Swift follow-up of the flaring FSRQ PKS 1127-14
ATel #14265; Filippo D'Ammando (INAF-IRA Bologna)
on 14 Dec 2020; 16:55 UT
Credential Certification: Filippo D'Ammando (dammando@ira.inaf.it)
Subjects: Optical, Ultra-Violet, X-ray, AGN, Black Hole, Blazar, Quasar
Following the flaring activity of the flat spectrum radio quasar PKS 1127-14 at redshift z = 1.184 (Wilkes 1986, MNRAS, 218, 331) detected in gamma rays by Fermi-LAT on 2020 December 10 (ATel #14260), a Swift target of opportunity observation was performed on 2020 December 13.
Swift-XRT data were taken in Photon Counting mode for a net exposure of about 1.1 ksec. Taking into consideration the Swift observations performed during 2007-2020, the source reached on 2020 December 13 one of the highest count rate observed in the 0.3-10 keV energy range (see Swift-XRT Monitoring of Fermi-LAT Sources of Interest).
The X-ray spectrum (0.3-10 keV) observed on 2020 December 13 can be fit by an absorbed power law model with a HI column density consistent with the Galactic value in the direction of the source (n_H = 3.5 x 10^20 cm^-2, Ben Bekhti N. et al., 2016, A&A, 594, A116) and a photon index of 1.26 +/- 0.19. The corresponding (observed) 0.3-10 keV flux is (9.5+/-1.2) x10^-12 erg cm^-2 s^-1. That flux is a factor of 3 higher than the value observed by Swift-XRT on 2011 April 3, (3.3+/-0.4) x10^-12 erg cm^-2 s^-1 (with a photon index of 1.31 +/- 0.20) and comparable to the value observed by Swift-XRT on 2007 March 24, (9.1+/-0.3) x10^-12 erg cm^-2 s^-1 (with a photon index of 1.18 +/- 0.05). The observations performed on 2011 April 3 and 2007 March 24 represent the lowest and highest X-ray flux observed by Swift-XRT so far.
Simultaneous Swift-UVOT observations were performed on 2020 December 13. The measured magnitudes in optical and UV bands show an intermediate brightness between the values observed on 2011 April 3 and 2007 March 24 (see table below).
| Band | 2020-12-13 | 2011-04-03 | 2007-03-24 |
|---|
| V | 16.87 +/- 0.14 | 16.99 +/- 0.14 | 16.44 +/- 0.04 |
| B | 17.05 +/- 0.09 | 17.23 +/- 0.10 | 16.68 +/- 0.04 |
| U | 15.99 +/- 0.08 | 16.17 +/- 0.08 | 15.61 +/- 0.05 |
| W1 | 15.88 +/- 0.08 | 16.07 +/- 0.09 | 15.45 +/- 0.05 |
| M2 | 15.91 +/- 0.07 | 16.08 +/- 0.06 | 15.56 +/- 0.06 |
| W2 | 16.19 +/- 0.08 | 16.30 +/- 0.08 | 15.80 +/- 0.05 |
In consideration of the ongoing activity of this source, confirmed also by the Swift observations, we encourage further multi-wavelength observations.
We would like to thank the Swift Team for making these observations possible, in particular B. Sbarufatti as the Swift Observatory Duty Scientist.
