NICER / ISS Science Nugget
for October 17, 2019

Recent AGN flare a member of a new class of transients

On 29 August 2019, the Zwicky Transient Facility discovered a new transient flare at the position of a known accreting supermassive black hole. Named AT2019pev, the flare in the broadband visible light was accompanied by strong Bowen (ionized oxygen) fluorescence features, which indicate the presence of high-velocity gas in the vicinity of the black hole. These strong flares from Active Galactic Nuclei (AGN) that produce Bowen fluorescence lines have recently been identified as a new class of objects (Trakhtenbrot et al. 2019, Nature Astronomy, vol. 3, p. 229; see NICER ISS weekly science update for 17 January, 2019). We do not yet know the origin of these extreme flares. Potentially, they are caused by an unknown instability in the accretion disc that causes gas to lose angular momentum and flood into the black hole. Or perhaps it is associated with the tidal disruption of a solar-type star, and we are watching the stellar debris quickly accrete into the black hole. While the interpretation remains uncertain, it is clear that these new and exotic transients associated with supermassive black holes are pushing our understanding of AGN accretion to new extremes.

The astronomical community was alerted to the presence of AT2019pev on 24 September 2019 (Gezari et al., Astronomer's Telegram #13127). In just 14 hours from the posting of the telegram, NICER was able to observe this quickly evolving transient. Amazingly, the 0.3-2 keV flux was very bright (10^-11 erg cm^-2 s^-1), nearly 10 times brighter than the X-ray flux measured by NASA's Swift observatory just one day prior, and 100 times brighter than an archival detection with ESA's ROSAT telescope.

NICER has accumulated approximately 30 ksec of exposure on AT2019pev since that initial observation. The source spectrum originally showed a 150 eV thermal component and a weak hard tail (similar to some other tidal disruption flares observed with NICER), but over a few days the hard tail became more pronounced and now appears to be similar to other unobscured AGN. We continue to monitor AT2019pev every few days, together with facilities at other wavelengths. This transient is just the fourth in this new class of AGN emissions including Bowen fluorescence lines, and so we continue to monitor with interest.

NICER's initial data was reported in Astronomer's Telegram #13132 by Kara et al.