X-ray absorption towards high-redshift sources: probing the intergalactic medium with blazars

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 ($\gtrsim10\,000$ 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 verify 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 perform 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 investigate the warm-hot IGM with a spectral model on the best candidates of our sample, obtaining an average IGM density of $n_0=1.01^{+0.53}_{-0.72}\times10^{-7}$ cm$^{-3}$ and temperature of $\log(T/\text{K})=6.45^{+0.51}_{-2.12}$. A more dedicated study is currently beyond reach, but our results can be used as a stepping stone for future more accurate analysis, involving Athena.Comment: 32 pages, 7 figures. Accepted, to be published in A&

Do stellar-mass and super-massive black holes have similar dining habits?

Through 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 disk and corona in the two source classes. We analyse 458 RXTE-PCA archival observations of the X-ray binary (XRB) GX339-4 focusing on the soft and soft-intermediate states, which have been suggested to be analogous to radiatively efficient AGN. The observed scatter in the $\log L_{disk}-\log L_{corona}$ relationship of GX339-4 is high ($\sim0.43\,$dex) and significantly larger than in a representative sample of radiatively-efficient, non- or weakly-jetted AGN ($\sim0.30\,$dex). On the face of it, this would appear contrary to the hypothesis that the systems simply scale with mass. On the other hand we also find that GX339-4 and our AGN sample show different $\dot{m}$ and $\Gamma$ distributions, with the latter being broader in GX339-4 (dispersion of $\sim0.16$ cf. $\sim0.08$ for AGN). GX339-4 also shows an overall softer slope, with mean $\sim2.20$ as opposed to $\sim2.07$ for the AGN sample. Remarkably, once similarly broad $\Gamma$ and $\dot{m}$ distributions are selected, the AGN sample overlaps nicely with GX339-4 observations in the mass-normalised $\log L_{disk}-\log L_{corona}$ plane, with a scatter of $\sim0.30-0.33\,$dex. This indicates that a mass-scaling of properties might hold after all, with our results being consistent with the disk-corona systems in AGN 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.Comment: Accepted for publication in Astronomy & Astrophysic

A Review of Web-Based Job Advertisements for Australian Event Management Positions

Strong growths in the Australian event management industry, ongoing technological changes and the internationalisation of the market place has spurred the need for appropriately educated and trained event managers and for a re-evaluation of educational and job training curriculum to meet these new challenges. In order for Australia to position itself as a world leader in event management, it is important to provide consistent high professional standards and event managers that not only meet, but exceed the demands of the industry. While there is some literature that focuses on the tourism and leisure job market (Crossley, 1992; Keung & Pine, 2000), and a small but developing literature base that focuses on event management training (Harris & Jago, 1999; Hawkins & Goldbatt, 1995) relatively little consideration has been given to a national agenda for event management skilling. To provide an indication of current employer requirements, a nationwide study of job advertisements in event management has commenced. The aims of the study are to further the understanding of the educational needs and training requirements of the industry; to ascertain the learned skills and personal attributes sought from event managers; to determine the compatibility of industry demands with current educational and vocational provisions; and to suggest post-secondary institutional avenues through which event management education and training needs can be pursued. This is an ongoing study and it is hoped that it will contribute towards a broad scale understanding of the event management job market. More importantly however, it can be used as the basis for curriculum evaluation and training needs, and create a better understanding and compatibility between event management education and industry practice. This paper reports the preliminary results from a content analysis of approximately 100 web-based job advertisements. Email alert accounts were established with several search engines to gather a sample of event management related job advertisements from around Australia. An analytical framework was devised for the analysis of the advertisements themselves. The results reveal several interesting trends including the geographical concentration of the event management job market, the range of industries that require event management specialists or event management skills, and a series of required skills and key attributes of event managers. The results of this study establish a platform from which to develop a classification of event management skills required by the industry

Disappearance of quasi-periodic-eruptions (QPEs) in GSN 069, simultaneous X-ray re-brightening, and predicted QPE re-appearance

We study the short- and long-timescale properties of quasi-periodic eruptions (QPEs) in GSN 069 and its overall X-ray evolution over the past 11 yr using 11 XMM-Newton and 1 Chandra observations from December 2010 to December 2021. QPEs are a transient phenomenon in GSN 069 last detected in January 2020 with a life-time between 1 and 5.5 yr. On short timescales, the QPE intensity and recurrence time oscillate defining alternating strong/weak QPEs and long/short recurrence times. The quiescent level variability in observations with QPEs exhibits a quasi-periodic oscillation (QPO) at the average observation-dependent recurrence time peaking with a delay of a few hr w.r.t. the preceding QPE. A significant late-time X-ray re-brightening starting with the QPE disappearance is observed in the long-term light curve of the quiescent emission, and the overall X-ray evolution follows the relation expected from constant-area blackbody emission. QPEs in GSN 069 are consistent with being produced by repeating tidal stripping events of a white dwarf (WD) donor in a highly eccentric orbit around the supermassive black hole, one QPE being produced at each pericenter passage. Our data suggest that the WD was partially disrupted when QPEs disappeared in GSN 069, giving rise to the observed X-ray re-brightening. We predict the re-appearance of QPEs in GSN 0699 in the near future with different recurrence times than currently detected QPEs, as the surviving core will again suffer a series of tidal stripping events at pericenter passage.Comment: Submitted to A&A. Comments welcom

