Swift follow-up of the flaring blazar B2 1633+38 (4C +38.41)

ATel #12037; F. D'Ammando (INAF-IRA Bologna), F. Longo (University and INFN, Trieste) on behalf of the Fermi Large Area Telescope Collaboration
on 14 Sep 2018; 20:13 UT
Credential Certification: Filippo D'Ammando (dammando@ira.inaf.it)

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Following the unprecedented optical outburst (ATel #12025) and the gamma-ray flare detected by AGILE (ATel #12026) and Fermi LAT (ATel #12027) on 2018 September 9, a Swift target of opportunity observation of the flat spectrum radio quasar B2 1633+38 (also known as 4C +38 41) was performed on September 13.

Swift/XRT data were taken in Photon Counting mode for a net exposure of about 1.9 ksec. The X-ray spectrum (0.3-10 keV) observed on 2018 September 13 can be fit by an absorbed power law model with an HI column density consistent with the Galactic value in the direction of the source (n_H = 1.1 x 10^20 cm^-2, Kalberla et al. 2005, A&A, 440, 775) and a photon index of 1.41+/-0.14. The net count rate observed is 0.127+/-0.010 cnt/s, corresponding to an observed 0.3-10 keV flux of (9.1+/-0.7) x10^-12 erg cm^-2 s^-1.The count rate and flux observed indicate a relatively high activity state in X-rays with respect e.g. to the Swift observations in 2009-2011 (Raiteri et al. 2012, A&A 545, A48). The hard photon index obtained on 2018 September 13 confirms the harder-when-brighter behavior in X-rays observed in 2011 during a previous gamma-ray flare of the source.

Simultaneous Swift/UVOT observations were performed on 2018 September 13. The measured magnitudes are: V = 16.29 +/- 0.10, B = 16.65 +/- 0.07, U = 15.83 +/- 0.06, W1 = 16.10 +/- 0.08, M2 = 17.30 +/- 0.13, W2 = 17.69 +/- 0.13, in agreement with the optical magnitudes reported in ATel #12034, and confirming the high activity state of the source in optical and UV.

In consideration of the ongoing activity of this source, confirmed also by the Swift observations, we encourage further multiwavelength observations. The Fermi LAT contact persons for this source are S. Ciprini (stefano.ciprini at ssdc.asi.it) and S. Buson (sara.buson at gmail.com).

We would like to thank the Swift Team for making these observations possible, in particular B. Sbarufatti as the Swift Observatory Duty Scientist.