SDSS J124602.54+011318.8: A Highly Variable Active Galactic Nucleus, Not an Orphan Gamma‐Ray Burst Afterglow

Abstract

ABSTRACTThe optically variable source SDSS J124602.54+011318.8 first appears in Sloan Digital Sky Survey (SDSS) data as a bright point source with nonstellar colors. Subsequent SDSS imaging and spectroscopy showed that the point source declined or disappeared, revealing an underlying host galaxy at redshift 0.385. Based on these properties, the source was suggested to be a candidate “orphan afterglow”: a moderately beamed optical transient associated with a gamma‐ray burst (GRB) whose highly beamed radiation cone does not include our line of sight. We present new imaging and spectroscopic observations of this source. When combined with a careful reanalysis of archival optical and radio data, the observations prove that SDSS J124602.54+011318.8 is in fact an unusual radio‐loud active galactic nucleus (AGN), probably in the BL Lac class. The object displays strong photometric variability on timescales of weeks to years, including several bright flares similar to the one reported initially. The SDSS observations are therefore almost certainly not related to a GRB. The optical spectrum of this object changes dramatically in correlation with its optical brightness. At the bright phase, weak, narrow oxygen emission lines and probably a broader Hα line are superposed on a blue continuum. As the flux decreases, the spectrum becomes dominated by the host galaxy light, with emerging stellar absorption lines, while both the narrow and broad emission lines have larger equivalent widths. We discuss briefly the implications of this discovery on the study of AGNs and other optically variable or transient phenomena.