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  • 1
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    The eROSITA Final Equatorial-Depth Survey (eFEDS) : Identification and characterization of the counterparts to point-like sources
    EDP Sciences ; 2022
    In:  Astronomy & Astrophysics Vol. 661 ( 2022-5), p. A3-
    Details
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 661 ( 2022-5), p. A3-
    Abstract: Context. In November 2019, eROSITA on board of the Spektrum-Roentgen-Gamma (SRG) observatory started to map the entire sky in X-rays. After the four-year survey program, it will reach a flux limit that is about 25 times deeper than ROSAT. During the SRG performance verification phase, eROSITA observed a contiguous 140 deg 2 area of the sky down to the final depth of the eROSITA all-sky survey (eROSITA Final Equatorial-Depth Survey; eFEDS), with the goal of obtaining a census of the X-ray emitting populations (stars, compact objects, galaxies, clusters of galaxies, and active galactic nuclei) that will be discovered over the entire sky. Aims. This paper presents the identification of the counterparts to the point sources detected in eFEDS in the main and hard samples and their multi-wavelength properties, including redshift. Methods. To identifyy the counterparts, we combined the results from two independent methods ( NWAY and ASTROMATCH ), trained on the multi-wavelength properties of a sample of 23k XMM-Newton sources detected in the DESI Legacy Imaging Survey DR8. Then spectroscopic redshifts and photometry from ancillary surveys were collated to compute photometric redshifts. Results. Of the eFEDS sources, 24 774 of 27 369 have reliable counterparts (90.5%) in the main sample and 231 of 246 sourcess (93.9%) have counterparts in the hard sample, including 2514 (3) sources for which a second counterpart is equally likely. By means of reliable spectra, Gaia parallaxes, and/or multi-wavelength properties, we have classified the reliable counterparts in both samples into Galactic (2695) and extragalactic sources (22 079). For about 340 of the extragalactic sources, we cannot rule out the possibility that they are unresolved clusters or belong to clusters. Inspection of the distributions of the X-ray sources in various optical/IR colour-magnitude spaces reveal a rich variety of diverse classes of objects. The photometric redshifts are most reliable within the KiDS/VIKING area, where deep near-infrared data are also available. Conclusions. This paper accompanies the eROSITA early data release of all the observations performed during the performance and verification phase. Together with the catalogues of primary and secondary counterparts to the main and hard samples of the eFEDS survey, this paper releases their multi-wavelength properties and redshifts.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    URL: Article
    DOI: 10.1051/0004-6361/202141631
    RVK:
    UA 1000
    RVK:
    US 1090
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2022
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
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  • 2
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    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?
    EDP Sciences ; 2023
    In:  Astronomy & Astrophysics Vol. 672 ( 2023-04), p. A167-
    Details
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 672 ( 2023-04), p. A167-
    Abstract: 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  × 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 0.2−2 keV /[erg s −1 ]) = 44.7 ± 0.1. 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 contribution. We present X-ray follow-up 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] λλ 4959,5007) and low-ionisation ([NII] λ 6585, [SII] λλ 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 ∼0.45 over 33 days and the UV shows a drop of ∼0.35 over the same period. 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 γ -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 the classification of this transient. 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.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    URL: Article
    DOI: 10.1051/0004-6361/202245078
    RVK:
    UA 1000
    RVK:
    US 1090
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2023
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
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  • 3
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    Deciphering the extreme X-ray variability of the nuclear transient eRASSt J045650.3−203750 : A likely repeating partial tidal disruption event
    EDP Sciences ; 2023
    In:  Astronomy & Astrophysics Vol. 669 ( 2023-01), p. A75-
    Details
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 669 ( 2023-01), p. A75-
    Abstract: 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 T in  ∼ 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.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    URL: Article
    DOI: 10.1051/0004-6361/202244805
    RVK:
    UA 1000
    RVK:
    US 1090
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2023
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
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  • 4
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    AT 2019avd: a novel addition to the diverse population of nuclear transients
    EDP Sciences ; 2021
    In:  Astronomy & Astrophysics Vol. 647 ( 2021-3), p. A9-
    Details
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 647 ( 2021-3), p. A9-
