A Runaway Black Hole in COSMOS: Gravitational Wave or Slingshot Recoil?
Finoguenov, A.; Aussel, H.; Sanders, D. B.; Taniguchi, Y.; Jahnke, K.; Silverman, J. D.; Salvato, M.; Fiore, F.; Bolzonella, M.; Vignali, C.; Gilli, R.; Piconcelli, E.; Comastri, A.; Cappelluti, N.; Brusa, M.; Mainieri, V.; Capak, P.; Koekemoer, A. M.; Civano, F.; Elvis, M.; McCracken, H. J.; Trump, J.; Scoville, N.; Frayer, D.; Lanzuisi, G.; Zamorani, G.; Puccetti, S.; Bressert, E.; Leauthaud, A.; Hao, H.; Bongiorno, A.; Lusso, E.; Impey, C. D.; Lilly, S. J.; Schiminovich, D.; Fruscione, A.; Le Floch, E.; Loeb, A.; Blecha, L.; Aldcroft, T.; Giodini, S.
United States, Italy, Germany, Switzerland, France, Japan
Abstract
We present a detailed study of a peculiar source detected in the COSMOS survey at z = 0.359. Source CXOC J100043.1+020637, also known as CID-42, has two compact optical sources embedded in the same galaxy. The distance between the two, measured in the HST/ACS image, is 0.495" ± 0.005" that, at the redshift of the source, corresponds to a projected separation of 2.46 ± 0.02 kpc. A large (~1200 km s-1) velocity offset between the narrow and broad components of Hβ 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 in COSMOS, 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-Newton data show that the absorption line is variable in energy by ΔE = 500 eV over four 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—the morphology, the velocity offset, and the inverted P-Cygni profile—occur in the same source is unlikely to be a coincidence. We envisage two possible explanations, both exceptional, for this system: (1) a gravitational wave (GW) recoiling black hole (BH), caught 1-10 Myr after merging; or (2) a Type 1/Type 2 system in the same galaxy where the Type 1 is recoiling due to the slingshot effect produced by a triple BH system. The first possibility gives us a candidate GW 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 active galactic nuclei (AGNs), and possibly universal in AGNs.