VLT/X-Shooter spectrum of the blazar TXS 0506+056 (located inside the IceCube-170922A error box)

ATel #10840; Alexis Coleiro (APC / IFIC) & Sylvain Chaty (AIM / Universit\'e Paris Diderot)
on 11 Oct 2017; 08:44 UT
Credential Certification: Alexis Coleiro (coleiro@apc.in2p3.fr)

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The blazar TXS 0506+056 (PMN J0509+0541) is currently reported to show increased gamma-ray and optical activity (ATel #10791, #10792, #10794, #10799, #10801, #10817, #10830, #10831, #10838) and has been proposed as the counterpart to the high-energy neutrino event IceCube-170922A (https://gcn.gsfc.nasa.gov/notices_amon/50579430_130033.amon).

We obtained a spectrum of TXS 0506+056 (starting on 2017-10-01 at 07:36:50 UTC) using the X-Shooter spectrograph on the ESO / Very Large Telescope (VLT).

The spectrum covers the wavelength range 3000 - 25,000 Angstrom with a resolution R~4300, ~7400 and ~5400 respectively in the UVB, VIS and NIR arms, and a total exposure time of 30 min.

With a preliminary data reduction, the spectrum shows Ca K&H (3933.7 AA, 3968.5 AA) and Na I D (5892.5 AA) absorption lines attributable to Galactic interstellar absorption. No other absorption lines are clearly identified. It is consistent with the spectrum of a non-thermally dominated blazar as reported in ATel #10799 and #10830. The UVB-VIS spectrum (wavelength range 3100 - 9000 AA), can be fitted by a single power law after preliminary flux calibration and dereddening. It confirms a bluer spectrum than the spectrum published in Halpern et al. (2003, AJ, 125, 572), as previously mentioned in ATel #10799.

The non-detection of Lyman alpha absorption in the X-Shooter spectrum indicates a rough upper limit on the redshift of TXS 0506+056: z < 1.6.

Further data analysis is ongoing.