads to different levels of absorption and Compton reflection (and/or scattering) among WLQs.

The nature of the extreme X-ray variability in the NLS1 1H 0707-495

ABSTRACT We examine archival XMM-Newton data on the extremely variable narrow-line Seyfert 1 active galactic nucleus (AGN) 1H 0707-495. We construct fractional excess variance (Fvar) spectra for each epoch, including the recent 2019 observation taken simultaneously with eROSITA. We explore both intrinsic and environmental absorption origins for the variability in different epochs, and examine the effect of the photoionized emission lines from outflowing gas. In particular, we show that the unusual soft variability first detected by eROSITA in 2019 is due to a combination of an obscuration event and strong suppression of the variance at 1 keV by photoionized emission, which makes the variance below 1 keV appear more extreme. We also examine the variability on long time-scales, between observations, and find that it is well described by a combination of intrinsic variability and absorption variability. We suggest that the typical extreme high frequency variability, which 1H 0707-495 is known for, is intrinsic to the source, but the large amplitude, low frequency variability that causes prolonged low-flux intervals is likely dominated by variable low-ionization, low-velocity absorption.

The frequency of extreme X-ray variability for radio-quiet quasars

ABSTRACT We analyse 1598 serendipitous Chandra X-ray observations of 462 radio-quiet quasars to constrain the frequency of extreme amplitude X-ray variability that is intrinsic to the quasar corona and innermost accretion flow. The quasars in this investigation are all spectroscopically confirmed, optically bright (mi ≤ 20.2), and contain no identifiable broad absorption lines in their optical/ultraviolet spectra. This sample includes quasars spanning z ≈ 0.1–4 and probes X-ray variability on time-scales of up to ≈12 rest-frame years. Variability amplitudes are computed between every epoch of observation for each quasar and are analysed as a function of time-scale and luminosity. The tail-heavy distributions of variability amplitudes at all time-scales indicate that extreme X-ray variations are driven by an additional physical mechanism and not just typical random fluctuations of the coronal emission. Similarly, extreme X-ray variations of low-luminosity quasars seem to be driven by an additional physical mechanism, whereas high-luminosity quasars seem more consistent with random fluctuations. The amplitude at which an X-ray variability event can be considered extreme is quantified for different time-scales and luminosities. Extreme X-ray variations occur more frequently at long time-scales (Δt ≳ 300 d) than at shorter time-scales and in low-luminosity quasars compared to high-luminosity quasars over a similar time-scale. A binomial analysis indicates that extreme intrinsic X-ray variations are rare, with a maximum occurrence rate of $\lt 2.4{{\ \rm per\ cent}}$ of observations. Finally, we present X-ray variability and basic optical emission-line properties of three archival quasars that have been newly discovered to exhibit extreme X-ray variability.

An Extreme X-Ray Variability Event of a Weak-line Quasar

Abstract We report the discovery of an extreme X-ray flux rise (by a factor of ≳20) of the weak-line quasar Sloan Digital Sky Survey (SDSS) J153913.47+395423.4 (hereafter SDSS J1539+3954) at z = 1.935. SDSS J1539+3954 is the most-luminous object among radio-quiet type 1 active galactic nuclei (AGNs) where such dramatic X-ray variability has been observed. Before the X-ray flux rise, SDSS J1539+3954 appeared X-ray weak compared with the expectation from its ultraviolet (UV) flux; after the rise, the ratio of its X-ray flux and UV flux is consistent with the majority of the AGN population. We also present a contemporaneous HET spectrum of SDSS J1539+3954, which demonstrates that its UV continuum level remains generally unchanged despite the dramatic increase in the X-ray flux, and its C iv emission line remains weak. The dramatic change only observed in the X-ray flux is consistent with a shielding model, where a thick inner accretion disk can block our line of sight to the central X-ray source. This thick inner accretion disk can also block the nuclear ionizing photons from reaching the high-ionization broad emission-line region, so that weak high-ionization emission lines are observed. Under this scenario, the extreme X-ray variability event may be caused by slight variations in the thickness of the disk. This event might also be explained by gravitational light-bending effects in a reflection model.

