arXiv subjects

D. Homan

Publications and source records attributed to D. Homan.

At least 19 recordsLinked to original sources

arXiv2026

SRG/eROSITA X-ray selected cataclysmic variable candidates observed in SDSS-V DR20

We report on the spectroscopic observations obtained during SDSS-V DR20 of accreting compact binaries (ACB), specifically the cataclysmic variables (CVs), that were identified as likely compact binary candidates from the SRG (Spectrum Roentgen Gamma) eROSITA eRASS1 and eRASS:3 observations. Our primary aim is to obtain a complete inventory of all CVs that were detected in eRASS1 and eRASS:3, with the goal to help better understand close-binary evolution, the population density, and demographics of these systems in the Galaxy. Previous population studies had their respective limitations, with volume-limited samples suffering from low-number statistics, while magnitude-limited observations were biased to only the brighter systems. All CVs are X-ray emitters, eROSITA, therefore, presents a unique opportunity to identify CVs based on their X-ray emission. Given eROSITA's sensitivity, we expect to find most X-ray active CVs within 500 pc, and magnetic CVs to several kpc. Using X-ray data from the eROSITA together with optical data from Gaia, three different approaches were explored to identify the Gaia optical counterpart to the eROSITA X-ray source of the likely ACB. From this, unique candidates were identified and were submitted in three different cartons to SDSS-V for optical spectroscopic observations as part of the Milky Way Mapper survey. From our submitted candidates, we found 538 likely CVs. We also identified CVs that were observed in other eROSITA based cartons, in which we identified an additional 49 systems that are likely CVs and which were not in our selection. We therefore identified 587 systems as CVs from the eROSITA based SDSS-V observations. We also attempted to sub classify the CVs as dwarf-novae, novalikes, or magnetic systems based on the eROSITA X-ray data, the emission line properties in the optical SDSS-V spectra, and other data in the public domain, if available.

astro-ph.SR↗

arXiv2026

Large-amplitude modulations and hours-timescale variability in the early X-ray light curve of a tidal disruption flare

We present new X-ray, optical, and UV observations of the tidal disruption event candidate eRASSt J234402.9-352640, (hereafter J2344). Between 50 and 60 days after peak optical brightness, J2344 exhibited large-amplitude modulations in its 0.2-2 keV emission, when the flux repeatedly dimmed and re-brightened by a factor of ~6, over a ~3-day timescale. These modulations exhibited harder-when-brighter behaviour but were not detected in high-cadence observations obtained 60-70 days and 170-200 days after peak optical brightness, when the system instead exhibited stochastic X-ray variability over timescales of hours. We discuss the different physical mechanisms responsible for such exotic X-ray variability and explore the possibility that the modulations in J2344 were caused by the Lense-Thirring precession of the inner accretion flow around the disrupting black hole.

astro-ph.HE↗

arXiv2025

BlazEr1: The eROSITA Blazar Catalog. Blazars and Blazar Candidates in the First eROSITA Survey

Aims. eROSITA, on board the Spectrum Roentgen Gamma (SRG) spacecraft, performed its first X-ray all-sky survey (eRASS1) between December 2019 and June 2020. It detected about 930000 sources, providing us with an unprecedented opportunity for a detailed blazar census. We present the properties of blazars and blazar candidates in eRASS1 and the compilation of the eROSITA blazar catalog. Methods. We compiled a list of blazar and blazar candidates from the literature and matched it with the eRASS1 catalog, constructing the Blazars in eRASS1 (BlazEr1) catalog. For sources with more than 50 counts, we obtained their X-ray spectral properties. We compiled multiwavelength data from the radio to the gamma-ray regimes for all sources, including multiwavelength spectral indices and redshifts. The full catalog is available online. Results. We present the BlazEr1 catalog, containing 5865 sources, of which 2106 are associated with confirmed blazars. For 2966 sources, eROSITA provides the first X-ray data. The contamination from non-blazar sources of the entire sample is less than 11%. Most candidates exhibit properties typical for blazars. We present the properties of the entire X-ray detected blazar population, including the distributions of X-ray luminosities and photon indices, multiwavelength properties, and the blazar log N-log S distribution. Our catalog provides follow up targets, such as potential MeV and TeV blazars. Conclusions. The BlazEr1 catalog provides a compilation of X-ray detected blazars and blazar candidates. The catalog serves as a starting point for exploiting further eROSITA surveys using the same methodology, enabling us to study the X-ray variability and a large number of spectral energy distributions of blazars in the future.

