The correlation observed between monochromatic X-ray and UV luminosities in radiatively-efficient active galactic nuclei (AGN) lacks a clear theoretical explanation despite being used for many ...applications. Such a correlation, with its small intrinsic scatter and its slope that is smaller than unity in log space, represents the compelling evidence that a mechanism regulating the energetic interaction between the accretion disk and the X-ray corona must be in place. This ensures that going from fainter to brighter sources the coronal emission increases less than the disk emission. We discuss here a self-consistently coupled disk-corona model that can identify this regulating mechanism in terms of modified viscosity prescriptions in the accretion disk. The model predicts a lower fraction of accretion power dissipated in the corona for higher accretion states. We then present a quantitative observational test of the model using a reference sample of broad-line AGN and modeling the disk-corona emission for each source in the LX − LUV plane. We used the slope, normalization, and scatter of the observed relation to constrain the parameters of the theoretical model. For non-spinning black holes and static coronae, we find that the accretion prescriptions that match the observed slope of the LX − LUV relation produce X-rays that are too weak with respect to the normalization of the observed relation. Instead, considering moderately-outflowing Comptonizing coronae and/or a more realistic high-spinning black hole population significantly relax the tension between the strength of the observed and modeled X-ray emission, while also predicting very low intrinsic scatter in the LX − LUV relation. In particular, this latter scenario traces a known selection effect of flux-limited samples that preferentially select high-spinning, hence brighter, sources.
The role played by the intergalactic medium (IGM) in the X-ray absorption towards high-redshift sources has recently drawn more attention in spectral analysis studies. Here, we study the X-ray ...absorption towards 15 flat-spectrum radio quasars at z > 2, relying on high counting statistic (≳10 000 photons) provided by XMM-Newton, with additional NuSTAR (and simultaneous Swift-XRT) observations when available. Blazars can be confidently considered to have negligible X-ray absorption along the line of sight within the host galaxy, likely swept by the kpc-scale relativistic jet. This makes our sources ideal for testing the absorption component along the IGM. Our new approach is to revisit the origin of the soft X-ray spectral hardening observed in high-z blazars in terms of X-ray absorption occurring along the IGM, with the help of a low-z sample used as comparison. We have verified that the presence of absorption in excess of the Galactic value is the preferred explanation to explain the observed hardening, while intrinsic energy breaks, predicted by blazars’ emission models, can easily occur out of the observing energy band in most sources. First, we performed an indirect analysis comparing the inferred amount of absorption in excess of the Galactic value with a simulated IGM absorption contribution, that increases with redshift and includes both a minimum component from diffuse IGM metals, and the additional contribution of discrete denser intervening regions. Then, we directly investigated the warm-hot IGM with a spectral model on the best candidates of our sample, obtaining an average IGM density of n0 = 1.01−0.72+0.53 × 10−7 $n_0=1.01^{+0.53}_{-0.72}\times10^{-7}$n0=1.01−0.72+0.53×10−7 cm−3 and temperature of log(T/K) = 6.45−2.12+0.51 $\log(T/\text{K})=6.45^{+0.51}_{-2.12}$log(T/K)=6.45−2.12+0.51 . A more dedicated study is currently beyond our reach, but our results can be used as a stepping stone for future more accurate analysis, involving Athena.
The ultra-soft narrow-line Seyfert 1 galaxy 1H 0707−495 is a well-known and highly variable active galactic nucleus (AGN), with a complex, steep X-ray spectrum, and has been studied extensively with
...XMM-Newton
. 1H 0707−495 was observed with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) aboard the Spectrum-Roentgen-Gamma (SRG) mission on October 11, 2019, for about 60 000 s as one of the first calibration and pointed verification phase (CalPV) observations. The eROSITA light curves show significant variability in the form of a flux decrease by a factor of 58 with a 1
σ
error confidence interval between 31 and 235. This variability is primarily in the soft band, and is much less extreme in the hard band. No strong ultraviolet variability has been detected in simultaneous
XMM-Newton
Optical Monitor observations. The UV emission is
L
UV
≈ 10
44
erg s
−1
, close to the Eddington limit. 1H 0707−495 entered the lowest hard flux state seen in 20 yr of
XMM-Newton
observations. In the eROSITA All-Sky Survey (eRASS) observations taken in April 2020, the X-ray light curve is still more variable in the ultra-soft band, but with increased soft and hard band count rates more similar to previously observed flux states. A model including relativistic reflection and a variable partial covering absorber is able to fit the spectra and provides a possible explanation for the extreme light-curve behaviour. The absorber is probably ionised and therefore more transparent to soft X-rays. This leaks soft X-rays in varying amounts, leading to large-amplitude soft-X-ray variability.