SDSS J075101.42+291419.1: A Super-Eddington Accreting Quasar with Extreme X-Ray Variability

Abstract We report the discovery of extreme X-ray variability in a type 1 quasar: SDSS J075101.42+291419.1. It has a black hole (BH) mass of 1.6 × 107 M ⊙ measured from reverberation mapping, and the BH is accreting with a super-Eddington accretion rate. Its XMM-Newton observation in 2015 May reveals a flux drop by a factor of ∼22 with respect to the Swift observation in 2013 May when it showed a typical level of X-ray emission relative to its UV/optical emission. The lack of correlated UV variability results in a steep X-ray-to-optical power-law slope (α OX) of −1.97 in the low X-ray flux state, corresponding to an X-ray weakness factor of 36.2 at rest-frame 2 keV relative to its UV/optical luminosity. The mild UV/optical continuum and emission-line variability also suggest that the accretion rate did not change significantly. A single power-law model modified by Galactic absorption describes the 0.3–10 keV spectra of the X-ray observations well in general. The spectral fitting reveals steep spectral shapes with Γ ≈ 3. We search for active galactic nuclei (AGNs) with such extreme X-ray variability in the literature and find that most of them are narrow-line Seyfert 1 galaxies and quasars with high accretion rates. The fraction of extremely X-ray variable objects among super-Eddington accreting AGNs is estimated to be ≈15%–24%. We discuss two possible scenarios, disk reflection and partial covering absorption, to explain the extreme X-ray variability of SDSS J075101.42+291419.1. We propose a possible origin for the partial covering absorber, which is the thick inner accretion disk and its associated outflow in AGNs with high accretion rates.

Lord of the Rings – Return of the King:Swift-XRT observations of dust scattering rings around V404 Cygni

On 2015 June 15, the black hole X-ray binary V404 Cygni went into outburst, exhibiting extreme X-ray variability which culminated in a final flare on June 26. Over the following days, the Swift-X-ray Telescope detected a series of bright rings, comprising five main components that expanded and faded with time, caused by X-rays scattered from the otherwise unobservable dust layers in the interstellar medium in the direction of the source. Simple geometrical modelling of the rings’ angular evolution reveals that they have a common temporal origin, coincident with the final, brightest flare seen by INTEGRAL's JEM X-1, which reached a 3–10?keV flux of ?25 Crab. The high quality of the data allows the dust properties and density distribution along the line of sight to the source to be estimated. Using the Rayleigh–Gans approximation for the dust scattering cross-section and a power-law distribution of grain sizes a, ??a?q, the average dust emission is well modelled by q=3.90+0.09?0.08 and maximum grain size of a+=0.147+0.024?0.004 ?m, though significant variations in q are seen between the rings. The recovered dust density distribution shows five peaks associated with the dense sheets responsible for the rings at distances ranging from 1.19 to 2.13?kpc, with thicknesses of ?40–80?pc and a maximum density occurring at the location of the nearest sheet. We find a dust column density of Ndust ? (2.0–2.5) × 1011?cm?2, consistent with the optical extinction to the source. Comparison of the inner rings’ azimuthal X-ray evolution with archival Wide-field Infrared Survey Explorer mid-IR data suggests that the second most distant ring follows the general IR emission trend, which increases in brightness towards the Galactic north side of the source.

Narrow-Line Seyfert 1 Galaxies: Observational and Theoretical Progress until 2003

Narrow-Line Seyfert 1 galaxies are an important class of Seyfert 1 galaxies. X-ray observations with ROSAT. XMM-Newton and Chandra have revealed many interesting and unexpected properties, e.g. steep soft (0.1-2.4) keV and hard (2-10) keV X-ray continua, a strong soft X-ray excess in many of these objects and unusually rapid and extreme X-ray variability. These X-ray properties are related to many optical and UV properties, such as the strength of the Fe II multiplet emission or the strength of the [OIII] emission. The unique combination of extreme observational properties in Narrow-Line Seyfert 1 galaxies allows for a more general understanding of the Seyfert phenomenon. This paper reviews the observational and theoretical progress made until October 2003.

VARIABILITY OF ACTIVE GALACTIC NUCLEI FROM THE OPTICAL TO X-RAY REGIONS

Some progress in understanding AGN variability is reviewed. Reprocessing of X-ray radiation to produce significant amounts of longer-wavelength continua seems to be ruled out. In some objects where there has been correlated X-ray and optical variability, the amplitude of the optical variability has exceeded the amplitude of X-ray variability. We suggest that accelerated particles striking material could be linking X-ray and optical variability (as in activity in the solar chromosphere). Beaming effects could be significant in all types of AGN. The diversity in optical/X-ray relationships at different times in the same object, and between different objects, might be explained by changes in geometry and directions of motion relative to our line of sight. Linear shot-noise models of the variability are ruled out; instead there must be large-scale organization of variability. Variability occurs on light-crossing timescales rather than viscous timescales and this probably rules out the standard Shakura-Sunyaev accretion disk. Radio-loud and radio-quiet AGNs have similar continuum shapes and similar variability properties. This suggests similar continuum origins and variability mechanisms. Despite their extreme X-ray variability, narrow-line Seyfert 1s (NLS1s) do not show extreme optical variability.

