Gamma-ray bursts (GRBs) are traditionally classified as either short GRBs with durations ≲2 s that are powered by compact object mergers or long GRBs with durations ≳2 s that are powered by the deaths of massive stars. Recent results, however, have challenged this dichotomy and suggest that there exists a population of merger-driven long bursts. One such example, GRB 191019A, has a
A tidal disruption event (TDE) occurs when a star is destroyed by a supermassive black hole. Broad-band radio spectral observations of TDEs trace the emission from any outflows or jets that are ejected from the vicinity of the supermassive black hole. However, radio detections of TDEs are rare, with <20 published to date, and only 11 with multi-epoch broad-band coverage. Here we present the radio detection of the TDE AT2020vwl and our subsequent radio monitoring campaign of the outflow that was produced, spanning 1.5 yr post-optical flare. We tracked the outflow evolution as it expanded between 1016 and 1017 cm from the supermassive black hole, deducing it was non-relativistic and launched quasi-simultaneously with the initial optical detection through modelling the evolving synchrotron spectra of the event. We deduce that the outflow is likely to have been launched by material ejected from stream-stream collisions (more likely), the unbound debris stream, or an accretion-induced wind or jet from the supermassive black hole (less likely). AT2020vwl joins a growing number of TDEs with well-characterized prompt radio emission, with future timely radio observations of TDEs required to fully understand the mechanism that produces this type of radio emission in TDEs.
more » « less- NSF-PAR ID:
- 10413122
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 522
- Issue:
- 4
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 5084-5097
- Size(s):
- ["p. 5084-5097"]
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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Abstract AT 2020mot is a typical UV/optical tidal disruption event (TDE) with no radio or X-ray signatures in a quiescent host. We find an
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ABSTRACT AT 2022cmc is a luminous optical transient (νLν ≳ 1045 erg s−1) accompanied by decaying non-thermal X-rays (peak duration tX ≲ days and isotropic energy EX,iso ≳ 1053 erg) and a long-lived radio/mm synchrotron afterglow, which has been interpreted as a jetted tidal disruption event (TDE). Both an equipartition analysis and a detailed afterglow model reveal the radio/mm emitting plasma to be expanding mildly relativistically (Lorentz factor $\Gamma \gtrsim \, \mathrm{ few}$ ) with an opening angle θj ≃ 0.1 and roughly fixed energy Ej,iso ≳ few × 1053 erg into an external medium of density profile n ∝ R−k with k ≃ 1.5–2, broadly similar to that of the first jetted TDE candidate Swift J1644+57 and consistent with Bondi accretion at a rate of ∼$10^{-3}\,\dot{M}_{\rm Edd}$ on to a 106 M⊙ black hole before the outburst. The rapidly decaying optical emission over the first days is consistent with fast-cooling synchrotron radiation from the same forward shock as the radio/mm emission, while the bluer slowly decaying phase to follow likely represents a separate thermal emission component. Emission from the reverse shock may have peaked during the first days, but its non-detection in the optical band places an upper bound Γj ≲ 100 on the Lorentz factor of the unshocked jet. Although a TDE origin for AT 2022cmc is indeed supported by some observations, the vast difference between the short-lived jet activity phase tX ≲ days and the months-long thermal optical emission also challenges this scenario. A stellar core-collapse event giving birth to a magnetar or black hole engine of peak duration ∼1 d offers an alternative model also consistent with the circumburst environment, if interpreted as a massive star wind.
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