Abstract
Observations of active galactic nuclei (AGN) and X-ray binaries have shown that relativistic jets are ubiquitous when compact objects accrete. One could therefore anticipate the launch of a jet after a star is disrupted and accreted by a massive black hole. This birth of a relativistic jet may have been observed recently in two stellar tidal disruption flares (TDFs), which were discovered in gamma-rays by Swift. Yet no transient radio emission has been detected from the tens of TDF candidates that were discovered at optical to soft X-ray frequencies. Because the sample that was followed-up at radio frequencies is small, the non-detections can be explained by Doppler boosting, which reduces the jet flux for off-axis observers. Plus, the existing followup observation are mostly within ∼ 10 months of the discovery, so the non-detections can also be due to a delay of the radio emission with respect to the time of disruption. To test the conjecture that all TDFs launch jets, we obtained 5 GHz follow-up observations with the Jansky VLA of six known TDFs. To avoid missing delayed jet emission, our observations probe 1–8 years since the estimated time of disruption. None of the sources are detected, with very deep upper limits at the 10 micro Jansky level. These observations rule out the hypothesis that these TDFs launched jets similar to radio-loud quasars. We also constrain the possibility that the flares hosted a jet identical to Sw 1644+57.
Highlights
A star that passes too close to a massive black hole will be torn apart by tidal forces
Our sample includes tidal disruption flares (TDFs) with existing radio upper limits, since the radio emission can be delayed with respect to the time of disruption, see previous section
The flux limit is set at twice the rms of the radio image of each TDF. (This is lower than the limit for a blinddetection experiment since we use the threshold to find the probability of a non-detection, not to claim a discovery.) In Table 2 we list the results of this exercise
Summary
A star that passes too close to a massive black hole will be torn apart by tidal forces. Tens of (candidate) stellar tidal disruption events have been found by searching for flares in soft X-ray (e.g., [2,3,4,5,6]), UV [7,8,9], or optical surveys [10,11,12], or based on spectra with extreme coronal lines [13, 14] None of these thermal flares are associated with a radio transient, but only a handful have been followed up at this frequency. We use these observations to constrain the jet models in sec. 4 and we close with a discussion in sec. 5
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