Optical Spectrum of XRTJ155712+382032, a candidate counterpart of the gamma-ray source 4FGL J1557.2+3822 and the IceCube neutrino event IC-240307A
ATel #16528; Simona Paiano (INAF/IASF-PA) , Aldo Treves (Uninsubria, INAF-OABrera), Renato Falomo (INAF-OAPD), Boris Sbarufatti (INAF-OABrera), Alberto Ulgiati (INAF/IASF-Palermo, Unipa), Paolo Padovani (ESO), Paolo Giommi (INAF-OAB), Riccardo Scarpa (IAC, Universidad de la Laguna)
on 14 Mar 2024; 13:14 UT
Credential Certification: Simona Paiano (simona.paiano@inaf.it)
Subjects: X-ray, Gamma Ray, Neutrinos, Blazar
A plausible association of the neutrino event IceCube-240307A is the gamma-ray source 4FGLJ1557.2+3822 (GCN 35898). The gamma-ray error box (see Figure ) contains only one Swift X-ray source (XRTJ155712+382032) of robust significance (> 6 sigma; see Ulgiati et al., 2024 for a description of the counterpart search procedure of unidentified Fermi sources). The counterparts in radio and optical bands are VLASS1QLCIRJ155712.27+382033.1 (2.3 mJy) and SDSSJ155712.26+382032.9 (g=20.25).
In 2020 May 09, we obtained an optical spectrum of the source (g=20.3) in the 4100-7700 Ang range with the Gran Telescopio Canarias+OSIRIS and the R1000B grism. The spectrum clearly shows absorption lines of an elliptical galaxy stellar population (CaII, G-band, Hbeta, MgI, and CaI+FeI) at z = 0.333.
On 2024 March 09, XRT was re-observed with Swift XRT: observed flux 2.2 (+1.1,-0.8)E-13 erg/cm^2/s. No substantial X-ray flux variation is detected with respect to the archival Swift observations (observed flux 2.4(+0.8,-0.5)E-13 erg/cm2/s).
Therefore the source appears as a galaxy dominated BL Lac object.
4FGLJ1557+3822_XRT_map