Cosmic hide and seek: the volumetric rate of X-ray quasi-periodic eruptions

Multi-wavelength 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 $\mathscr{R}_{{\rm vol}} = 0.60_{-0.43}^{+4.73} \times 10^{-6}\,$Mpc$^{-3}$ above an intrinsic average $\log L_{\rm 0.5-2.0\,keV}^{\rm peak} > 41.7$. Since the exact lifetime of QPEs ($\tau_{\rm life}$) is currently not better defined than between a few years or few decades, we convert this to a formation rate of $\mathscr{R}_{\rm vol}/\tau_{\rm life}\approx 0.6 \times 10^{-7} (\tau_{\rm life}/10\,\mathrm{y})^{-1}\,$Mpc$^{-3}\,$year$^{-1}$. As a comparison, this value is a factor $\sim10\,\tau_{\rm life}$ 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.Comment: Accepted for publication in A&A. Comments welcom

Tormund's return: Hints of quasi-periodic eruption features from a recent optical tidal disruption event

Quasi-periodic eruptions (QPEs) are repeating thermal X-ray bursts associated with accreting massive black holes, the precise underlying physical mechanisms of which are still unclear. We present a new candidate QPE source, AT 2019vcb (nicknamed Tormund by the ZTF collaboration), which was found during an archival search for QPEs in the XMM-Newton archive. It was first discovered in 2019 as an optical tidal disruption event (TDE) at $z=0.088$, and its X-ray follow-up exhibited QPE-like properties. Our goals are to verify its robustness as QPE candidate and to investigate its properties to improve our understanding of QPEs. We performed a detailed study of the X-ray spectral behaviour of this source over the course of the XMM-Newton archival observation. We also report on recent Swift and NICER follow-up observations to constrain the source's current activity and overall lifetime, as well as an optical spectral follow-up. The first two Swift detections and the first half of the 30 ks XMM-Newton exposure of Tormund displayed a decaying thermal emission typical of an X-ray TDE. However, the second half of the exposure showed a dramatic rise in temperature (from 53 to 114 eV) and 0.2-2 keV luminosity (from $3.2\times10^{42}$ to $1.2\times10^{44}$ erg s$^{-1}$). The late-time NICER follow-up indicates that the source is still X-ray bright more than three years after the initial optical TDE. Although only a rise phase was observed, Tormund's strong similarities with a known QPE source (eRO-QPE1) and the impossibility to simultaneously account for all observational features with alternative interpretations allow us to classify Tormund as a candidate QPE. If confirmed as a QPE, it would further strengthen the observational link between TDEs and QPEs. It is also the first QPE candidate for which an associated optical TDE was directly observed, constraining the formation time of QPEs.Comment: Accepted for publication in A&A, 20 pages, 17 figure

Ticking away: The long-term X-ray timing and spectral evolution of eRO-QPE2

Quasi-periodic eruptions (QPEs) are repeated X-ray flares from galactic nuclei that recur every few hours to days, depending on the source. Despite some diversity in the recurrence and amplitude of eruptions, their striking regularity has motivated theorists to associate QPEs with orbital systems. Among the known QPE sources, eRO-QPE2 has shown the most regular flare timing and luminosity since its discovery. We report here on its long-term evolution over 3.3 yr from discovery and find that: i) the average QPE recurrence time per epoch has decreased over time, albeit not at a uniform rate; ii) the distinct alternation between consecutive long and short recurrence times found at discovery has not been significant since; iii) the spectral properties, namely flux and temperature of both eruptions and quiescence components, have remained remarkably consistent within uncertainties. We attempted to interpret these results as orbital period and eccentricity decay coupled with orbital and disk precession. However, since gaps between observations are too long, we are not able to distinguish between an evolution dominated by just a decreasing trend, or by large modulations (e.g. due to the precession frequencies at play). In the former case, the observed period decrease is roughly consistent with that of a star losing orbital energy due to hydrodynamic gas drag from disk collisions, although the related eccentricity decay is too fast and additional modulations have to contribute too. In the latter case, no conclusive remarks are possible on the orbital evolution and the nature of the orbiter due to the many effects at play. However, these two cases come with distinctive predictions for future X-ray data: in the case of a decreasing trend, we expect all future observations to show a shorter recurrence time than the latest epoch, while in the case of large-amplitude modulations we expect some future observations to be found with a larger recurrence, hence an apparent temporary period increase