astro-ph.HE↗

arXiv2025

Counterpart identification and classification for eRASS1 and characterisation of the AGN content

[abridged] Accurately accounting for the AGN phase in galaxy evolution requires a large, clean AGN sample. This is now possible with SRG/eROSITA. The public Data Release 1 (DR1, Jan 31, 2024) includes 930,203 sources from the Western Galactic Hemisphere. The data enable the selection of a large AGN sample and the discovery of rare sources. However, scientific return depends on accurate characterisation of the X-ray emitters, requiring high-quality multiwavelength data. This paper presents the identification and classification of optical and infrared counterparts to eRASS1 sources using Gaia DR3, CatWISE2020, and Legacy Survey DR10 (LS10) with the Bayesian NWAY algorithm and trained priors. Sources were classified as Galactic or extragalactic via a Machine Learning model combining optical/IR and X-ray properties, trained on a reference sample. For extragalactic LS10 sources, photometric redshifts were computed using Circlez. Within the LS10 footprint, all 656,614 eROSITA/DR1 sources have at least one possible optical counterpart; about 570,000 are extragalactic and likely AGN. Half are new detections compared to AllWISE, Gaia, and Quaia AGN catalogues. Gaia and CatWISE2020 counterparts are less reliable, due to the surveys shallowness and the limited amount of features available to assess the probability of being an X-ray emitter. In the Galactic Plane, where the overdensity of stellar sources also increases the chance of associations, using conservative reliability cuts, we identify approximately 18,000 Gaia and 55,000 CatWISE2020 extragalactic sources. We release three high-quality counterpart catalogues, plus the training and validation sets, as a benchmark for the field. These datasets have many applications, but in particular empower researchers to build AGN samples tailored for completeness and purity, accelerating the hunt for the Universes most energetic engines.

astro-ph.HE↗

arXiv2025

Searching for supermassive black holes binaries within SRG/eROSITA-De I: Properties of the X-ray selected candidates

Abridged: Supermassive black hole binaries (SMBHBs) separated by (sub)-pc scales represent one of the latest stages of hierarchical galaxy assembly. However, many of these objects are hidden behind large columns of gas and dust at the center of galaxies and are difficult to detect. The X-ray and UV emission in these systems are predicted to vary regularly on timescales comparable to that of the orbital period of the binary. This is the first of a series of papers where we aim to find SMBHB candidates based on quasi-periodic light curves from the soft X-ray instrument eROSITA on board the Spectrum-Roentgen-Gamma (SRG) observatory and X-ray follow-up. We searched the multi-epoch SRG/eROSITA all-sky surveys for extragalactic sources that show an `up-down-up-down' or `down-up-down-up' profile (from scan to scan) in their 0.2--2.3 keV flux light curves. We compiled a sample of 16 sources that are suitable for X-ray follow-up campaigns given their brightness and significant variability between bright and faint SRG/eROSITA flux levels. We triggered extensive Swift-XRT and NICER monitoring campaigns on the best SMBHB candidates to confirm or discard their tentative periodicities. Optical spectroscopic observations confirmed the nuclear and extragalactic nature of 15/16 objects and enabled single-epoch SMBH mass measurements and BPT classifications. Our most promising candidate, eRASSt J0530-4125, shows X-ray quasi-periodic variability with a typical time scale of one year in the observed frame. By stacking the X-ray observations of each source in our sample, we find that 14/15 sources can be modeled by a power law with a photon index ranging from $\Gamma\sim1.8-2.8$. Based on our selection, we estimate an optimistic upper limit on the fraction of SMBHB candidates to be $\sim 0.05$ per galaxy. We emphasize that further observational evidence is needed to confirm the SMBHB nature of our sources.

astro-ph.HE↗

arXiv2024

An X-ray flaring event and a variable soft X-ray excess in the Seyfert LCRS B040659.9-385922 as detected with eROSITA