Extreme X-ray variability in the luminous quasar PDS 456

We present evidence from BeppoSAX and XMM–Newton of extreme X-ray variability in the high-luminosity radio-quiet quasar PDS 456, the most luminous known active galactic nucleus (AGN) at z < 0.3. Repeated X-ray flaring is found in PDS 456, over the duration of the 340-ks long BeppoSAX observation. The X-ray flux doubles in just 30 ks, whilst the total energy output of the flaring events is as high as 1051 erg. Under the assumption of isotropic emission at the Eddington limit, this implies that the size of the X-ray emitting region in PDS 456 is less than 3 Schwarzschild radii, for a 109-M⊙ black hole. From the rates of change of luminosity observed during the X-ray flares, we calculate lower limits for the radiative efficiency limit between 0.06 and 0.41, implying that accretion on to a Kerr black hole is likely in PDS 456. We suggest that the rapid variability is from X-ray flares produced through magnetic reconnection above the disc and calculate that the energetics and time-scale of the flares are plausible if the quasar is accreting near to the maximum Eddington rate. A similar mechanism may account for the extreme, rapid X-ray variability observed in many narrow line Seyfert 1s. In the case of PDS 456, we show that the X-ray flaring could be reproduced through a self-induced cascade of ∼1000 individual flares over a time-scale of the order 1 d.

ROSAT High-Resolution Imager monitoring of extreme X-ray variability in the narrow-line quasar PHL 1092

We report results from an 18-d ROSAT High-Resolution Imager (HRI) monitoring campaign on the ultrasoft narrow-line Seyfert 1 (NLS1) class quasar PHL 1092. This luminous, radio-quiet quasar showed strong X-ray variability in a short ROSAT Position-Sensitive Proportional Counter observation, and ROSAT HRI monitoring of the similar object IRAS 13224-3809 revealed extreme variability on intermediate time-scales. We wanted to determine whether remarkable X-ray variability persistently occurs in PHL 1092, and we also wanted to search for outstanding variability events that constrain emission processes. Given the large luminosity of PHL 1092 (∼ 5 × 1045 erg s-1 in the HRI band), we detect extremely rapid and large-amplitude X-ray variability throughout our monitoring. The maximum observed variability amplitude is a factor of ≈14, and in the most rapid variability event the HRI count rate increases by a factor of ≈ 3.8 in a rest-frame time interval of 1.3 × 1042 erg s-2, making it the most extreme such event we are aware of from a radio-quiet quasar. Standard `radiative efficiency limit' arguments imply a radiative efficiency larger than can be achieved by accretion on to a Kerr black hole rotating at the maximum plausible rate, although we point out that such arguments depend upon the geometry of initial radiation release. Relativistic motions of the X-ray source are probably causing the radiative efficiency limit to break down; such relativistic motions have also been inferred in the similar NLS1-class quasar PKS 0558-504.

Extreme X-Ray Variability in the Narrow-Line QSO PHL 1092

view Abstract Citations (60) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Extreme X-Ray Variability in the Narrow-Line QSO PHL 1092 Forster, Karl ; Halpern, Jules P. Abstract A ROSAT observation of the narrow-line Fe II QSO PHL 1092 shows rapid variability that requires an efficiency of at least 0.13, exceeding the theoretical maximum for an accretion disk around a non-rotating black hole. Plausible explanations for this high efficiency incorporate anisotropic emission and/or accretion onto a rapidly rotating black hole, the latter being recently suggested by Kwan et al. as a mechanism for generating the strong Fe II lines of PHL 1092 by mechanical heating in an accretion disk. The soft X-ray luminosity of PHL 1092 had also increased by a factor of 21 over the weak Einstein detection, to more than 5 x 10^46^ ergs s^-1^. Its photon spectral index of 4.2 is among the steepest of any active galactic nucleus. These X-ray properties are characteristic of narrow-line Seyfert 1 galaxies, of which PHL 1092 is evidently a very luminous member. Narrow-line QSOs also extend the significant correlation between X-ray luminosity and X-ray spectral index which we have found among a large sample of optically selected, narrow-line Seyfert 1 galaxies observed by ROSAT, Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177715 arXiv: arXiv:astro-ph/9603152 Bibcode: 1996ApJ...468..565F Keywords: GALAXIES: SEYFERT; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: PG 1404+226; X-RAYS: GALAXIES; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: PHL 1092; GALAXIES: ACTIVE; Astrophysics E-Print: LaTeX file, 13 pages including 3 tables and 3 postscript figures. To appear in The Astrophysical Journal full text sources arXiv | ADS | data products SIMBAD (13) NED (9) HEASARC (1)