Rare blue optical transient points to possible new class of cosmic explosion

Chinese astronomers report the detection of a peculiar optical transient event that occurred in the center of a dwarf galaxy. The newly discovered event, designated AT2019ijn, is a fast-rising, slow-decaying blue optical transient with exceptionally bright radio emission. The finding was detailed in a paper published July 6 in The Astrophysical Journal Letters.

One of the rarest transients

Among astronomical transients are events with extreme parameters, such as fast-evolving blue optical transients (FBOTs). They are extremely rare and characterized by a rapid rise to peak brightness within 10 days and blue colors near the peak, followed by an exponential decay to the post-flare level within a month.

AT2019ijn was first identified on May 31, 2019, by the Zwicky Transient Facility (ZTF) as an optical transient in the nucleus of a dwarf galaxy at a redshift of 0.27. New observations of this source conducted by a team of astronomers led by Hucheng Ding of Anhui Normal University in China indicate that AT2019ijn is a peculiar FBOT.

“In this Letter, we present the discovery and results from the multi-wavelength analysis of AT2019ijn, a peculiar optical transient that occurred in the center of a dwarf galaxy,” the researchers write.

What do the new observations reveal?

The observations found that AT2019ijn rapidly rose in the optical band to a peak luminosity of −21.05 mag in 5.26 days, followed by a slow decline over more than a month. Moreover, the transient has a persistently blue color. These properties generally resemble those of typical luminous FBOTs, with the exception of the slow decline, which is more consistent with superluminous supernovae (SLSNe) or tidal disruption events (TDEs).

In the radio band, the emission from AT2019ijn was found to be luminous and long-lasting, peaking 641 days after its optical discovery. The peak radio luminosity of AT2019ijn was more than one order of magnitude higher than that of any other known LFBOT or SLSN, but comparable to that of jetted TDEs.

The astronomers noted that the radio emission from AT2019ijn can be explained by an off-axis relativistic jet that spreads into our line of sight at late times, giving rise to the delayed peak in the light curve. They estimate that the jet’s kinetic energy is 700 sexdecillion ergs, which, together with the luminous optical emission, suggests the presence of an active central engine.

The nature and classification

The authors of the paper assume that the results favor the TDE scenario as an explanation for the nature of AT2019ijn. Therefore, this transient is most likely a jetted TDE involving an intermediate-mass black hole with a mass of around 132,000 solar masses. However, they do not completely rule out the hypothesis that AT2019ijn may be a jetted magnetar.

The researchers concluded that the peculiar properties of AT2019ijn may mean that it belongs to a new class of optical transients.

“Our results suggest that AT2019ijn may represent a new class of relativistic optical transients, paving a new way for their statistical studies by combining optical and radio time-domain surveys,” the scientists conclude.

Who’s behind this story?

Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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