Extreme continuum variability in AGNs can indicate extreme changes in accretion flows onto supermassive black holes. We explore the multiwavelength nature of a continuum flare in the Seyfert LCRS B040659.9$-$385922. The all-sky X-ray surveys conducted by the eROSITA showed that its X-ray flux increased by a factor of roughly five over six months, and concurrent optical photometric monitoring with the ATLAS showed a simultaneous increase. We triggered a multiwavelength follow-up monitoring program (XMM, NICER; optical spectroscopy) to study the evolution of the accretion disk, broad-line region, and X-ray corona. During the campaign, X-ray and optical continuum flux subsided over roughly six months. We detected a soft X-ray excess near the flare peak and after it subsided, both exhibiting a power-law (nonthermal) behavior. We modeled the broadband optical/UV/X-ray spectral energy distribution at both the flare peak and post-flare times with the AGNSED model, incorporating thermal disk emission into the optical/UV and warm thermal Comptonization in the soft X-rays. Additionally, we find that the broad Heii $\lambda$4686 emission line fades significantly as the optical/UV/X-ray continuum fades, which could indicate a substantial flare of disk emission above 54 eV. We also observed a redshifted broad component in the H${\beta}$ emission line that is present during the high flux state of the source and disappears in subsequent observations. We witnessed a likely sudden strong increase in local accretion rate, which manifested itself via an increase in accretion disk emission and thermal Comptonization emission in the soft X-rays, followed by a decrease in accretion and Comptonized luminosity. The physical processes leading to such substantial variations are still an open question, and future continuous monitoring along with multi-wavelength studies will shed some light on it.

astro-ph.HE↗

arXiv2024

SDSS1335+0728: The awakening of a $\sim 10^6 M_{\odot}$ black hole

The galaxy SDSS1335+0728, which had exhibited no prior optical variations during the preceding two decades, began showing significant nuclear variability in the Zwicky Transient Facility (ZTF) alert stream from December 2019 (as ZTF19acnskyy). Its behaviour suggests that SDSS1335+0728 hosts a $\sim 10^6 M_{\odot}$ black hole (BH) that is currently in the process of `turning on'. We present a multi-wavelength photometric analysis and spectroscopic follow-up performed with the aim of better understanding the origin of the nuclear variations detected in SDSS1335+0728. We used archival photometry and spectroscopic data to study the state of SDSS1335+0728 prior to December 2019, and new observations from Swift, SOAR/Goodman, VLT/X-shooter, and Keck/LRIS taken after its turn-on to characterise its current state. We find that: (a) since 2021, the UV flux is four times brighter than the flux reported by GALEX in 2004; (b) since June 2022, the mid-infrared flux has risen more than two times, and the W1-W2 WISE colour has become redder; (c) since February 2024, the source has begun showing X-ray emission; (d) the narrow emission line ratios are now consistent with a more energetic ionising continuum; (e) broad emission lines are not detected; and (f) the [OIII] line increased its flux $\sim 3.6$ years after the first ZTF alert, which implies a relatively compact narrow-line-emitting region. We conclude that the variations observed in SDSS1335+0728 could be either explained by an AGN that is just turning on or by an exotic tidal disruption event (TDE). If the former is true, SDSS1335+0728 is one of the strongest cases of an AGN observed in the process of activating. If the latter, it would correspond to the longest and faintest TDE ever observed (or another class of still unknown nuclear transient). Future observations of SDSS1335+0728 are crucial to further understand its behaviour.

astro-ph.GA↗

arXiv2024

A radio flare associated with the nuclear transient eRASSt J234403-352640: an outflow launched by a potential tidal disruption event

We present an extensive radio monitoring campaign of the nuclear transient eRASSt J234403-352640 with the Australia Telescope Compact Array, one of the most X-ray luminous TDE candidates discovered by the SRG/eROSITA all-sky survey. The observations reveal a radio flare lasting more than 1000 d, coincident with the X-ray, UV, optical, and infra-red flare of this transient event. Through modelling of the 10 epochs of radio spectral observations obtained, we find that the radio emission is well-described by an expanding synchrotron emitting region, consisting of a single ejection of material launched coincident with the optical flare. We conclude that the radio flare properties of eRASSt J234403-352640 are consistent with the population of radio-emitting outflows launched by non-relativistic tidal disruption events, and that the flare is likely due to an outflow launched by a tidal disruption event (but could also be a due to a new AGN accretion event) in a previously turned-off AGN.

