A radio jet from the optical and x-ray bright stellar tidal disruption flare ASASSN-14li

Abstract

The tidal disruption of a star by a supermassive black hole leads to a shortlived thermal flare. Despite extensive searches, radio follow-up observations of known thermal stellar tidal disruption flares (TDFs) have not yet produced a conclusive detection. We present a detection of variable radio emission from a thermal TDF, which we interpret as originating from a newly launched jet.The multi-wavelength properties of the source present a natural analogy with accretion state changes of stellar mass black holes, which suggests all TDFs could be accompanied by a jet. In the rest frame of the TDF, our radio observations are an order of magnitude more sensitive than nearly all previous upper limits, explaining how these jets, if common, could thus far have escaped detection. Although radio jets are a ubiquitous and well-studied feature of accreting compact objects, it remains unclear why only a subset of active galactic nuclei (AGN) are radio-loud. A stellar tidal disruption flare (TDF) presents a novel method with which to study jet production in accreting supermassive black holes. These flares occur after perturbations to a star's orbit have brought it to within a few tens of Schwarzschild radii of the central supermassive black hole and the star gets torn apart by the black hole's tidal force. A large amount of gas is suddenly injected close to the black hole event horizon and we therefore anticipate the launch of a relativistic jet as this stellar debris gets accreted (1, 2). About two dozen TDFs have so far been discovered at soft x-ray, UV, and optical wavelengths, e.g., (3)(4)(5). All of these flares can be described by black body emission, hence their description as thermal TDFs. Hard x-ray emission from a relativistic jet launched after a stellar disruption has been observed in three cases (6-10).These so-called relativistic TDFs are readily detected at radio frequencies [the best-studied source, Swift J1644+57, reached a peak flux of 30 mJy at 22 GHz (11,12)]. Surprisingly, radio observations of thermal TDFs show no signs of equally powerful jets (13,14), bringing into question the universality of jet production triggered by large changes in the accretion rate (15).On December 2 2014, the All-Sky Automated Survey for Supernovae (ASAS-SN) reported the discovery of , an optical transient with a blue continuum in Swift UV/Optical Telescope (UVOT) follow-up observations, located in the nucleus of a galaxy at redshift z = 0.021. These properties prompted this transient to be classified as a potential stellar tidal disruption flare. The source was also detected in Swift x-ray Telescope (XRT) observations, but only at soft x-ray energies (0.3-1 keV; Fig. 1). We began a radio monitoring campaign with the Arcminute Microkelvin Imager (AMI) at 15.7 GHz 22 days after the first Swift observation and obtained two observations with the Westerbork Synthesis Radio Telescope (WSRT) at 1.4 GHz [see Supporting Material (SM), table S1]. The 15.7 GHz light curve shows a monotonic decay (factor 5 decrease in 140 days; Fig. 2), ...

XMMSL1 J074008.2-853927: a tidal disruption event with thermal and non-thermal components

“…At a distance of 73 Mpc, this corresponds to a luminosity L ν = (7.7 ± 0.4) × 10 27 erg s −1 Hz −1 . This is slightly lower than the radio luminosity of the nearby thermal TDE ASSASN-14li (likely produced by a non-relativistic outflow; Alexander et al 2016;van Velzen et al 2016), an order of magnitude below the radio luminosity of the candidate TDE IGR J12580+0134 (possibly due to an off-axis jet; Irwin et al 2015;Lei et al 2016), and several orders of magnitude below the radio luminosities of relativistic TDEs like Swift J164449.3+573451 (ascribed to an on-axis relativistic jet; Zauderer et al 2011). Additional radio observations of XMMSL1 J0740-85 are planned and the time-evolution of the emission will be discussed in a future paper (Alexander et al, in prep.…”

MUSE REVEALS A RECENT MERGER IN THE POST-STARBURST HOST GALAXY OF THE TDE ASASSN-14li

“…In particular, ASASSN-14li at z = 0.0206 (D 90 Mpc), discovered in November 2014, has been the best studied TDE to date at all wavelengths, including the optical/UV Cenko et al 2016;Brown et al 2016b), radio (Alexander et al 2016;van Velzen et al 2016;Romero-Cañizales et al 2016), X-rays Miller et al 2015), and mid-infrared (Jiang et al 2016), as well as with theoretical modeling (Krolik et al 2016). The archival, nuclear SDSS spectrum of the host galaxy of ASASSN-14li, PGC 043234 (VIII Zw 211; M 3 × 10 9 M ), shows strong Balmer lines in absorption and no strong evidence for current star-formation .…”

Radio Observations of the Tidal Disruption Event XMMSL1 J0740–85

“…However, only four TDEs with associated radio emission have been published to date: two jetted events discovered by Swift (Sw J1644+57 and Sw J2058+0516), IGR J1258+0134, claimed to have an off-axis relativistic jet (Irwin et al 2015;Lei et al 2016), and ASASSN14li, which produced less luminous radio emission arising from a non-relativistic outflow (Alexander et al 2016;van Velzen et al 2016b). Radio upper limits for an additional 15 events rule out Sw J1644+57-like jets in most cases, but cannot rule out slower, non-relativistic outflows as seen in ASASSN-14li (Komossa 2002;Bower et al 2013;van Velzen et al 2013;Chornock et al 2014;Arcavi et al 2014).…”

“…Radio observations of TDEs also provide an independent measurement of the event energy, the size of the emitting region, and the magnetic field strength (e.g. Zauderer et al 2011;Berger et al 2012;Zauderer et al 2013;Alexander et al 2016;van Velzen et al 2016b;Lei et al 2016).…”