Context.
Quasi-periodic X-ray eruptions (QPEs) are a recently discovered phenomenon, the nature of which remains unclear. Based on their discovery in active galactic nuclei (AGNs), explanations ...related to an AGN accretion disk or potentially a stellar tidal disruption event (TDE) have been put forward. Alternatives, including highly unequal mass compact object binaries, have also been proposed to explain their properties.
Aims.
We perform a systematic study of the five known QPE host galaxies with the aim of providing new insights as to their nature.
Methods.
We analysed new and archival medium resolution optical spectroscopy of the QPE hosts. We measured emission (and absorption) line fluxes, their ratios, and equivalent widths (EWs) to locate the QPE hosts on diagnostic diagrams. We also measured the velocity dispersion of the stellar absorption lines to estimate their black hole masses.
Results.
All QPE host galaxies show emission lines in their optical spectra. Based on their ratios and EWs, we find evidence for the presence of an AGN in all sources, including those previously reported as passive. We measure velocity dispersions between 36 and 90 km s
−1
, implying the presence of low mass (10
5−6.7
M
⊙
) black holes, consistent with literature findings. Finally, we find a significant over-representation (two out of the five sources, or a factor of 13
−10
+13
) of quiescent Balmer strong (post-starburst) galaxies among QPE hosts.
Conclusions.
The presence of a narrow line region consistent with an AGN in all QPE host galaxies implies that a long-lived accretion flow likely plays an integral part in the QPE phenomenon. The strong over-representation of quiescent Balmer strong galaxies among QPE hosts can be naturally explained in both the TDE and interacting extreme mass ratio inspiral hypotheses.
Over the years, numerous attempts have been made to connect the phenomenology and physics of mass accretion onto stellar-mass and super-massive black holes in a scale-invariant fashion. In this ...paper, we explore this connection at the radiatively efficient (and non-jetted) end of accretion modes by comparing the relationship between the luminosity of the accretion disc and corona in the two source classes. Motivated by the apparently tight relationship between these two quantities in active galactic nuclei (AGNs), we analyse 458 RXTE-PCA archival observations of the X-ray binary (XRB) GX 339–4, using this object as an exemplar for the properties of XRBs in general. We focus on the soft and soft-intermediate states, which have been suggested to be analogous to radiatively efficient AGNs. The observed scatter in the log
L
disc
− log
L
corona
relationship of GX 339–4 is high (∼0.43 dex) and significantly larger than in a representative sample of radiatively efficient, non- or weakly jetted AGNs (∼0.30 dex). At first glance, this would appear contrary to the hypothesis that the systems simply scale with mass. On the other hand, we also find that GX 339–4 and our AGN sample show different accretion rate and power-law index distributions, with the latter in particular being broader in GX 339–4 (dispersion of ∼0.16 cf. ∼0.08 for AGN). GX 339–4 also shows an overall softer slope, with a mean value of ∼2.20 as opposed to ∼2.07 for the AGN sample. Remarkably, once similarly broad Γ and
ṁ
distributions are selected, the AGN sample overlaps nicely with GX 339–4 observations in the mass-normalised log
L
disc
− log
L
corona
plane, with a scatter of ∼0.30 − 0.33 dex in both cases. This indicates that a mass-scaling of properties might hold after all, with our results being consistent with the disc-corona systems in AGNs and XRBs exhibiting the same physical processes, albeit under different conditions for instance in terms of temperature, optical depth and/or electron energy distribution in the corona, heating-cooling balance, coronal geometry and/or black hole spin.
