NICER follow-up observations of the flaring blazar TXS 0646-176
ATel #15000; Filippo D'Ammando (INAF-IRA Bologna) and Giulia Migliori (INAF-IRA Bologna)
on 26 Oct 2021; 15:48 UT
Credential Certification: Filippo D'Ammando (dammando@ira.inaf.it)
Subjects: X-ray, AGN, Black Hole, Blazar, Quasar
Following the recent gamma-ray flaring activity of the flat spectrum radio quasar TXS 0646-176 (ATel #14980, ATel #14981, ATel #14993), the source has been observed by NICER for a ToO request (NICER GO Cycle 3 ID: 4115, PI: D'Ammando) between 2021 October 19 21:08 UTC and October 25 05:53 UTC for an effective time of 61.5 ks.
The source is detected above the background across the energy interval 0.5-6.0 keV in the quick look data, with an average count rate of 2.0 counts/s. Variations of the count rate up to a factor of 2.5 has been observed across the observation, with possible short flares.
Preliminary analysis shows that the NICER spectrum collected over the entire period in the 0.5-6.0 keV energy range can be described by an absorbed power-law model with a HI column density consistent with the Galactic value in the direction of the source (n_H = 2.37 x 10^21 cm^-2, Ben Bekhti et al., 2016, A&A, 594, A116) and a photon index of 1.22 +/- 0.01. The corresponding (observed) 0.3-10 keV flux is (7.9 +/- 0.1)e-12 erg cm^-2 s^-1. The photon index is similar to the ones reported for the X-ray observations carried out by Swift-XRT (ATel #14987) and SRG/eROSITA (ATel #14991) on 2021 October 18 and 2021 October 14-15, respectively. The X-ray flux estimated by NICER over the period 2021 October 19-25 is slightly lower than the values recently observed by Swift-XRT and SRG/eROSITA.
In consideration of the ongoing activity of this source, confirmed also by optical observations performed by REM (ATel #14999), we encourage further multi-wavelength observations.
We thank the NICER PI, Keith Gendreau, and the operation team for the rapid approval and execution of the ToO request. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.