    Abstract: We report on SRG/eROSITA, ZTF, ASAS-SN, Las Cumbres, NEOWISE-R, and Swift XRT/UVOT observations of the unique ongoing event AT 2019avd, located in the nucleus of a previously inactive galaxy at z = 0.029. eROSITA first observed AT 2019avd on 2020-04-28 during its first all sky survey, when it was detected as an ultra-soft X-ray source ( kT ~ 85 eV) that was ≳90 times brighter in the 0.2−2 keV band than a previous 3 σ upper flux detection limit (with no archival X-ray detection at this position). The ZTF optical light curve in the ~450 days preceding the eROSITA detection is double peaked, and the eROSITA detection coincides with the rise of the second peak. Follow-up optical spectroscopy shows the emergence of a Bowen fluorescence feature and high-ionisation coronal lines ([Fe  X ] 6375 Å, [Fe  XIV ] 5303 Å), along with persistent broad Balmer emission lines ( FWHM ~ 1400 km s −1 ). Whilst the X-ray properties make AT 2019avd a promising tidal disruption event (TDE) candidate, the optical properties are atypical for optically selected TDEs. We discuss potential alternative origins that could explain the observed properties of AT 2019avd, such as a stellar binary TDE candidate, or a TDE involving a super massive black hole binary.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    URL: Article
    DOI: 10.1051/0004-6361/202039681
    RVK:
    UA 1000
    RVK:
    US 1090
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2021
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
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  • 5
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    The eROSITA Final Equatorial-Depth Survey (eFEDS) : Variability catalogue and multi-epoch comparison
    EDP Sciences ; 2022
    In:  Astronomy & Astrophysics Vol. 661 ( 2022-5), p. A8-
    Details
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 661 ( 2022-5), p. A8-
    Abstract: The 140-square-degrees Final Equatorial-Depth Survey (eFEDS) field, observed with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma mission, provides a first look at the variable eROSITA sky. We analysed the intrinsic X-ray variability of the eFEDS sources and provide X-ray light curves and tables with variability test results in the 0.2–2.3 keV (soft) and 2.3–5.0 keV (hard) bands. We performed variability tests using the traditional normalised excess variance and maximum amplitude variability methods (as performed for the 2RXS catalogue), and we present results from the Bayesian excess variance and Bayesian block methods. We identified 65 sources as being significantly variable in the soft band. In the hard band, only one source is found to vary significantly. For the most variable sources, the light curves are well fit by an empirical stellar flare model and reveal extreme flare properties. A few highly variable active galactic nuclei have also been detected. About half of the variable eFEDS sources are detected in the X-rays for the first time with eROSITA. Comparison with 2RXS and XMM-Newton observations provides variability information on timescales of years to decades.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    URL: Article
    DOI: 10.1051/0004-6361/202141155
    RVK:
    UA 1000
    RVK:
    US 1090
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2022
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
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  • 6
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    eRASSt J074426.3 + 291606: prompt accretion disc formation in a ‘faint and slow’ tidal disruption event
    Oxford University Press (OUP) ; 2023
    In:  Monthly Notices of the Royal Astronomical Society Vol. 520, No. 3 ( 2023-02-15), p. 4209-4225
    Details
    In: Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP), Vol. 520, No. 3 ( 2023-02-15), p. 4209-4225
    Abstract: We 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.
    Type of Medium: Online Resource
    ISSN: 0035-8711 , 1365-2966
    URL: Article
    DOI: 10.1093/mnras/stad046
    Language: English
    Publisher: Oxford University Press (OUP)
    Publication Date: 2023
    detail.hit.zdb_id: 2016084-7
    SSG: 16,12
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  • 7
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    eROSITA detection rates for tidal disruptions of white dwarfs by intermediate mass black holes
    Oxford University Press (OUP) ; 2019
    In:  Monthly Notices of the Royal Astronomical Society Vol. 489, No. 4 ( 2019-11-11), p. 5413-5423
    Details
    In: Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP), Vol. 489, No. 4 ( 2019-11-11), p. 5413-5423
    Abstract: White dwarf black hole tidal disruption events (herein WTDEs) present an opportunity to probe the quiescent intermediate mass black hole population in the Universe. We run an extensive set of Monte Carlo based simulations to explore SRG/eROSITA’s detection sensitivity to WTDEs as a function of black hole mass, redshift, and time offset between event flaring and it first being observed. A novel estimate of WTDE rate densities from globular clusters and dwarf galaxies is also presented. We combine this with estimated detection sensitivities to infer the rate of eROSITA detecting these events. Depending on the estimate of the intrinsic rate of WTDEs, we anticipate that eROSITA may detect three events over its 4 yr all-sky survey. eROSITA will be most sensitive to systems with black hole masses above $10^4\, \mathrm{M}_{\odot }$, and is most likely to catch these within 5 d of flaring.
