Abstract
We present a detailed study of a peculiar source in the COSMOS survey at z=0.359. Source CXOCJ100043.1+020637 (CID-42) presents two compact optical sources embedded in the same galaxy. The distance between the 2, measured in the HST/ACS image, is 0.495" that, at the redshift of the source, corresponds to a projected separation of 2.46 kpc. A large (~1200 km/s) velocity offset between the narrow and broad components of Hbeta has been measured in three different optical spectra from the VLT/VIMOS and Magellan/IMACS instruments. CID-42 is also the only X-ray source having in its X-ray spectra a strong redshifted broad absorption iron line, and an iron emission line, drawing an inverted P-Cygni profile. The Chandra and XMM data show that the absorption line is variable in energy by 500 eV over 4 years and that the absorber has to be highly ionized, in order not to leave a signature in the soft X-ray spectrum. That these features occur in the same source is unlikely to be a coincidence. We envisage two possible explanations: (1) a gravitational wave recoiling black hole (BH), caught 1-10 Myr after merging, (2) a Type 1/ Type 2 system in the same galaxy where the Type 1 is recoiling due to slingshot effect produced by a triple BH system. The first possibility gives us a candidate gravitational waves recoiling BH with both spectroscopic and imaging signatures. In the second case, the X-ray absorption line can be explained as a BAL-like outflow from the foreground nucleus (a Type 2 AGN) at the rearer one (a Type 1 AGN), which illuminates the otherwise undetectable wind, giving us the first opportunity to show that fast winds are present in obscured AGN.
Highlights
Double super-massive black holes (SMBHs) within a single galaxy are predicted from the combination of hierarchical models of structure formation, and the observed link between black hole (BH) mass and host galaxy bulge mass in the local universe (e.g., Magorrian et al 1998; Gebhardt 2000)
We report in this paper the properties of a peculiar and unique source at z = 0.359, CID-42, in a survey of X-ray sources with Advanced Camera for Surveys (ACS) counterparts in Cosmic Evolution Survey (COSMOS)
Using the bolometric luminosity and BH mass of the SE source, we find that the Eddington ratio is L/LEdd ∼ 0.04, consistent with either the nuclear activity of this source having just been triggered or already declining
Summary
Our estimated Mstar and SFR are meant to be upper limits because the nuclear component can still contribute to the said quantities. Given this mass and SFR, CID-42 is actively star forming, with an SFR consistent with the overall X-ray selected AGN population (Silverman et al 2009). No significant variability has been detected, as expected, given that the optical SED is dominated by the host galaxy light. The full four-year X-ray light curve (Figure 2, right middle) shows a factor of 3 variability in the full (0.5–10 keV) band flux, while in the soft (0.5–2 keV) and hard (2–10 keV) bands the variability reaches a factor 4 and 4.5, respectively. No significant variability in the spectral slope has been measured (Figure 2, right bottom)
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