Swift observations of the flaring TeV blazar 1ES 1959+650: the outburst continues in X-rays, and is also detected in the UV and optical

ATel #16941; S. Komossa (MPIfR, NAOC), D. Grupe (NKU), J. Y. Wei (NAOC), D. W. Xu (NAOC)
on 12 Dec 2024; 19:26 UT
Credential Certification: St. Komossa (stefanie.komossa@gmx.de)

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We report on Swift XRT and UVOT observations of the flaring TeV blazar 1ES 1959+650 (RA=19:59:59.85, DEC=+65:08:54.65). A major outburst of the blazar was detected by SVOM on December 6 (ATel #16935), and an increase in VHE emission was reported as well (ATel #16939). Swift observed 1ES 1959+650 on 2024 December 12. Given the high source brightness, the XRT observations were carried out in WT mode to avoid pile-up. The data confirm that the outburst is ongoing but has already declined. With the XRT, we measure an absorbed flux of f = 4.74(+/-0.08) 10^-10 erg/cm^2/s in the observed 0.3-10 keV band. That flux 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 = 2.03 +/- 0.03. This flux is still a factor 5.2 higher compared to a pre-outburst observation of 1ES 1959+650 carried out with Swift on 2024 August 8 where f = 9.1(+/-0.2) 10^-11 erg/cm^2/s and Gamma_x = 2.35 +/- 0.04.

1ES 1959+650 was observed with the Swift UVOT as well. We selected all filters to obtain an SED and its evolution. We find the following observed Vega magnitudes of V=14.78+/-0.04, U=14.43+/-0.05, W1=14.48+/-0.04, M2=14.71+/-0.05, and W2=14.65+/-0.04 mag. These are up to 0.9 Vega magnitudes brighter than the values of 2024 August 8, when we measured V=15.34+/-0.05, B=15.84+/-0.04, U=15.22+/-0.04, W1=15.38+/-0.06, M2=15.50+/-0.06, and W2=15.53+/-0.05 mag.

Finally, the optical V-band position of the outburst source measured with the UVOT is at RA=19:59:59.87 and DEC=+65:08:54.7 (J2000), with a positional uncertainty better than 3", confirming 1ES 1959+650 as the counterpart.

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.