Swift follow-up observations of the gamma-ray flaring FSRQ PMN J1903-6749

ATel #13774; A. Gokus (Remeis Observatory Bamberg & ECAP, University of Wuerzburg) & R. Angioni (SSDC/INFN)
on 2 Jun 2020; 17:46 UT
Credential Certification: Roberto Angioni (r.angioni90@gmail.com)

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Following the recent report on enhanced gamma-ray activity from the FSRQ PMN J1903-6749 (ATel #13767), we requested high-urgency ToO observations with the instruments on board of the Neil Gehrels Swift Observatory. Two observations were performed on May 29 and May 31 with a combined exposure of 3.9 ks.

We analyzed the X-ray data taken with Swift-XRT in the aforementioned ToO observations, as well as archival data from 2011 and 2012, fitting an absorbed power-law model (N_H = 4.1e20 cm^-2; HI4PI Collaboration et al., 2016, A&A, 594, A116), and obtained the following measurements (fluxes are in 10^-12 erg/cm2/s, count rates in cts/s):

MJD | Photon index | Flux | Count rate |
55706.3 | 1.2 +/- 0.4 | 2.0 +/- 0.7 | 0.04 |
55708.5 | 1.4 +/- 0.7 | 1.3 +/- 0.8 | 0.03 |
56156.2 | 0.8 +/- 0.6 | 1.9 +/- 0.9 | 0.03 |
56166.7 | 1.2 +/- 0.5 | 2.3 +/- 1.0 | 0.04 |
58998 | 1.35 +/- 0.18 | 6.5 +/- 1.1 | 0.13 |
59000 | 1.31 +/- 0.19 | 5.4 +/- 1.0 | 0.11 |

Optical/UV photometry was performed with all filters available for Swift-UVOT for both archival and current observations. We reduced the data following standard procedures. The reported flux density is dereddened by E(B-V)=0.074 in the direction of the source. The optical flux density in mJy for all Swift-UVOT observations is reported here:

MJD | B band | V band | U band |
55706.7 | 0.05 +/- 0.04 | -------------------- | 0.061 +/- 0.019 |
55708.5 | 0.06 +/- 0.04 | -------------------- | 0.071 +/- 0.022 |
56157.6 | 0.07 +/- 0.04 | 0.01 +/- 0.08 | 0.041 +/- 0.009 |
56166.7 | 0.05 +/- 0.04 | 0.16 +/- 0.08 | 0.048 +/- 0.019 |
58998.6 | 1.38 +/- 0.07 | 2.57 +/- 0.14 | 0.61 +/- 0.03 |
58998.8 | 1.55 +/- 0.07 | -------------------- | 0.77 +/- 0.04 |
59000.6 | 0.63 +/- 0.04 | 0.92 +/- 0.14 | 0.30 +/- 0.022 |

The corresponding UV flux density in mJy is:

MJD | W1 band | M2 band | W2 band |
55706.7 | 0.020 +/- 0.011 | ----------------------- | 0.019 +/- 0.006 |
55708.5 | 0.023 +/- 0.013 | 0.023 +/- 0.011 | 0.013 +/- 0.007 |
56157.6 | 0.012 +/- 0.011 | 0.011 +/- 0.010 | 0.015 +/- 0.006 |
56166.7 | 0.015 +/- 0.012 | ----------------------- | 0.017 +/- 0.006 |
58998.6 | 0.294 +/- 0.026 | 0.176 +/- 0.021 | 0.142 +/- 0.013 |
58998.8 | 0.411 +/- 0.025 | ----------------------- | 0.188 +/- 0.018 |
59000.6 | 0.155 +/- 0.014 | 0.087 +/- 0.012 | 0.077 +/- 0.009 |

The current X-ray flux shows an increase with respect to archival measurements by a factor between ~2 and ~5, while the optical/UV flux density increased by at least an order of magnitude. The maximum flux density in both the optical and UV bands was observed on 29 May (MJD 58998), while it had already decreased by a factor of at least 2 in the following observation, showing a faster flare decay with respect to the X-ray flux. Overall, these observations confirm the existence of an X-ray and optical flare close in time with the enhanced gamma-ray state of PMN J1903-6749.

We thank the Swift Team for making these observations possible, in particular, N. J. Klingler as the Swift Observatory Duty Scientist.