SDSS J124602.54+011318.8: A Highly Variable AGN, Not an Orphan GRB Afterglow

Abstract

The 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 re-analysis of archival optical and radio data, the observations prove that SDSS J124602.54+011318.8 is in fact an unusual radio-loud AGN, probably in the BL Lac class. The object displays strong photometric variability on time scales of weeks to years, including several bright flares, similar to the one initially reported. The SDSS observations are therefore almost certainly not related to a GRB. The optical spectrum of this object dramatically changes in correlation with its optical brightness. At the bright phase, weak, narrow oxygen emission lines and probably a broader H$\alpha$ 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 briefly discuss the implications of this discovery on the study of AGNs and other optically variable or transient phenomena.

14 pages, 5 figures, AASTEX 5.0.2, slight modifications following referee's report, PASP, in press

References in corpus (6)

• The u'g'r'i'z' Standard Star Network
• Off-Axis Afterglow Emission from Jetted Gamma-Ray Bursts
• The Deep X-ray Radio Blazar Survey (DXRBS). II. New Identifications
• Not Color Blind: Using Multiband Photometry to Classify Supernovae
• The difficulty in using orphan afterglows to measure gamma-ray burst beaming
• An AGN Identification for 3EG J2006-2321

Cited by in corpus (20)

• The Physics of Gamma-Ray Bursts
• Optical discovery of probable stellar tidal disruption flares
• A Description of Quasar Variability Measured Using Repeated SDSS and POSS Imaging
• Orphan GRB radio afterglows: Candidates and constraints on beaming
• Science with a wide-field UV transient explorer
• A Complete Survey of the Transient Radio Sky and Implications for Gamma-Ray Bursts, Supernovae, and other Relativistic Explosions
• The Diversity of Extremely Red Objects
• The Deep Lens Survey Transient Search I : Short Timescale and Astrometric Variability
• Discovery of a Cosmological, Relativistic Outburst via its Rapidly Fading Optical Emission
• PTF10iya: A short-lived, luminous flare from the nuclear region of a star-forming galaxy
• Orphan Afterglows of Collimated Gamma-Ray Bursts: Rate Predictions and Prospects for Detection
• Dirty Fireballs and Orphan Afterglows: A Tale of Two Transients
• Supernovae in Low-Redshift Galaxy Clusters: Observations by the Wise Observatory Optical Transient Search (WOOTS)
• Discovery of a Transient U-band Dropout in a Lyman-Break Survey: A Tidally-Disrupted Star at z = 3.3?
• SDSS J124602.54+011318.8: A Highly Luminous Optical Transient at z=0.385
• An Untriggered Search for Optical Bursts
• The CFHTLS Real Time Analysis System "Optically Selected GRB Afterglows"
• Optical Limits on Precursor Emission from Gamma-Ray Bursts with Known Redshift
• An XMM-Newton look at the strongly variable radio-weak BL Lac Fermi J1544-0639
• A Spatially Resolved Study of the GRB 020903 Host Galaxy