astro-ph.HE↗

arXiv2024

The eROSITA Upper Limits: Description and access to the data

The soft X-ray instrument eROSITA on board the Spectrum-Roentgen-Gamma (SRG) observatory has successfully completed four of the eight planned all-sky surveys, detecting almost one million X-ray sources during the first survey (eRASS1). The catalog of this survey will be released as part of the first eROSITA data release (DR1). Based on X-ray aperture photometry, we provide flux upper limits for eRASS1 in several energy bands. We cover galactic longitudes between $180^{\circ}\lesssim l \lesssim 360^{\circ}$ (eROSITA-DE). These data are crucial for studying the X-ray properties of variable and transient objects, as well as non-detected sources in the eROSITA all-sky survey data. We performed aperture photometry on every pixel of the SRG/eROSITA standard pipeline data products for all available sky tiles in the single detection band ($0.2 - 2.3$ keV). Simultaneously, we performed the same analysis in the three-band detection at soft ($0.2-0.6$ keV), medium ($0.6-2.3$ keV), and hard ($2.3-5.0$ keV) energy bands. Based on the combination of products for the individual bands, we are also able to provide aperture photometry products and flux upper limits for the $0.2 - 5.0$ keV energy band. The upper limits were calculated based on a Bayesian approach that utilizes detected counts and background within the circular aperture. The final data products consist of tables with the aperture photometry products (detected counts, background counts, and exposure time), a close-neighbor flag, and the upper flux limit based on an absorbed power-law spectral model ($\Gamma=2.0, \; N_{\rm H}=3\times10^{20}$ cm$^{-2}$). The upper limits are calculated using the one-sided $3\sigma$ confidence interval (CL) of a normal distribution, representing CL = 99.87\%. The aperture photometry products allow for an easy computation of upper limits at any other confidence interval and spectral model. ...

astro-ph.HE↗

arXiv2024

The SRG/eROSITA all-sky survey: First X-ray catalogues and data release of the western Galactic hemisphere

The eROSITA telescope array aboard the Spektrum Roentgen Gamma (SRG) satellite began surveying the sky in December 2019, with the aim of producing all-sky X-ray source lists and sky maps of an unprecedented depth. Here we present catalogues of both point-like and extended sources using the data acquired in the first six months of survey operations (eRASS1; completed June 2020) over the half sky whose proprietary data rights lie with the German eROSITA Consortium. We describe the observation process, the data analysis pipelines, and the characteristics of the X-ray sources. With nearly 930000 entries detected in the most sensitive 0.2-2.3 keV energy range, the eRASS1 main catalogue presented here increases the number of known X-ray sources in the published literature by more than 60%, and provides a comprehensive inventory of all classes of X-ray celestial objects, covering a wide range of physical processes. A smaller catalogue of 5466 sources detected in the less sensitive but harder 2.3-5 keV band is the result of the first true imaging survey of the entire sky above 2 keV. We show that the number counts of X-ray sources in eRASS1 are consistent with those derived over narrower fields by past X-ray surveys of a similar depth, and we explore the number counts variation as a function of the location in the sky. Adopting a uniform all-sky flux limit (at 50% completeness) of F_{0.5-2 keV} > 5 \times 10^{-14}$ erg\,s$^{-1}$\,cm$^{-2}$, we estimate that the eROSITA all-sky survey resolves into individual sources about 20% of the cosmic X-ray background in the 1-2 keV range. The catalogues presented here form part of the first data release (DR1) of the SRG/eROSITA all-sky survey. Beyond the X-ray catalogues, DR1 contains all detected and calibrated event files, source products (light curves and spectra), and all-sky maps. Illustrative examples of these are provided.

astro-ph.HE↗

arXiv2024

Rapid evolution of the recurrence time in the repeating partial tidal disruption event eRASSt J045650.3-203750

