Chandra Observations of Abell 2261 Brightest Cluster Galaxy, a Candidate Host to a Recoiling Black Hole

TL;DR

This study uses Chandra X-ray observations to search for evidence of a recoiling supermassive black hole in Abell 2261's brightest galaxy, finding no X-ray emission but setting limits on possible low-level accretion.

Contribution

It provides the first deep X-ray constraints on the presence and activity of a potential recoiling black hole in Abell 2261-BCG, a candidate with a large stellar core.

Findings

No X-ray point source detected in the galaxy's core.

Limits on black hole accretion rate set at a bolometric Eddington fraction of about 10^{-6}.

X-ray gas morphology suggests a recent large dynamic event.

Abstract

We use Chandra X-ray observations to look for evidence of a recoiling black hole from the brightest cluster galaxy in Abell 2261 (A2261-BCG). A2261-BCG is a strong candidate for a recoiling black hole because of its large, flat stellar core, revealed by Hubble Space Telescope imaging observations. We took 100-ksec observations with Chandra and combined it with 35 ksec of archival observations to look for low-level accretion onto a black hole of expected mass $M\sim10^{10}\ M_{\scriptscriptstyle \odot}$ that could possibly be located in one of four off-center stellar knots near the galaxy's center or else in the optical center of the galaxy or in the location of radio emission. We found no X-ray emission arising from a point source in excess of the cluster gas and can place limits on the accretion of any black hole in the central region to a 2-7 keV flux below $4.3 \times 10^{-16}\ \mathrm{erg\ s^{-1}\ cm^{-2}}$, corresponding to a bolometric Eddington fraction of about $10^{-6}$. Thus there is either no $10^{10}\ M_{\scriptscriptstyle \odot}$ black hole in the core of A2261-BCG, or it is accreting at a low level. We also discuss the morphology of the X-ray emitting gas in the cluster and how its asymmetry is consistent with a large dynamic event.