Optical view of neutrino emitter candidate PKS 0735 +178
Renato Falomo (INAF/OAPD), Aldo Treves (Universita dell Insubria, INAF/OABrera), Simona Paiano (INAF/OAR, INAF/IASF-PA)
PKS0735+178 is the possible counterpart of the neutrino event IceCube 211208 (GCN#31191), and a Baikal neutrino (ATEL#15112). The event corresponded too high activity of source in gamma rays (ATEL#15099), X-rays (ATEL#15102, #15108, #15109, #15113), optical (ATEL 15021), and radio (ATEL#15105). In spite of several attempts to detect intrinsic absorption and/or emission lines in the optical spectrum of PKS0735+178, no such features were detected thus a firm determination of the redshift is not available. However, since 1974 an intervening absorption system was detected (Carswell et al. 1974, ApJ,190,101) at ~3980 Ang and associated to MgII 2800 Ang doublet. This sets a firm lower limit to the redshift of z > 0.424. This absorption was confirmed in follow up spectroscopy adding other absorption features at the same redshift (Falomo and Ulrich 2000, A&A,357,91 and Rector et al 2001, AJ,122,565). The optical spectrum obtained at VLT+FORS (Landoni et al 2013, AJ,145,114) is reproduced in https://web.oapd.inaf.it/zbllac/extra/atel0735_fig.jpg. From this spectrum we can set an upper limit for the emission line of [OII] 3727 Ang in the spectral range 5300-6700 Ang (corresponding to a redshift range limit z < 0.8). No emission lines are detected in this range down to an equivalent width EWmin = 0.2 Ang. Therefore the upper limit of the line flux is F ~ 3 x 10^(-16) erg/cm2/s. This corresponds to line luminosity upper limits in the range 1.5 to 6 x 10^(41) erg/s. Various attempts were also performed to resolve the host galaxy of this source both from ground (Stickel et al 1993; Falomo & Ulrich 2000, A&A,357,91) and space using HST (Scarpa et al 2000,ApJ,544,258, Urry et al 2000,ApJ,532,816). In all cases optical imaging failed to detect the surrounding nebulosity. Assuming the host galaxy is typical of BL Lac objects (e.g. Falomo, Pian and Treves 2014,A&Av,22,73) a lower limit (z > 0.5) to the redshift was estimated from the non detection of the host (Falomo & Ulrich 2000,A&A,357,91). Deep images of the field around this source revealed a number of galaxies in the immediate environment (see Figure in https://web.oapd.inaf.it/zbllac/extra/atel0735_fig.jpg ). In particular the companion galaxy at 7" NW of the BL Lac object is at z = 0.645 corresponding to a projected distance from the target of only 50 kpc. Another fainter galaxy is found at ~ 3.5" East. If these galaxies form a small group associated to the target, a plausible redshift is z ~ 0.65.
radio,optical,x-ray,gamma ray,neutrinos,blazar