Chandra High resolution Observations of CID-42, a candidate recoiling SMBH

TL;DR

This study uses high-resolution Chandra observations to investigate CID-42, a candidate recoiling supermassive black hole, revealing that only one optical source hosts an unobscured AGN, supporting the gravitational wave recoil scenario.

Contribution

First high-resolution X-ray imaging clarifies the nature of CID-42's optical sources, favoring the recoiling SMBH hypothesis over a dual AGN system.

Findings

Only one optical source shows X-ray emission, indicating an unobscured AGN.

The second source likely hosts a star-forming region, not an obscured SMBH.

A new X-ray feature was discovered near the main source.

Abstract

We present Chandra High Resolution Camera observations of CID-42, a candidate recoiling supermassive black hole (SMBH) at z=0.359 in the COSMOS survey. CID-42 shows two optical compact sources resolved in the HST/ACS image embedded in the same galaxy structure and a velocity offset of ~1300 km/s between the H\beta\ broad and narrow emission line, as presented by Civano et al. (2010). Two scenarios have been proposed to explain the properties of CID-42: a GW recoiling SMBH and a double Type 1/ Type 2 AGN system, where one of the two is recoiling because of slingshot effect. In both scenario, one of the optical nuclei hosts an unobscured AGN, while the other one, either an obscured AGN or a star forming compact region. The X-ray Chandra data allow to unambiguously resolve the X-ray emission, and unveil the nature, of the two optical sources in CID-42. We find that only one of the optical nuclei is responsible for the whole X-ray unobscured emission observed and a 3sigma upper limit on the flux of the second optical nucleus is measured. The upper limit on the X-ray luminosity plus the analysis of the multiwavelength spectral energy distribution indicate the presence of a starforming region in the second source rather than an obscured SMBH, thus favoring the GW recoil scenario. However the presence of a very obscured SMBH cannot be fully ruled-out. A new X-ray feature, in a SW direction with respect to the main source, is discovered and discussed.