Swift and INTEGRAL observation of 1ES 1959+650

ATel #1317; V. Beckmann, W. Ishibashi (ISDC), E. Bottacini, M. Ajello, J. Greiner (MPE)
on 1 Dec 2007; 19:32 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Volker Beckmann (Volker.Beckmann@obs.unige.ch)

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Following the report of a bright hard X-ray state of 1ES 1959+650 as seen in INTEGRAL IBIS/ISGRI (ATel #1315: 3.2e-11 erg/cm**2/sec at 20-40 keV), Swift performed a pointed observation of the source. The observation started at 2007-11-30T17:57 with an effective exposure time of the XRT of 1279 sec. The XRT data are well represented by a single power law model with photon index 1.76+-0.04 plus Galactic absorption (NH = 1.0e21 1/cm**2). The 2-10 keV flux is 1.3e-10 erg/cm**2/sec, corresponding to a luminosity of Lx = 6.8e44 erg/sec. Swift/UVOT data show that 1ES 1959+650 had UVW2 = 14.89 +- 0.02 mag (0.75 mJy), UVW1 = 14.72 +- 0.01 mag (1.16 mJy), U = 14.61 +- 0.02 mag (2.01 mJy), and B = 15.42 +- 0.02 mag (2.79 mJy) during the observation.

Simultaneously the source was again observed by INTEGRAL IBIS/ISGRI from 2007-11-28T20:16 until 2007-12-01T09:37 with an effective exposure time of 205 ksec. The blazar is not detectable anymore, with a 3 sigma upper limit of 1.5e-11 erg/cm**2/sec in the 20-40 keV energy band. This upper limit is 4 times lower than the expected flux when extrapolating the Swift/XRT spectrum to the IBIS/ISGRI band. This might imply that the peak of the synchrotron emission during the observation occurred between the two energy bands, i.e. in the range ~3 to 20 keV. In order to estimate the energy of the synchrotron peak, the Swift/XRT data can be fit with an exponential cutoff power law with photon index 1.5 +- 0.1 and a high energy cut off at Ec = 10 +- 5 keV. This model gives a 20-40 keV flux of 1.3e-11 erg/cm**2/sec, consistent with the IBIS/ISGRI upper limit, and the peak of the synchrotron emission would be around 5 keV.