Swift detects an outburst of IGR J17407-2808/CXOU J174042.0-280724

ATel #3685; P. Romano (INAF-IASF Palermo), S. D. Barthelmy (GSFC), H. A. Krimm (CRESST/GSFC/USRA), P. Esposito (INAF-OAC), V. Mangano (INAF-IASF Palermo), M. Siegel (PSU), D. N. Burrows (PSU), P. A. Evans (LU), R. Farinelli (INAF-IASF Palermo, U Ferrara), J. A. Kennea (PSU), D. M. Palmer (LANL), S. Vercellone (INAF-IASF Palermo), N. Gehrels (GSFC)
on 15 Oct 2011; 18:08 UT
Credential Certification: Pat Romano (romano@ifc.inaf.it)

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On 2011 October 15 at 01:12:40 UT the Swift BAT triggered on an outburst from the fast hard X-ray transient IGR J17407-2808 (Kretschmar et al. 2004,ATel #345). Sguera et al (2006,ApJ,646,452) proposed IGR J17407-2808 as a candidate Supergiant Fast X-ray Transient (SFXT), based on its hard X-ray behaviour, although they noted the peculiarly quick strong flares (20-60 keV, peak flux 800 mCrab, lasting a couple of minutes), thus not ruling out an alternative source nature.

We analized the BAT observations using the data set from T-239 to T+502 s from recent telemetry downlinks. The partial coding was 92%. The BAT ground-calculated position is RA, Dec = 265.209, -28.090 deg, or
RA(J2000) = 17h 40m 50.1s
Dec(J2000) = -28d 05' 25.8"
with an uncertainty of 3.2 arcmin, (radius, sys+stat, 90% c.l.). The mask-weighted light curve shows a single peak starting at ~T-20 s, peaking at ~T+30 s, and ending at ~T+90 s. There is a possible increase (at ~T+900 s) as the source left the BAT FoV (at T+950 s) due to a spacecraft slew. T90 (15-350 keV) is 80+/-36 s (including systematics).

The BAT time-averaged spectrum from T-31.0 to T+65.0 s is best fit by a simple power-law model, with a photon index of 1.89+/-0.38. The 1-s peak photon flux measured from T+40.5 s is 0.3+/-0.1 ph/cm2/s (15-150 keV). All quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/505516/BA/ .

Swift slewed immediately. The XRT began observing the field 131.3 s after the BAT trigger. Using the first 772 s of Photon Counting (PC) mode data, and correcting for the astrometric errors by utilizing Swift/UVOT data (methods of Evans et al. 2009,MNRAS,397,1177), we find the source at the following location: RA,Dec (J2000) = 265.17532, -28.12403, or
RA(J2000) = 17h 40m 42.08s,
Dec(J2000) = -28d 07m 26.5s,
with an estimated error of 2.4 arcsec radius (90% c.l.). This position is 1.6 arcsec from CXOU J174042.0-280724 (Tomsick et al. 2008,ApJ,685,1143) that Heinke et al. (2009,ApJ,701,1627) proposed as the likely counterpart of IGR J17407-2808. The XRT position is inconsistent (1.4 arcmin) with the position of 2RXP J174040.9-280852, previously indicated as the soft X-ray counterpart of IGR J17407-2808. The simultaneous BAT and XRT data can therefore unequivocally establish CXOU J174042.0-280724 as the soft X-ray counterpart of IGR J17407-2808.

The XRT light curve shows a sharp rise to a bright flare that reached ~10 counts/s at ~T+900 s, and a total dynamic range of 100. The XRT spectrum (PC data, T+139 s to T+911 s, 772 s integration) can be fitted with an absorbed power law, with a photon index of -0.06-0.70+0.79, an absorbing column density of NH=(1.46-1.33+2.20)E+22 cm-2 (consistent with the Galactic value of 7.30E21 cm-2, Kalberla et al. 2005) and an average flux of ~6E-11 erg/cm2/s (2-10 keV, unabsorbed).

The UVOT began observing the field 137 s after the BAT trigger. We do not detect the source in any of the 7 UV/optical filters down to a limit in the white filter of 20.7 mag. Galactic extinction along this line of sight is expected to be E(B-V) = 1.92 (Schlegel et al. 1998).

In the soft X-ray, the present outburst is several times brighter than the brightest Chandra flare of CXOU J174042.0-280724 ((8-2+4)E-12 erg/cm2/s, Heinke et al. 2009), observed in 2007 while the source was not in outburst. As the faintest Chandra flux reported is 1.7E-13 erg/cm2/s, the soft X-ray dynamic range of this source is at least 200. We also note that the flares observed by BAT are shorter than the flares we typically observe in confirmed SFXTs, as previously reported by Sguera et al (2006) for INTEGRAL observations.