Possible Swift Counterpart of the Gamma-ray Transient Fermi J1717-5156

ATel #4042; T.-N. Lu, K. L. Li, A. K.H. Kong (National Tsing Hua University, Taiwan)
on 15 Apr 2012; 10:45 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

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We report follow-up Swift observations on the newly discovered gamma-ray transient Fermi J1717-5156 (Atel #4023). We combined three Swift XRT PC mode datasets resulting in a total exposure time of ~17.2ksec. A bright X-ray source is clearly detected at the gamma-ray transient location with R.A.=17:17:34.47, Decl.=-51:55:31.5 (J2000) and an error radius of 3.5 arcsec (90% confidence). We determined it as the primary X-ray counterpart. Its X-ray spectrum could be fitted with an absorbed power law model with a photon index of 1.59 (+0.14, -0.13). The fitted column density is 2.99 (+0.65, -0.60) E+21 cm^-2 and the model predicted unabsorbed flux is 5.40 (+0.36, -0.33) E-12 erg cm^-2 s^-1 (0.3-10 keV). Among the 17.2ks observations, there is no obvious flux change and hardness ratio change. We could not find a source in the previous ROSAT observations (exposure time ~620s). With the X-ray flux from Swift XRT data, PIMMS predicts a flux = 6.129E-02 cps for ROSAT PSPC with energy range 0.1-2.0 keV.

Apart from the primary X-ray counterpart, there are three other X-ray sources within the 0.18 deg error circle (95% confidence) of Fermi J1717-5156 (Atel #4023).

17:17:26.80, -51:56:52.8 (J2000), error radius = 4.1 arcsec (offset from the best-fit LAT position: 0.03 deg)
17:17:24.82, -51:57:45.9 (J2000), error radius = 4.0 arcsec (offset from the best-fit LAT position: 0.05 deg)
17:17:28.16, -51:58:27.1 (J2000), error radius = 4.1 arcsec (offset from the best-fit LAT position: 0.05 deg)

These three sources are too faint for spectral analysis. We could not rule out their possibilities of being the real counterpart, although the primary X-ray counterpart we mentioned above is the brightest and closest to the gamma-ray center.

We also checked the Swift UVOT data and spotted an faint UVOT source corresponding to the primary X-ray counterpart. There is an indication of optical variability among the three UVOT datasets as seen below (and we adopt Vega magnitudes here):

2012-04-09
UW1: 18.64 +/- 0.07
U: 17.98 +/- 0.06
B: 18.59 +/- 0.07
V: 17.96 +/- 0.10

2012-04-11
UW1: 18.09 +/- 0.07
U: 17.51 +/- 0.05
B: 18.07 +/- 0.06
V: 17.48 +/- 0.06

2012-04-13
UW1: 18.43 +/- 0.08
U: 17.76 +/- 0.06
B: 18.30 +/- 0.08
V: 17.77 +/- 0.10

Inspecting other possible counterparts, we found no 2MASS counterpart and a counterpart from the SuperCOSMOS Sky Survey with B-band = 19.091, R-band = 18.366, and I = 17.874 near the position of the UVOT source. This suggests that the source has brightened by 1 magnitude in the B-band.

For now, there are multiwavlength counterparts reported for Fermi J1717-5156 and all of them have slight positional offsets from the Fermi LAT center: WISE J171734.65-515532.0 is 0.4 arcsec; the radio source PMN J1717-5155 is 0.03 arcsec; Swift XRT primary counterpart is 2.1 arcsec; Swift UVOT primary counterpart is 0.16 arcsec.

Based on the primary X-ray counterpart spectrum and previous reports on the WISE counterpart of this source (Atel #4029), we agree with Massaro et al. (Atel #4029) that Fermi J1717-5156 is a gamma-ray blazar. Sambruna et al. (2010) suggested a correlation between the gamma-ray and hard X-ray spectrum for blazars and separated them into two groups. According to our Swift X-ray observations, the fitted photon index (~1.6) is consistent with a flat spectrum radio quasar (FSRQ), belonging to the luminous blazar group in Sambruna et al. (2010).

We acknowledge the use of public (quick-look) data from the Swift data archive.