    Type of Medium: Online Resource
    ISSN: 0035-8711 , 1365-2966
    URL: Article
    DOI: 10.1093/mnras/stz2520
    Language: English
    Publisher: Oxford University Press (OUP)
    Publication Date: 2019
    detail.hit.zdb_id: 2016084-7
    SSG: 16,12
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  • 8
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    The rebrightening of a ROSAT -selected tidal disruption event: repeated weak partial disruption flares from a quiescent galaxy?
    Oxford University Press (OUP) ; 2023
    In:  Monthly Notices of the Royal Astronomical Society Vol. 520, No. 3 ( 2023-02-15), p. 3549-3559
    Details
    In: Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP), Vol. 520, No. 3 ( 2023-02-15), p. 3549-3559
    Abstract: 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 ± 0.1) × 10−12 erg s−1 cm−2], ultra-soft (kT = 0.11 ± 0.03 keV) X-ray flare from a quiescent galaxy (z = 0.051 89). During its fifth all-sky survey (eRASS5) in 2022, Spectrum-Roentgen-Gamma (SRG)/ eROSITA detected the repeated flaring of J1331, where it had rebrightened to an observed 0.2–2 keV flux of (6.0 ± 0.7) × 10−13 erg s−1 cm−2, with spectral properties (kT = 0.115 ± 0.007 keV) consistent with the ROSAT-observed flare ∼30 yr earlier. In this work, we report on X-ray, ultraviolet, optical, and radio observations of this system. During a pointed XMM observation ∼17 d 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 ≳40 over this period. Given the extremely low probability (∼5 × 10−6) of observing two independent full TDEs from the same galaxy over a 30 yr 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 time-scales [$\mathcal {O}(\mathrm{d})$] compared to standard TDE candidates, with negligible ongoing accretion at late times post-disruption between outbursts.
    Type of Medium: Online Resource
    ISSN: 0035-8711 , 1365-2966
    URL: Article
    DOI: 10.1093/mnras/stad022
    Language: English
    Publisher: Oxford University Press (OUP)
    Publication Date: 2023
    detail.hit.zdb_id: 2016084-7
    SSG: 16,12
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  • 9
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    X-ray quasi-periodic eruptions from two previously quiescent galaxies
    Springer Science and Business Media LLC ; 2021
    In:  Nature Vol. 592, No. 7856 ( 2021-04-29), p. 704-707
    Details
    In: Nature, Springer Science and Business Media LLC, Vol. 592, No. 7856 ( 2021-04-29), p. 704-707
    Abstract: Quasi-periodic eruptions (QPEs) are very-high-amplitude bursts of X-ray radiation recurring every few hours and originating near the central supermassive black holes of galactic nuclei 1,2 . 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 1,2 , with emission lines in their optical spectra classifying their nuclei as hosting an actively accreting supermassive black hole 3,4 . Here we report observations of QPEs in two further galaxies, obtained with a blind and systematic search of 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 that is typical of active galactic nuclei is not required to trigger these events. Indeed, the periods, amplitudes and profiles of the QPEs reported here are inconsistent with current models that invoke radiation-pressure-driven instabilities in the accretion disk 5–9 . 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 that of the main body 10 , and future X-ray observations may constrain possible changes in their period owing to orbital evolution. This model could make QPEs a viable candidate for the electromagnetic counterparts of so-called extreme-mass-ratio inspirals 11–13 , with considerable implications for multi-messenger astrophysics and cosmology 14,15 .
    Type of Medium: Online Resource
    ISSN: 0028-0836 , 1476-4687
    URL: Article
    DOI: 10.1038/s41586-021-03394-6
    RVK:
    TA 1000
    RVK:
    UA 1000
    RVK:
    WA 15000
    Language: English
    Publisher: Springer Science and Business Media LLC
    Publication Date: 2021
    detail.hit.zdb_id: 120714-3
    detail.hit.zdb_id: 1413423-8
    SSG: 11
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  • 10
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    A radio flare associated with the nuclear transient eRASSt J234403-352640: An outflow launched by a potential tidal disruption event
    Oxford University Press (OUP) ; 2024
    In:  Monthly Notices of the Royal Astronomical Society
    Details
    In: Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP)
    Abstract: We present an extensive radio monitoring campaign of the nuclear transient eRASSt J234402.9-352640 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 & gt;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 J234402.9-352640 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.
    Type of Medium: Online Resource
    ISSN: 0035-8711 , 1365-2966
    URL: Article
    DOI: 10.1093/mnras/stae362
    Language: English
    Publisher: Oxford University Press (OUP)
    Publication Date: 2024
    detail.hit.zdb_id: 2016084-7
    SSG: 16,12
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