Swift monitoring of the high-z FSRQ PKS 0438-43

ATel #15027; Filippo D'Ammando (INAF-IRA Bologna)
on 9 Nov 2021; 17:10 UT
Credential Certification: Filippo D'Ammando (dammando@ira.inaf.it)

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We are carried out an X-ray monitoring of the high-redshift (z = 2.852) flat spectrum radio quasar PKS 0438-43 with the Neil Gehrels Swift satellite. Observations have been performed on 2021 October 2, 9, 25, 29, and November 8.

Swift-XRT data were taken in Photon Counting mode on 2021 October 2, 9, 25, 29 and November 8 for a net exposure of about 2.7, 2.9, 2.5, 2.4, and 1.8 ksec, respectively. The X-ray spectrum (0.3-10 keV) 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 = 1.28 x 10^21 cm^-2, Ben Bekhti N. et al., 2016, A&A, 594, A116) and a photon index of 1.68 +/- 0.14, 1.68 +/- 0.11, 1.64 +/- 0.11, 1.68 +/- 0.15, and 1.55 +/- 0.12, respectively. The corresponding unabsorbed 0.3-10 keV flux observed on 2021 October 2, 9, 25, 29, and November 8 is (6.7 +/- 0.5)x10^-12, (10.4 +/- 0.6)x10^-12, (10.7 +/- 0.7)x10^-12, (9.7 +/- 0.7)x10^-12, and (16.8 +/- 1.8)x10^-12 erg cm^-2 s^-1, respectively.

As a comparison, the summed spectrum of the source collected between 2010 December 26 and 2011 January 7 (for a total exposure of 4.7 ks) and between 2016 December 15-21 (for a total exposure of 2.9 ks) can be well fitted by an absorbed power-law with a photon index of 1.61 +/- 0.26 and 1.37 +/- 0.12, respectively. The corresponding unabsorbed 0.3-10 keV flux of the source is (1.2 +/- 0.2)x10^-12 and (9.6 +/- 0.6)x10^-12 erg cm^-2 s^-1, respectively.

PKS 0438-43 has been observed by Swift-XRT in 2021 October during a high X-ray activity period, with fluxes comparable to the value observed on 2018 December, at the time of a gamma-ray flaring activity (ATel #9854), and a relatively softer photon index. On 2021 November 8, the source reached the historical maximum in X-rays.

In consideration of the ongoing activity of this source, confirmed also by the Swift-XRT observations, we encourage further multi-wavelength observations. Further Swift observations of the source are planned on 2021 November 12 and 16.

We would like to thank the Swift Team, the Swift ODS, and the PI for making these observations possible.