Quasi-Periodic Eruptions (QPEs) are extreme high-amplitude bursts of X-ray radiation recurring every few hours and originating near the central supermassive black holes in galactic nuclei. It is ...currently unknown what triggers these events, how long they last and how they are connected to the physical properties of the inner accretion flows. Previously, only two such sources were known, found either serendipitously or in archival data, with emission lines in their optical spectra classifying their nuclei as hosting an actively accreting supermassive black hole. Here we present the detection of QPEs in two further galaxies, obtained with a blind and systematic search over half of the X-ray sky. The optical spectra of these galaxies show no signature of black hole activity, indicating that a pre-existing accretion flow typical of active nuclei is not required to trigger these events. Indeed, the periods, amplitudes and profiles of the newly discovered QPEs are inconsistent with current models that invoke radiation-pressure driven accretion disk instabilities. Instead, QPEs might be driven by an orbiting compact object. Furthermore, their observed properties require the
mass of the secondary object to be much smaller than the main body and future X-ray observations may constrain possible changes in the period due to orbital evolution. This scenario could make QPEs a viable candidate for the electromagnetic counterparts of the so-called extreme mass ratio inspirals, with considerable implications for multi-messenger astrophysics and cosmology.
Abstract
Stars that interact with supermassive black holes (SMBHs) can be either completely or partially destroyed by tides. In a partial tidal disruption event (TDE), the high-density core of the ...star remains intact, and the low-density outer envelope of the star is stripped and feeds a luminous accretion episode. The TDE AT 2018fyk, with an inferred black hole mass of 10
7.7±0.4
M
⊙
, experienced an extreme dimming event at X-ray (factor of >6000) and UV (factor of ∼15) wavelengths ∼500–600 days after discovery. Here we report on the reemergence of these emission components roughly 1200 days after discovery. We find that the source properties are similar to those of the predimming accretion state, suggesting that the accretion flow was rejuvenated to a similar state. We propose that a repeated partial TDE, where the partially disrupted star is on an ∼1200 day orbit about the SMBH and periodically stripped of mass during each pericenter passage, powers its unique light curve. This scenario provides a plausible explanation for AT 2018fyk’s overall properties, including the rapid dimming event and the rebrightening at late times. We also provide testable predictions for the behavior of the accretion flow in the future; if the second encounter was also a partial disruption, then we predict another strong dimming event around day 1800 (2023 August) and a subsequent rebrightening around day 2400 (2025 March). This source provides strong evidence of the partial disruption of a star by an SMBH.
Context.
High-redshift quasars signpost the early accretion history of the Universe. The penetrating nature of X-rays enables a less absorption-biased census of the population of these luminous and ...persistent sources compared to optical/near-infrared colour selection. The ongoing SRG/eROSITA X-ray all-sky survey offers a unique opportunity to uncover the bright end of the high-
z
quasar population and probe new regions of colour parameter space.
Aims.
We searched for high-
z
quasars within the X-ray source population detected in the contiguous ~140 deg
2
field observed by eROSITA during the performance verification phase. With the purpose of demonstrating the unique survey science capabilities of eROSITA, this field was observed at the depth of the final all-sky survey. The blind X-ray selection of high-redshift sources in a large contiguous, near-uniform survey with a well-understood selection function can be directly translated into constraints on the X-ray luminosity function (XLF), which encodes the luminosity-dependent evolution of accretion through cosmic time.
Methods.
We collected the available spectroscopic information in the eFEDS field, including the sample of all currently known optically selected
z
> 5.5 quasars and cross-matched secure Legacy DR8 counterparts of eROSITA-detected X-ray point-like sources with this spectroscopic sample.
Results.
We report the X-ray detection of eFEDSU J083644.0+005459, an eROSITA source securely matched to the well-known quasar SDSS J083643.85+005453.3 (
z
= 5.81). The soft X-ray flux of the source derived from eROSITA is consistent with previous
Chandra
observations. The detection of SDSS J083643.85+005453.3 allows us to place the first constraints on the XLF at
z
> 5.5 based on a secure spectroscopic redshift. Compared to extrapolations from lower-redshift observations, this favours a relatively flat slope for the XLF at
z
~ 6 beyond
L
*
, the knee in the luminosity function. In addition, we report the detection of the quasar with LOFAR at 145 MHz and ASKAP at 888 MHz. The reported flux densities confirm a spectral flattening at lower frequencies in the emission of the radio core, indicating that SDSS J083643.85+005453.3 could be a (sub-) gigahertz peaked spectrum source. The inferred spectral shape and the parsec-scale radio morphology of SDSS J083643.85+005453.3 indicate that it is in an early stage of its evolution into a large-scale radio source or confined in a dense environment. We find no indications for a strong jet contribution to the X-ray emission of the quasar, which is therefore likely to be linked to accretion processes.
