5.0 GHz Continuum eEVN Observations of the Recoiling Supermassive Black Hole Candidate SDSSS J113323.97+550415.8

ATel #7388; M. Perez-Torres (IAA-CSIC, Granada / CEFCA, Teruel), N. Ramirez-Olivencia (IAA-CSIC, Granada) E. Piconcelli (INAF, Rome), A. Alberdi (IAA-CSIC, Granada), S. Komossa (MPIfR, Bonn), R. Herrero-Illana (IAA-CSIC, Granada)
on 14 Apr 2015; 05:48 UT
Credential Certification: Miguel A. Perez-Torres (torres@iaa.es)

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We report electronic European VLBI Network (eEVN) radio observations of the recoiling supermassive black hole (SMBH) candidate SDSSS J113323.97+550415.8 (=SDSS1133), in the outskirts of the nearby (D=28.9 Mpc) galaxy Mrk 177 (Koss et al. MNRAS, 2014). We carried out the highest angular resolution (milliarcsecond) radio observations of this object ever, on 12 December 2014. The eEVN array consisted of five radio antennas, including Effelsberg, Jodrell Mk2, Onsala, Torun, and Shanghai. We used a phase-referencing scheme, given the expected faintness of SDSS1133, typically spending 3.5 min on the target and 1 min on the bright, nearby (1.9 deg apart) QSO 1124+571, for almost 2 hr. The array observed at a sky frequency of 1.66 GHz, and used a total bandwidth of 256 MHz, which resulted in a synthesized Gaussian beam of (21 x 18) sq. milliarcseconds, corresponding to a spatial resolution of 3 pc at the distance of Mrk 177. We imaged a region of about (2 x 2) sq. arcseconds around SDSS1133.

We found no evidence of radio emission above a 3-sigma limit of 150 microJy/beam in the region around the recoiling SMBH candidate. This value corresponds to a 3-sigma upper limit of the monochromatic 5.0 GHz luminosity of 5.2e24 erg/s/Hz. We note that this upper limit is a factor of three better than that provided by existing archival FIRST radio observations at 1.4 GHz (3-sigma upper limit of about 450 microJy/beam for SDSS1333, corresponding to a monochromatic luminosity of 1.6e25 erg/s/Hz). Our radio observations place a stringent upper limit to the radio emission from the putative recoiling SMBH, especially if it is a type I AGN. Koss et al. (2014) also suggested a possible alternative scenario, namely that of a type IIn SN that exploded in 2001. Those supernovae tend to have peak radio luminosities as high as a few times 1e28 erg/s/Hz, and rising times of the order of 1-4 years (e.g., SN1986J in NGC 891, or SN2000ft in NGC7469), sometimes even longer (e.g., some of the recent supernovae in Arp 299A or Arp 220). While we cannot rule out completely the SN scenario--due to the lack of radio observations between 2001 and December 2014--the radio-silence of this object seems to be at odds with a type IIn nature, as supernovae may exhibit luminosities well above 1e25 erg/s/Hz, even 13 years after its explosion. Deeper radio observations would further constrain the nature of this puzzling object.

The European VLBI Network (EVN) is a joint facility of European, Chinese, South African, and other radio astronomy institutes funded by their national research councils. MPT, NRO, AA, RHI, and FC acknowledge funding support from the Spanish MINECO through grant AYA2012-38491-C02-02.