In this letter, we present the results from further X-ray and UV observations of the nuclear transient eRASSt J045650.3-203750 (hereafter J0456-20). We detected five repeating X-ray and UV flares from J0456-20, making it one of the most promising repeating partial tidal disruption event (pTDE) candidates. More importantly, we also found rapid changes in the recurrence time $T_\text{recur}$ of the X-ray flares by modelling the long-term X-ray light curve of J0456-20. $T_\text{recur}$ first decreased rapidly from about 300 days to around 230 days. It continued to decrease to around 190 days with an indication of a constant $T_\text{recur}$ evidenced from the latest three cycles. Our hydrodynamic simulations suggest that, in the repeating pTDE scenario, such rapid evolution of $T_\text{recur}$ could be reproduced if the original star is a $1~\mathrm{M}_\odot$ main-sequence star near the terminal age and loses nearly 80-90% of its mass during the initial encounter with a supermassive black hole (SMBH) of mass around $10^5~\mathrm{M}_\odot$. The inferred mass loss of 0.8-0.9 $\mathrm{M}_\odot$ is higher than the estimated value of around 0.13 $\mathrm{M}_\odot$ from observation, which could be explained if the radiation efficiency is low (i.e. $\ll0.1$). Our results indicate that repeating pTDEs could be effective tools to explore the dynamics around supermassive black holes beyond our own Galaxy.

astro-ph.HE↗

arXiv2023

Multiwavelength study of extreme variability in LEDA 1154204: A changing-look event in a type 1.9 Seyfert

Context. Multiwavelength studies of transients in actively accreting supermassive black holes have revealed that large-amplitude variability is frequently linked to significant changes in the optical spectra -- a phenomenon known as changing-look AGN (CLAGN).} Aims. In 2020, the Zwicky Transient Facility detected a transient flaring event in the type 1.9 AGN LEDA 1154204, wherein brightness sharply increased by 0.55 mag in one month, then began to decay. Spectrum Roentgen Gamma (SRG)/eROSITA also observed the object as part of its all-sky X-ray surveys, after the flare had started decaying. Methods. We performed a three-year, multiwavelength follow-up campaign to track the source's spectral and temporal characteristics, during the post-flare fading. This campaign included optical spectroscopy, X-ray spectroscopy and photometry, and UV, optical, and IR continuum photometry. Results. Optical spectra taken near the flare peak revealed a broad double-peaked H$\beta$ emission and a blue continuum, both undetected in a 2005 archival spectrum; broad H$\beta$ had increased by a factor $>$5--6. Then, from late 2020 through 2023, broad Balmer line flux faded as the continuum faded, with Balmer decrement increasing by $\sim$2.2, consistent with the expected ionization response. The X-ray spectrum exhibits no significant spectral variability despite dramatic flux variation -- a factor of 17. There is no evidence of a soft X-ray excess, indicating an energetically unimportant warm corona. Conclusions. The transient event was likely triggered by a disk instability in a pre-existing AGN-like accretion flow, culminating in the observed multiwavelength variability -- X-rays via thermal Comptonization, BLR illumination, and IR dust echo -- and CLAGN event.

astro-ph.HE↗

arXiv2023

Still alive and kicking: A significant outburst in changing-look AGN Mrk 1018

Changing-look active galactic nuclei (CL-AGN) have been observed to change optical spectral type. Mrk 1018 is unique: first classified as a type 1.9 Seyfert galaxy, it transitioned to a type 1 before returning to its initial classification after approximately 30 years. We present a high-cadence monitoring programme that caught a major outburst in 2020. Due to sunblock, only the decline could be observed. We studied X-ray, UV, optical, and IR before and after the outburst to investigate the responses of the AGN structures. We derived a u'-band light curve of the AGN contribution alone. The flux increased by a factor of the order of 13. We confirmed this in other optical bands and determined the shape and speed of the decline in each waveband. The shapes of H beta and H alpha were analysed before and after the event. Two XMM-Newton observations from before and after the outburst were also exploited. The outburst is asymmetric, with a swifter rise than decline. The decline is best fit by a linear function, ruling out a tidal disruption event. The optical spectrum shows no change approximately 8 months before and 17 months after. The UV flux increased slightly after the outburst but the X-ray primary flux is unchanged. However, the 6.4 keV Iron line has doubled in strength. IR data taken 13 days after the observed optical peak show an increased emission level. Calculating the distance of the broad-line region and inner edge of the torus from the supermassive black hole can explain the multi-wavelength response to the outburst, in particular: i) the unchanged H beta and H alpha lines, ii) the unchanged primary X-ray spectral components, iii) the rapid and extended infrared response, as well as iv) the enhanced emission of the reflected 6.4 keV line. The outburst was due to a dramatic and short-lasting change in the intrinsic accretion rate. We discuss different models as potential causes.