Conclusions.
Our results indicate that the population of X-ray luminous AGNs at high redshift may be larger than previously thought. From our XLF constraints, we make the conservative prediction that eROSITA will detect ~90 X-ray luminous AGNs at redshifts 5.7 <
z
< 6.4 in the full-sky survey (De+RU). While subject to different jet physics, both high-redshift quasars detected by eROSITA so far are radio-loud; a hint at the great potential of combined X-ray and radio surveys for the search of luminous high-redshift quasars.
X-ray quasi-periodic eruptions (QPEs) are intense repeating soft X-ray bursts from the nuclei of nearby galaxies. Their physical origin is still largely unconstrained, and several theoretical models ...have been proposed ranging from disc instabilities to impacts between an orbiting companion and the existing accretion disc around the primary, or episodic mass transfer at pericentre in an extreme mass-ratio binary. We present here results from a recent
XMM-Newton
observation of GSN 069, the galactic nucleus where QPEs were first discovered. After about two years of absence, QPEs have reappeared in GSN 069, and we detect two consecutive QPEs separated by a much shorter recurrence time than ever before. Moreover, their intensites and peak temperatures are remarkably different, a novel addition to the QPE phenomenology. We study the QPE spectral properties from all
XMM-Newton
observations assuming QPEs to either represent an additional emission component superimposed on that from the disc, or the transient evolution of the disc emission itself. In the former scenario, QPEs are consistent with black-body emission from a region that expands by a factor of 2–3 during the individual QPE evolution with radius ≃5 − 10 × 10
10
cm at QPE peak. In the alternative non-additive scenario, QPEs originate from a region with an area ∼6 − 30 times smaller than the quiescent state X-ray emission, with the smallest regions corresponding to the hottest and most luminous eruptions. The QPE reappearance reveals that eruptions are only present below a quiescent luminosity threshold corresponding to an Eddington ratio
λ
thresh
≃ 0.4 ± 0.2 for a 10
6
M
⊙
black hole. The disappearance of QPEs above
λ
thresh
is most likely driven by the ratio of QPE to quiescence temperatures,
kT
QPE
/
kT
quiesc
, approaching unity at high quiescent luminosity, making QPE detection challenging, if not impossible, above threshold. We briefly discuss some of the consequences of our results on the proposed models for the QPE physical origin.
Multiwavelength extragalactic nuclear transients, particularly those detectable as multi-messengers, are among the primary drivers for the next-generation observatories. X-ray quasi-periodic ...eruptions (QPEs) are the most recent and perhaps most peculiar addition to this group. Here, we report a first estimate of the volumetric rate of QPEs based on the first four discoveries with the eROSITA X-ray telescope onboard the Spectrum Roentgen Gamma observatory. Under the assumption, supported by a suite of simulated light curves, that these four sources sample the intrinsic population somewhat homogeneously, we correct for their detection efficiency and compute a QPE abundance of R vol = 0.60 −0.43 +4.73 × 10 −6 Mpc −3 above an intrinsic average log L 0.5−2.0 keV peak > 41.7. Since the exact lifetime of QPEs ( τ life ) is currently not better defined than between a few years or few decades, we convert this to a formation rate of ℛ vol / τ life ≈ 0.6 × 10 −7 ( τ life /10 yr) −1 Mpc −3 yr −1 . As a comparison, this value is a factor ∼10 τ life times smaller than the formation rate of tidal disruption events. The origin of QPEs is still debated, although lately most models suggest that they are the electromagnetic counterpart of extreme mass ratio inspirals (EMRIs). In this scenario, the QPE rate would thus be the first ever constraint (i.e. a lower limit) to the EMRI rate from observations alone. Future discoveries of QPEs and advances in their theoretical modeling will consolidate or rule out their use for constraining the number of EMRIs detectable by the LISA mission.
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