New Swift observations of the SVOM-discovered outburst of the TeV blazar 1ES 1959+650: X-rays are rising again

ATel #16955; S. Komossa (MPIfR), D. Grupe (NKU), J. Y. Wei (NAOC), F. Cangemi (APC), A. Coleiro (APC), A. Foisseau (APC)
on 16 Dec 2024; 16:03 UT
Credential Certification: St. Komossa (stefanie.komossa@gmx.de)

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We report on new Swift XRT and UVOT observations of the TeV blazar 1ES 1959+650 (RA=19:59:59.85, DEC=+65:08:54.65) that was discovered in an X-ray outburst with SVOM on December 6 (ATel #16935) and also showed enhanced VHE emission (ATel #16939). The first Swift follow-up observations of 2024 December 12 (Atel #16941) showed that the X-rays were no longer at the high level of December 6, but were still a factor of 5 higher than during previous Swift observations of August 2024. The UV and optical emission had increased by up to 0.9 mag.

Swift re-observed 1ES 1959+650 on December 14 with the XRT and UVOT. The XRT observation, carried out in WT mode to avoid pile up, shows that the X-ray emission has increased again, and that the outburst is ongoing and may be long lasting. We obtain an absorbed flux of f = 7.37(+/-0.10) 10^-10 erg/cm^2/s in the observed 0.3-10 keV band. That measurement is based on a single powerlaw fit with absorption fixed at the Galactic value of N_H=1.01 10^21 cm^-2 and a photon index Gamma_x = 1.85+/-0.02. We also inspected an archival Swift XRT observation of 2024 December 15, and find Gamma_x = 1.86 +/-0.01 and f = 9.93(+/-0.08) 10^-10 erg/cm^2/s. This flux is a factor of 2.1 higher than the one measured 3 d earlier on December 12 (Atel #16941).

The Swift UVOT re-observed 1ES 1959+650 on December 14 in all filters. The UV and optical emission only mildly increased by 0.1 mag, insignificant within the uncertainties when compared to the data of December 12. The B filter observation of December 12 could not be used. On December 14, we found B=15.08+/-0.04 mag which is brighter than the August 8 observation of B=15.84+/-0.04 mag.

We would like to thank Brad Cenko for approving our observing request and the Swift team for carrying out the observations. S.K. would like to thank the SVOM team at NAOC Beijing for their hospitality during a visit in November/December.