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1. An outflow powers the optical rise of the nearby, fast-evolving tidal disruption event AT2019qiz

   https://doi.org/10.1093/mnras/staa2824  

   Nicholl, M ; Wevers, T ; Oates, S R ; Alexander, K D ; Leloudas, G ; Onori, F ; Jerkstrand, A ; Gomez, S ; Campana, S ; Arcavi, I ; et al ( October 2020 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT At 66 Mpc, AT2019qiz is the closest optical tidal disruption event (TDE) to date, with a luminosity intermediate between the bulk of the population and the faint-and-fast event iPTF16fnl. Its proximity allowed a very early detection and triggering of multiwavelength and spectroscopic follow-up well before maximum light. The velocity dispersion of the host galaxy and fits to the TDE light curve indicate a black hole mass ≈106 M⊙, disrupting a star of ≈1 M⊙. By analysing our comprehensive UV, optical, and X-ray data, we show that the early optical emission is dominated by an outflow, with a luminosity evolution L ∝ t2, consistent with a photosphere expanding at constant velocity (≳2000 km s−1), and a line-forming region producing initially blueshifted H and He ii profiles with v = 3000–10 000 km s−1. The fastest optical ejecta approach the velocity inferred from radio detections (modelled in a forthcoming companion paper from K. D. Alexander et al.), thus the same outflow may be responsible for both the fast optical rise and the radio emission – the first time this connection has been observed in a TDE. The light-curve rise begins 29 ± 2 d before maximum light, peaking when the photosphere reaches the radius where optical photons can escape. The photosphere then undergoes a sudden transition, first cooling at constant radius then contracting at constant temperature. At the same time, the blueshifts disappear from the spectrum and Bowen fluorescence lines (N iii) become prominent, implying a source of far-UV photons, while the X-ray light curve peaks at ≈1041 erg s−1. Assuming that these X-rays are from prompt accretion, the size and mass of the outflow are consistent with the reprocessing layer needed to explain the large optical to X-ray ratio in this and other optical TDEs, possibly favouring accretion-powered over collision-powered outflow models. 

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2. An elliptical accretion disk following the tidal disruption event AT 2020zso

   https://doi.org/10.1051/0004-6361/202142616  

   Wevers, T. ; Nicholl, M. ; Guolo, M. ; Charalampopoulos, P. ; Gromadzki, M. ; Reynolds, T. M. ; Kankare, E. ; Leloudas, G. ; Anderson, J. P. ; Arcavi, I. ; et al ( October 2022 , Astronomy & Astrophysics)

   Aims. The modelling of spectroscopic observations of tidal disruption events (TDEs) to date suggests that the newly formed accretion disks are mostly quasi-circular. In this work we study the transient event AT 2020zso, hosted by an active galactic nucleus (AGN; as inferred from narrow emission line diagnostics), with the aim of characterising the properties of its newly formed accretion flow. Methods. We classify AT 2020zso as a TDE based on the blackbody evolution inferred from UV/optical photometric observations and spectral line content and evolution. We identify transient, double-peaked Bowen (N  III ), He  I , He  II, and H α emission lines. We model medium-resolution optical spectroscopy of the He  II (after careful de-blending of the N  III contribution) and H α lines during the rise, peak, and early decline of the light curve using relativistic, elliptical accretion disk models. Results. We find that the spectral evolution before the peak can be explained by optical depth effects consistent with an outflowing, optically thick Eddington envelope. Around the peak, the envelope reaches its maximum extent (approximately 10 15 cm, or ∼3000–6000 gravitational radii for an inferred black hole mass of 5−10 × 10 5 M ⊙ ) and becomes optically thin. The H α and He  II emission lines at and after the peak can be reproduced with a highly inclined ( i  = 85 ± 5 degrees), highly elliptical ( e  = 0.97 ± 0.01), and relatively compact ( R in = several 100 R g and R out = several 1000 R g ) accretion disk. Conclusions. Overall, the line profiles suggest a highly elliptical geometry for the new accretion flow, consistent with theoretical expectations of newly formed TDE disks. We quantitatively confirm, for the first time, the high inclination nature of a Bowen (and X-ray dim) TDE, consistent with the unification picture of TDEs, where the inclination largely determines the observational appearance. Rapid line profile variations rule out the binary supermassive black hole hypothesis as the origin of the eccentricity; these results thus provide a direct link between a TDE in an AGN and the eccentric accretion disk. We illustrate for the first time how optical spectroscopy can be used to constrain the black hole spin, through (the lack of) disk precession signatures (changes in inferred inclination). We constrain the disk alignment timescale to > 15 days in AT2020zso, which rules out high black hole spin values ( a  < 0.8) for M BH  ∼ 10 6 M ⊙ and disk viscosity α  ≳ 0.1. 