astro-ph.HE↗

arXiv2023

Improving the selection of changing-look AGNs through multi-wavelength photometric variability

We present second epoch optical spectra for 30 changing-look (CL) candidates found by searching for Type-1 optical variability in a sample of active galactic nuclei (AGNs) spectroscopically classified as Type 2. We use a random-forest-based light curve classifier and spectroscopic follow-up, confirming 50 per cent of candidates as turning-on CLs. In order to improve this selection method and to better understand the nature of the not-confirmed CL candidates, we perform a multi-wavelength variability analysis including optical, mid-infrared (MIR) and X-ray data, and compare the results from the confirmed and not-confirmed CLs identified in this work. We find that most of the not-confirmed CLs are consistent with weak Type 1s dominated by host-galaxy contributions, showing weaker optical and MIR variability. On the contrary, the confirmed CLs present stronger optical fluctuations and experience a long (from five to ten years) increase in their MIR fluxes and the colour W1-W2 over time. In the 0.2-2.3 keV band, at least four out of 11 CLs with available SRG/eROSITA detections have increased their flux in comparison with archival upper limits. These common features allow us to select the most promising CLs from our list of candidates, leading to nine sources with similar multi-wavelength photometric properties to our CL sample. The use of machine learning algorithms with optical and MIR light curves will be very useful to identify CLs in future large-scale surveys.

astro-ph.GA↗

arXiv2023

Discovery of the luminous X-ray ignition eRASSt J234402.9$-$352640; I. Tidal disruption event or a rapid increase in accretion in an active galactic nucleus?

In November 2020, a new, bright object, eRASSt J234402.9$-$352640, was discovered in the second all-sky survey of SRG/eROSITA. The object brightened by a factor of at least 150 in 0.2--2.0 keV flux compared to an upper limit found six months previous, reaching an observed peak of $1.76_{-0.24}^{+0.03} \times 10^{-11}$ erg cm$^{-2}$ s$^{-1}$. The X-ray ignition is associated with a galaxy at $z=0.10$, making the peak luminosity log$_{10}(L_{\rm 0.2-2keV}/[\textrm{erg s}^{-1}])$=$44.7\pm0.1$. Around the time of the rise in X-ray flux, the nucleus of the galaxy brightened by approximately 3 mag. in optical photometry, after correcting for the host. We present data from Swift, XMM-Newton, and NICER, which reveal a very soft spectrum as well as strong 0.2--2.0 keV flux variability on multiple timescales. Optical spectra taken in the weeks after the ignition event show a blue continuum with broad, asymmetric Balmer emission lines, and high-ionisation ([OIII]$\lambda\lambda$4959,5007) and low-ionisation ([NII]$\lambda$6585, [SII]$\lambda\lambda$6716,6731) narrow emission lines. Following the peak in the optical light curve, the X-ray, UV, and optical photometry all show a rapid decline. The X-ray light curve shows a decrease in luminosity of $\sim$0.45 over 33 days and the UV shows a drop of $\sim$0.35. eRASSt J234402.9$-$352640 also shows a brightening in the mid-infrared, likely powered by a dust echo of the luminous ignition. We find no evidence in Fermi-LAT $\gamma$-ray data for jet-like emission. The event displays characteristics of a tidal disruption event (TDE) as well as of an active galactic nucleus (AGN), complicating its classification. Based on the softness of the X-ray spectrum, the presence of high-ionisation optical emission lines, and the likely infrared echo, we find that a TDE within a turned-off AGN best matches our observations.

astro-ph.HE↗

arXiv2023

The rebrightening of a ROSAT-selected tidal disruption event: repeated weak partial disruption flares from a quiescent galaxy?

