The Large Area Telescope ( LAT ) on board the Fermi Gamma-ray Space Telescope discovered a rapid ( \sim 5 days duration ) , high-energy ( E > 100 MeV ) gamma-ray outburst from a source identified with the blazar PKS 1502+106 ( OR 103 , S3 1502+10 , z=1.839 ) starting on August 05 , 2008 ( \sim 23 UTC , MJD 54683.95 ) , and followed by bright and variable flux over the next few months .
Results on the gamma-ray localization and identification , as well as spectral and temporal behavior during the first months of the Fermi all-sky survey are reported here in conjunction with a multi-waveband characterization as a result of one of the first Fermi multi-frequency campaigns .
The campaign included a Swift ToO ( followed up by 16-day observations on August 07-22 , MJD 54685-54700 ) , VLBA ( within the MOJAVE program ) , Owens Valley ( OVRO ) 40m , Effelsberg-100m , Metsähovi-14m , RATAN-600 and Kanata-Hiroshima radio/optical observations .
Results from the analysis of archival observations by INTEGRAL , XMM- Newton and Spitzer space telescopes are reported for a more complete picture of this new gamma-ray blazar .
PKS 1502+106 is a sub-GeV peaked , powerful flat spectrum radio quasar ( luminosity at E > 100 MeV , L _ { \gamma } , is about 1.1 \times 10 ^ { 49 } erg s ^ { -1 } , and black hole mass likely close to 10 ^ { 9 } M _ { \sun } ) , exhibiting marked gamma-ray bolometric dominance , in particular during the asymmetric outburst ( L _ { \gamma } / L _ { opt } \sim 100 , and 5-day averaged flux F _ { E > 100 ~ { } \mathrm { MeV } } = 2.91 \pm 1.4 \times 10 ^ { -6 } ph cm ^ { -2 } s ^ { -1 } ) , which was characterized by a factor greater than 3 of flux increase in less than 12 hours .
The outburst was observed simultaneously from optical to X-ray bands ( F _ { 0.3 - 10 ~ { } \mathrm { keV } } = 2.18 _ { -0.12 } ^ { +0.15 } \times 10 ^ { -12 } erg cm ^ { -2 } s ^ { -1 } , and hard photon index \sim 1.5 , similar to past values ) with a flux increase of less than one order of magnitude with respect to past observations , and was likely controlled by Comptonization of external-jet photons produced in the broad line region ( BLR ) in the gamma-ray band .
No evidence of a possible blue bump signature was observed in the optical-UV continuum spectrum , while some hints for a possible 4-day time-lag with respect to the gamma-ray flare were found .
Nonetheless , the properties of PKS 1502+106 and the strict optical/UV , X- and gamma-ray cross correlations suggest the contribution of the synchrotron self Compton ( SSC ) , in-jet , process should dominate from radio to X-rays .
This mechanism may also be responsible for the consistent gamma-ray variability observed by the LAT on longer timescales , after the ignition of activity at these energies provided by the BLR-dissipated outburst .
Modulations and subsequent minor , rapid flare events were detected , with a general fluctuation mode between pink-noise and a random-walk .
The averaged gamma-ray spectrum showed a deviation from a simple power-law , and can be described by a log-parabola curved model peaking around 0.4-0.5 GeV .
The maximum energy of photons detected from the source in the first four months of LAT observations was 15.8 GeV , with no significant consequences on extragalactic background light predictions .
A possible radio counterpart of the gamma-ray outburst can be assumed only if a delay of more than 3 months is considered on the basis of opacity effects at cm and longer wavelengths .
The rotation of the electric vector position angle observed by VLBA from 2007 to 2008 could represent a slow field ordering and alignment with respect to the jet axis , likely a precursor feature of the ejection of a superluminal radio knot and the high-energy outburst .
This observing campaign provides more insight into the connection between MeV-GeV flares and the moving , polarized structures observed by the VLBI .