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3. Neutrino follow-up with the Zwicky transient facility: results from the first 24 campaigns

   https://doi.org/10.1093/mnras/stad767  

   Stein, Robert ; Reusch, Simeon ; Franckowiak, Anna ; Kowalski, Marek ; Necker, Jannis ; Weimann, Sven ; Kasliwal, Mansi M. ; Sollerman, Jesper ; Ahumada, Tomas ; Amaro Seoane, Pau ; et al ( March 2023 , Monthly Notices of the Royal Astronomical Society)

   The Zwicky Transient Facility (ZTF) performs a systematic neutrino follow-up programme, searching for optical counterparts to high-energy neutrinos with dedicated Target-of-Opportunity (ToO) observations. Since first light in March 2018, ZTF has taken prompt observations for 24 high-quality neutrino alerts from the IceCube Neutrino Observatory, with a median latency of 12.2 h from initial neutrino detection. From two of these campaigns, we have already reported tidal disruption event (TDE) AT 2019dsg and likely TDE AT 2019fdr as probable counterparts, suggesting that TDEs contribute >7.8 per cent of the astrophysical neutrino flux. We here present the full results of our programme through to December 2021. No additional candidate neutrino sources were identified by our programme, allowing us to place the first constraints on the underlying optical luminosity function of astrophysical neutrino sources. Transients with optical absolutes magnitudes brighter that −21 can contribute no more than 87 per cent of the total, while transients brighter than −22 can contribute no more than 58 per cent of the total, neglecting the effect of extinction and assuming they follow the star formation rate. These are the first observational constraints on the neutrino emission of bright populations such as superluminous supernovae. None of the neutrinos were coincident with bright optical AGN flares comparable to that observed for TXS 0506+056/IC170922A, with such optical blazar flares producing no more than 26 per cent of the total neutrino flux. We highlight the outlook for electromagnetic neutrino follow-up programmes, including the expected potential for the Rubin Observatory.

    

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4. First optical identification of the SRG /eROSITA-detected supernova remnant G 116.6 − 26.1. I. Preliminary results

   https://doi.org/10.1093/mnras/stac1599  

   Palaiologou, E. V. ; Leonidaki, I. ; Kopsacheili, M. ( June 2022 , Monthly Notices of the Royal Astronomical Society)

   The supernova remnant (SNR) candidate G 116.6 − 26.1 is one of the few high Galactic latitude (|b| > 15°) remnants detected so far in several wavebands. It was discovered recently in the SRG/eROSITA all-sky X-ray survey and also displays a low-frequency weak radio signature. In this study, we report the first optical detection of G 116.6 − 26.1 through deep, wide-field, and higher resolution narrowband imaging in H$\alpha$, $[\rm S\,{\small{\rm II}}]$ and $[\rm O\,{\small{\rm III}}]$ light. The object exhibits two major and distinct filamentary emission structures in a partial shell-like formation. The optical filaments are found in an excellent positional match with available X-ray, radio, and UV maps, can be traced over a relatively long angular distance (38 and 70 arcmin) and appear unaffected by any strong interactions with the ambient interstellar medium. We also present a flux-calibrated, optical emission spectrum from a single location, with Balmer and several forbidden lines detected, indicative of emission from shock excitation in a typical evolved SNR. Confirmation of the most likely SNR nature of G 116.6 − 26.1 is provided from the observed value of the line ratio [S ii] / H$\alpha$ = $0.56\, \pm \, 0.06$, which exceeds the widely accepted threshold 0.4, and is further strengthened by the positive outcome of several diagnostic tests for shock emission. Our results indicate an approximate shock velocity range 70–100 km s−1 at the spectroscopically examined filament, which, when combined with the low emissivity in H$\alpha$ and other emission lines, suggest that G 116.6 − 26.1 is a SNR at a mature evolutionary stage.

    

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5. A joint SRG/eROSITA + ZTF search: Discovery of a 97-min period eclipsing cataclysmic variable with evidence of a brown dwarf secondary

   https://doi.org/10.1093/mnras/stae012  

   Galiullin, Ilkham ; Rodriguez, Antonio C. ; Kulkarni, Shrinivas R. ; Sunyaev, Rashid ; Gilfanov, Marat ; Bikmaev, Ilfan ; Yungelson, Lev ; van Roestel, Jan ; Gänsicke, Boris T. ; Khamitov, Irek ; et al ( January 2024 , Monthly Notices of the Royal Astronomical Society)

   Cataclysmic variables (CVs) that have evolved past the period minimum during their lifetimes are predicted to be systems with a brown dwarf donor. While population synthesis models predict that around 40–70 per cent of the Galactic CVs are post-period minimum systems referred to as ‘period bouncers’, only a few dozen confirmed systems are known. We report the study and characterization of a new eclipsing CV, SRGeJ041130.3+685350 (SRGeJ0411), discovered from a joint SRG/eROSITA and ZTF programme. The optical spectrum of SRGeJ0411 shows prominent hydrogen and helium emission lines, typical for CVs. We obtained optical high-speed photometry to confirm the eclipse of SRGeJ0411 and determine the orbital period to be Porb ≈ 97.530 min. The spectral energy distribution suggests that the donor has an effective temperature of ≲ 1800 K. We constrain the donor mass with the period–density relationship for Roche lobe-filling stars and find that Mdonor ≲ 0.04 M⊙. The binary parameters are consistent with evolutionary models for post-period minimum CVs, suggesting that SRGeJ0411 is a new period bouncer. The optical emission lines of SRGeJ0411 are single-peaked despite the system being eclipsing, which is typically only seen due to stream-fed accretion in polars. X-ray spectroscopy hints that the white dwarf in SRGeJ0411 could be magnetic, but verifying the magnetic nature of SRGeJ0411 requires further investigation. The lack of optical outbursts has made SRGeJ0411 elusive in previous surveys, and joint X-ray and optical surveys highlight the potential for discovering similar systems in the near future.

    

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