The ROSAT-selected tidal disruption event (TDE) candidate RX J133157.6-324319.7 (J1331), was detected in 1993 as a bright (0.2-2 keV flux of $(1.0 \pm 0.1) \times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$), ultra-soft ($kT=0.11 \pm 0.03$ keV) X-ray flare from a quiescent galaxy ($z=0.05189$). During its fifth All-Sky survey (eRASS5) in 2022, SRG/eROSITA detected the repeated flaring of J1331, where it had rebrightened to an observed 0.2-2 keV flux of $(6.0 \pm 0.7) \times 10^{-13}$ erg s$^{-1}$ cm$^{-2}$, with spectral properties ($kT=0.115 \pm 0.007$ keV) consistent with the ROSAT-observed flare $\sim$30 years earlier. In this work, we report on X-ray, UV, optical, and radio observations of this system. During a pointed XMM observation $\sim$17 days after the eRASS5 detection, J1331 was not detected in the 0.2-2 keV band, constraining the 0.2-2 keV flux to have decayed by a factor of $\gtrsim$40 over this period. Given the extremely low probability ($\sim5\times 10^{-6}$) of observing two independent full TDEs from the same galaxy over a 30 year period, we consider the variability seen in J1331 to be likely caused by two partial TDEs involving a star on an elliptical orbit around a black hole. J1331-like flares show faster rise and decay timescales ($\mathcal{O}(\mathrm{days})$) compared to standard TDE candidates, with neglible ongoing accretion at late times post-disruption between outbursts.

astro-ph.HE↗

arXiv2023

eRASSt J074426.3+291606: prompt accretion disc formation in a 'faint and slow' tidal disruption event

We report on multi-wavelength 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 $\sim 4$) 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 $\sim$160 relative to an archival 3$\sigma$ upper limit inferred from a serendipitous Chandra pointing in 2011. The transient was also independently found in the optical by the Zwicky Transient Factory (ZTF), with the eRASS2 detection occurring only $\sim$20 days after the peak optical brightness, suggesting that the accretion disc formed promptly in this TDE. Continued X-ray monitoring over the following $\sim$400 days by eROSITA, NICER XTI and Swift XRT showed a net decline by a factor of $\sim$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 $\sim$50 during an 80 day period. Contemporaneous Swift UVOT observations during this extreme X-ray variability reveal a relatively smooth decline, which persists over $\sim$400 days post-optical peak. The peak observed optical luminosity (absolute $g$-band magnitude $\sim -16.8$ 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 timescale $\sim$120~days), making J0744 the first member of a potential new class of `faint and slow' TDEs.

astro-ph.HE↗

arXiv2022

The long-term broad-line responsivity in MKN 110

We examine the long-term history of the optical spectrum of the extremely variable Active Galactic Nucleus (AGN) MKN 110. By combining various archival data with new data, we cover an unprecedented long period of $\sim$30 years (1987 - 2019). We find that the He II $\lambda 4686$ emission line changes by a factor of forty and varies more strongly than the optical continuum. Following Ferland et al. (2020), we take He II $\lambda 4686$ as a proxy for the FUV continuum and compare the flux of several other line species against it. This comparison reveals a clear pattern, whereby lines respond close to linearly at low FUV fluxes, and saturate at high FUV fluxes. The saturation level of the response appears to depend on the excitation energy of the line species. In addition to this global pattern, we note changes among observational epochs, indicating a structural evolution in the broad line region (BLR). The line profiles in our spectra show an offset between the narrow and broad components of the He II $\lambda 4686$ and H$\beta$ lines. This offset shows a significant negative correlation with the FUV flux and a positive correlation with the line velocity width. Our analysis reveals a complex BLR response to a changing continuum. The clear presence of a non-responsive component of the broad lines indicates the existence of multiple contributions to the line emission. We find there are several kinematic models of the BLR and inner regions of the AGN that match our data.

astro-ph.GA↗