Fermi and Swift observations of correlated outburst activity from the BL Lac object OT 081 (PKS 1749+096)

ATel #9260; Stefano Ciprini (Rome ASDC/ Perugia INFN, Italy), Josefa Becerra Gonzalez (NASA/GSFC/UMCP/CRESST, USA), Giovanna Pivato (Pisa INFN/Univ., Italy), David J. Thompson (GSFC/NASA, USA), on behalf of the Fermi Large Area Telescope Collaboration
on 24 Jul 2016; 10:54 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@asdc.asi.it)

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We report a strong multi-wavelength outburst in the BL Lac object OT 081 (also known as PKS 1749+096, 4C +09.57) with the radio counterpart position R.A.: 267.88674 deg, Dec.: 9.65020 deg (J2000.0, Lanyi et al. 2010, AJ, 139, 1695) and with redshift z=0.322 (Stickel, Fried, Kuehr 1988, A&A, 191, 16). Gamma ray and X-ray flares reached peaks during the interval 2016 July 16 and 20 simultaneous with an optical outburst (Balonek et al, 2016, ATel #9259).

The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, on 2016 July 16 and July 20 observed the highest daily-averaged gamma-ray fluxes (E>100 MeV) from the BL Lac object OT 081 (also known as PKS 1749+096 and 4C +09.57) since the beginning of its all-sky survey, (3.9+/-0.4) X 10^-6 photons cm^-2 s^-1 and (4.3+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only) respectively. These are about 75 times greater than its four-year average flux reported in the third Fermi-LAT source catalog (3FGL J1751.5+0939, Acero et al. 2015, ApJS, 218, 23). The corresponding daily averaged photon spectral indexes (E>100 MeV) of 2.0+/-0.1 and 1.89+/-0.04 (statistical uncertainty only) are harder than the value of 2.25 for a power-law fit reported in the 3FGL Catalog. On July 20, high-energy photons of 18 and 34 GeV were seen, positionally consistent with the source. 6-hour integrated gamma-ray fluxes (E>100 MeV) in these two days span the range (3.5 - 6.8) X 10^-6 photons cm^-2 s^-1. The beginning of the gamma-ray activity on July 9 was already reported in ATel#9231.

High-urgency follow-up ToO Swift observations were performed on 2016 July 17 and July 20 close to these peaks of unprecedented gamma ray activity. Swift XRT data were taken in Photon Counting mode. The preliminary X-ray spectrum (0.3-10 keV) can be fit by an absorbed power law model with an HI column density set to the Galactic value of 9.4 X 10^20 cm^-2 (Kalberla et al. 2005, A&A, 440, 775). The unabsorbed 0.3-10 keV fluxes of July 17 and July 20 were respectively (1.5+/-0.2)X10^-11 erg cm^-2 s^-1 and (1.7+/-0.1)X10^-11 erg cm^-2 s^-1, comparable to the peak X-ray flux detected by Swift XRT in 2007 February 26 of (1.3+/-0.2)X10^-11 erg cm^-2 s^-1. The corresponding hard photon spectral indexes respectively of 1.3+/-0.1 and 1.2+/-0.1 suggest a dominant inverse Compton emission for the X-ray spectrum, related to the high-level GeV activity. Simultaneous Swift UVOT observations obtained using all the filters are given in observed magnitudes. For July 17: V=15.38+/-0.05, B=16.09+/-0.04, U=15.41+/-0.04, UV-W1=15.87+/-0.05, UV-M2=16.66+/-0.06, UV-W2=16.41+/-0.05. For July 20: V=15.53+/-0.05, B=16.23+/-0.04, U=15.61+/-0.04, UV-W1=15.88+/-0.05, UV-M2=16.88+/-0.09, UV-W2=16.54+/-0.05. These UVOT values are about 1 magnitude fainter than the values observed on 2007 February 26. The time-correlated gamma-ray/X-ray/UV/ and optical (ATel #9259) outburst also confirms the source identification of 3FGL J1751.5+0939 with the BL Lac object OT 081 (PKS 1749+096).

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of this source, we encourage multiwavelength observations. For this source the Fermi LAT contact person is J. Becerra Gonzalez (josefa.becerra@nasa.gov). The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

We thank the Swift Team for making these observations possible, in particular D. Malesani as the Swift Observatory Duty Scientist. The NASA Swift gamma-ray burst explorer is a MIDEX Gamma Ray Burst mission led by NASA with participation of Italy and the UK.