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3FGL J1557.0-4225 is likely an AGN

ATel #10324; Xuejie Dai, Zhongxiang Wang, Liang Chen (SHAO)
on 28 Apr 2017; 03:01 UT
Credential Certification: Z. Wang (wangzx@shao.ac.cn)

Subjects: Gamma Ray, >GeV, AGN, Variables

In our analysis of Fermi LAT unidentified sources at high Galactic latitudes, we found that the source 3FGL J1557.0-4225 had two flaring events. We re-analyzed the Fermi LAT data, and determined its position at RA=238.846 deg, Decl.=-41.839 deg (J2000.0), with 1-sigma uncertainty of 0.036 deg. The two flares occurred during approximately MJD 55943-56003 and 56363-56383, and the flux reached 60.5+/-8.6 x 10^-9 ph/s/cm^2 and 98.7+/-16.2 x 10^{-9} ph/s/cm^2, respectively. Comparing to the average flux of 17.1+/-1.1 x 10^-9 ph/s/cm^2, the flares are significant at a 5-sigma level. In addition, the source emission can be well described by a simple power law, with photon index 2.82+/-0.08 for the whole data. During the two flares, the emission was significantly harder, with photon index being 2.38+/-0.11 and 2.47+/-0.16, respectively. We searched for counterparts in the error circle, there is only one radio source, PMN J1555-4150, approximately 1 arcmin away. Given these, we suggest that 3FGL J1557.0-4225 is likely an AGN. Further X-ray and radio observations of the source are encouraged.