A Quasar Turns On

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The intermediate Palomar Transient Factory (iPTF) has discovered a quasar — the brightly-shining, active nucleus of a galaxy — abruptly turning on in what appears to be the fastest such transition ever seen in such an object.

A Rapid Transition

Quasars are expected to show variations in brightness on timescales of hours to millions of years, but it’s not often that we get to study their major variability in real time! So far, we’ve discovered only a dozen “changing-look” quasars — active galactic nuclei that exhibit major changes in their spectral class and brightness between observations. Roughly half of these were quasars that turned on and half were quasars that turned off, generally on timescales of maybe 5 or 10 years.

spectral change

The dramatic change in spectrum of iPTF 16bco between the archival SDSS data from 2004 (bottom) and the follow-up spectroscopy from Keck 2+DEIMOS in 2016 (top). [Adapted from Gezari et al. 2017]

In June 2016, however, a team of scientists led by Suvi Gezari (University of Maryland) discovered iPTF 16bco, a nuclear transient that wasn’t there the last time Palomar checked in 2012. A search through archival Sloan Digital Sky Survey and GALEX data — in addition to some follow-up X-ray imaging and spectroscopic observations — told the team what they needed to know: iPTF 16bco is a quasar that only just turned on within the 500 days preceding the iPTF observations.

This source, in fact, is a 100-million-solar-mass black hole located at the center of a galaxy at a redshift of= 0.237. In just over a year, the source changed classification from a galaxy with weak narrow-line emission to a quasar with characteristic strong, broad emission lines and a ten-fold increase in continuum brightness! What caused this sudden transition?

Instabilities at Fault?

changing look quasars

iPTF 16bco and the other known changing-look quasars with disappearing (red circles) and appearing (blue circles) broad-line emission. [Adapted from Gezari et al. 2017]

Gezari and collaborators used the large number of recent and archival observations of the galaxy to explore several scenarios that might be responsible for the rapid change in its brightness and spectral appearance. They found that the data disfavor variable obscuration by an absorber between us and the galaxy, microlensing of a background object, and tidal disruption of a star.

Instead, the authors conclude that the best-fitting explanation is one in which the galaxy’s nucleus already had a preexisting accretion disk, but the disk recently developed an instability. That instability caused more gas to rapidly feed onto the black hole, bumping the accretion rate up a notch and resulting in the quasar suddenly brightening.

Continued observations of iPTF 16bco will certainly help us to better understand what’s happening in this unusual source. In the meantime, its rapid change of state pushes the limits of accretion disk theory and presents us with an intriguing challenge to our understanding of quasars.

Citation

S. Gezari et al 2017 ApJ 835 144. doi:10.3847/1538-4357/835/2/144

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