INTEGRAL sees brightening of NGC 2617

ATel #5103; S. Tsygankov (1, 2, 3), R. Krivonos (1, 4), S. Sazonov (1), A. Lutovinov (1), R. Sunyaev (1, 5) (1: IKI, Moscow; 2: FINCA, Turku; 3: University of Oulu; 4: University of California, Berkeley; 5: MPA, Garching)
on 4 Jun 2013; 16:10 UT
Credential Certification: Roman Krivonos (krivonos@iki.rssi.ru)

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During the Galactic Latitude Scan observations of the Puppis region performed by the INTEGRAL observatory between April 22 and June 2, 2013, we have discovered a new source, IGR J08357-0404, with the coordinates R.A.=08 35 42.48, Decl.=-04 04 37.2 (equinox 2000.0, uncertainty 4.2 arcmin). The source was registered at a 9 sigma significance level with the flux 3.3+/-0.4 mCrab (4.7e-11 erg/cm2/s) in the 17-60 keV energy range.

The position of the source coincides with the nearby Seyfert galaxy NGC 2617 (z=0.014213), which has recently exhibited an optical outburst and shown a dramatic spectral evolution over the last decade with a transition from Seyfert Type 2 to Seyfert Type 1 (ATel #5010). An extensive Swift/XRT monitoring in May 2013 showed a gradual increase of the 0.2 - 12 keV flux of NGC 2617, with a nearly constant X-ray spectral slope (ATel #5059).

Adopting the known distance to NGC 2617, the 17-60 keV luminosity of IGR J08357-0404 is Log(L)=43.27. The source was under the detection threshold (1-sigma upper limit is 1.6 mCrab in this position) in the INTEGRAL 9-year Galactic plane survey (Krivonos et al., 2012, private comm.) and is not present in the 70-month Swift/BAT survey either (Baumgartner et al., 2012).

Taking into account the order of magnitude increase of both the soft and hard X-ray flux (ATel #5059, and this report) and the emergence of broad emission lines in the optical spectrum (ATel #5010), NGC 2617 appears to have experienced a strong outburst accompanied by changes in the structure of the obscuring torus. Continued follow-up at all possible wavelengths is encouraged in order to further track the source luminosity and its properties.