Rapid late time radio flux density changes of the tidal disruption event ASASSN-15oi 9 years after disruption

ATel #16502; Akash Anumarlapudi (UWM), David Kaplan (UWM), Dougal Dobie (Swinburne/OzGrav), Assaf Horesh (HUJI), Emil Lenc (CSIRO), and Tara Murphy (U. Sydney) on behalf of the VAST collaboration.
on 4 Mar 2024; 21:06 UT
Credential Certification: Akash Anumarlapudi (aakash@uwm.edu)

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We report Australian SKA Pathfinder (ASKAP) observations of the tidal disruption event ASASSN-15oi taken as part of the Rapid ASKAP Continuum Survey (RACS; McConnell et al. 2020, PASA, 37, e048) and Variable And Slow Transients Survey (VAST; Murphy et al. 2021, PASA, 38, e054) at 887.5 MHz. ASASSN-15oi was radio quiet for 6 months after the initial disruption, until a radio flare on November 12, 2015 was reported by Horesh et al. (2021 Nature Astronomy, 5, 491) that decayed over an year. They also reported a late time re-brightening (4 years after the disruption) in data from the VLA Sky Survey (Lacy et al. 2020, PASP, 132, 035001).

We observed the field of ASASSN-15oi with a cadence of roughly every two months since mid 2023 as part of VAST, with additional observations as part of RACS. The source was clearly detected in all epochs of RACS and VAST data, which confirms the 4-year old radio rebrightening of this source which started in 2019 but then began to decline in the VAST observations starting in June 2023.

Recently, though, this source showed a sudden dip in the flux density by almost a factor of 2 around October 2023. Subsequent observations showed a rise back to the previous level and then (in February 2024) another decline (see the table below and the figure attached). Observations of several other sources in the field showed no such variations.

Flux density measurements for the latest VAST observations of the source are given below.

Start time of observation (UTC) | Peak flux density (mJy) | Error (mJy) |

2023-08-30T14:49:54.1 | 6.51 | 0.20 |

2023-10-28T08:50:47.2 | 3.69 | 0.18 |

2023-12-26T05:27:50.3 | 6.65 | 0.27 |

2024-02-26T01:53:16.0 | 4.70 | 0.30 |

A more detailed long-term analysis of this event is underway. We encourage multi-frequency observations to better constrain the evolution of this source.

This work uses data obtained from Inyarrimanha Ilgari Bundara / the Murchison Radio-astronomy Observatory. We acknowledge the Wajarri Yamaji People as the Traditional Owners and native title holders of the Observatory site. CSIRO’s ASKAP radio telescope is part of the Australia Telescope National Facility (https://ror.org/05qajvd42). Operation of ASKAP is funded by the Australian Government with support from the National Collaborative Research Infrastructure Strategy. ASKAP uses the resources of the Pawsey Supercomputing Research Centre. Establishment of ASKAP, Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory and the Pawsey Supercomputing Research Centre are initiatives of the Australian Government, with support from the Government of Western Australia and the Science and Industry Endowment Fund.

The ASKAP light curve for this event can be obtained here