Quasi-periodic X-ray eruptions and tidal disruption events prefer similar host galaxies

In the past five years, six quasi-periodic X-ray eruption (QPE) sources have been discovered in the nuclei of nearby galaxies. Their origin remains an open question. We present MUSE integral field spectroscopy of five QPE host galaxies to characterize their properties. We find that 3/5 galaxies host extended emission line regions (EELRs) up to 10 kpc in size. The EELRs are photo-ionized by a non-stellar continuum, but the current nuclear luminosity is insufficient to power the observed emission lines. The EELRs are decoupled from the stars both kinematically and in projected sky position, and the low velocities and velocity dispersions ($<$ 100 km s$^{-1}$ and $\lesssim 75$ km s$^{-1}$ respectively) are inconsistent with being AGN- or shock-driven. The origin of the EELRs is likely a previous phase of nuclear activity. The QPE host galaxy properties are strikingly similar to those of tidal disruption events (Wevers et al. submitted). The preference for a very short-lived (the typical EELR lifetime is $\sim$15000 years), gas-rich phase where the nucleus has recently faded significantly suggests that TDEs and QPEs may share a common formation channel, disfavoring AGN accretion disk instabilities as the origin of QPEs. In the assumption that QPEs are related to extreme mass ratio inspiral systems (EMRIs; stellar-mass objects on bound orbits about massive black holes), the high incidence of EELRs and recently faded nuclear activity can be used to aid in the localization of the host galaxies of EMRIs discovered by low frequency gravitational wave observatories.Comment: Submitted to ApJ Letters. In a companion paper we present an IFU analysis of TDE host galaxies (Wevers & French

Live to die another day: the rebrightening of AT2018fyk as a repeating partial tidal disruption event

Stars that interact with supermassive black holes (SMBHs) can either be 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 AT2018fyk, with an inferred black hole mass of $10^{7.7\pm0.4}$ M$_{\odot}$, experienced an extreme dimming event at X-ray (factor of $>$6000) and UV (factor $\sim$15) wavelengths $\sim$500--600 days after discovery. Here we report on the re-emergence of these emission components roughly 1200 days after discovery. We find that the source properties are similar to those of the pre-dimming accretion state, suggesting that the accretion flow was rejuvenated to a similar state. We propose that a repeating partial TDE, where the partially disrupted star is on a $\sim 1200$ day orbit about the SMBH and is periodically stripped of mass during each pericenter passage, powers its unique lightcurve. This scenario provides a plausible explanation for AT2018fyk'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 (August 2023), and a subsequent rebrightening around day 2400 (March 2025). This source provides strong evidence of the partial disruption of a star by a SMBH.Comment: ApJ Letters, accepted for publicatio

X-ray emission from a rapidly accreting narrow-line Seyfert 1 galaxy at z = 6.56

The space density of X-ray-luminous, blindly selected active galactic nuclei (AGN) traces the population of rapidly accreting super-massive black holes through cosmic time. It is encoded in the X-ray luminosity function, whose bright end remains poorly constrained in the first billion years after the Big Bang as X-ray surveys have thus far lacked the required cosmological volume. With the eROSITA Final Equatorial-Depth Survey (eFEDS), the largest contiguous and homogeneous X-ray survey to date, X-ray AGN population studies can now be extended to new regions of the luminosity-redshift space (L2-10 keV &gt; 1045 erg s-1 and z &gt; 6). Aims. The current study aims at identifying luminous quasars at z &gt; 5:7 among X-ray-selected sources in the eFEDS field in order to place a lower limit on black hole accretion well into the epoch of re-ionisation. A secondary goal is the characterisation of the physical properties of these extreme coronal emitters at high redshifts. Methods. Cross-matching eFEDS catalogue sources to optical counterparts from the DESI Legacy Imaging Surveys, we confirm the low significance detection with eROSITA of a previously known, optically faint z = 6:56 quasar from the Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs) survey. We obtained a pointed follow-up observation of the source with the Chandra X-ray telescope in order to confirm the low-significance eROSITA detection. Using new near-infrared spectroscopy, we derived the physical properties of the super-massive black hole. Finally, we used this detection to infer a lower limit on the black hole accretion density rate at z &gt; 6. Results. The Chandra observation confirms the eFEDS source as the most distant blind X-ray detection to date. The derived X-ray luminosity is high with respect to the rest-frame optical emission of the quasar.With a narrow Mgii line, low derived black hole mass, and high Eddington ratio, as well as its steep photon index, the source shows properties that are similar to local narrow-line Seyfert 1 galaxies, which are thought to be powered by young super-massive black holes. In combination with a previous high-redshift quasar detection in the field, we show that quasars with L2-10 keV &gt; 1045 erg s-1 dominate accretion onto super-massive black holes at z _ 6