NuSTAR and NICER follow-up observations of the flaring blazar BL Lacertae
ATel #14096; Filippo D'Ammando (INAF-IRA Bologna)
on 15 Oct 2020; 11:18 UT
Credential Certification: Filippo D'Ammando (dammando@ira.inaf.it)
Subjects: X-ray, AGN, Black Hole, Blazar
Following the historical maximum flux of the blazar BL Lacertae observed in optical, UV, X-ray, and gamma-rays (ATel #14065, #14069, #14072), NuSTAR observed the source for a DDT request (PI: D'Ammando) between 2020 October 11 14:05:18 UTC and October 12 07:15:00 UTC, for an effective exposure time of 30.7 ks. The source is detected across the energy interval 3-79 keV in the quick look data, at a net count rate of about 0.23 counts/s for both the focal plane modules.
The data were reduced by performing the standard data reduction procedure, using the NuSTAR Data Analysis Software nustardas v1.9.2 with a caldb version 20200912 and assuming 70 arcsec circle as the source region. The count rate shows an increasing trend in the second part of the NuSTAR observation. Preliminary analysis has been performed fitting the 3-79 keV X-ray spectra with an absorbed power-law with the Galactic absorption corresponding to a hydrogen column density of n_H = 3.4 x 10^21 cm^-2 (Raiteri et al. 2010, A&A, 524, 43). The fit results in a photon index of 1.69 +/- 0.03 and an observed (i.e. not corrected for Galactic extinction) flux in the 3-79 keV energy range of (2.53 +/- 0.06) x10^-11 erg cm^-2 s^-1. The flux observed on 2020 October 11-12 is a factor of two higher than the values observed by NuSTAR during 2019 September 14-19 (Weaver et al. 2020, ApJ, 900, 137), confirming the high activity state of the source. The photon index estimated on 2020 October 11-12 is harder than the value estimated in 2019 over the entire observing period.
BL Lacertae has been observed also by NICER for a DDT request (PI: D'Ammando) between 2020 October 11 17:35:04 UTC and October 12 23:13:06 UTC for an effective time of 14.7 ks. The source is detected across the energy interval 0.3-5.0 keV in the quick look data, with an average count rate of 3.85 counts/s. Variations of the count rate up to a factor of two has been observed across the observation. Preliminary analysis shows that the joint NICER and NuSTAR spectrum in the 0.3-79 keV energy range is better described by a broken power-law than a single power-law. The spectral index obtained below and above the break is 2.41 +/- 0.06 and 1.56 +/- 0.03, respectively, with a break energy of 2.32 (+0.10, -0.12) keV. Leaving the Galactic absorption value free to vary we found n_H = (2.7 +/- 0.1) x 10^21 cm^-2.
We thank the NuSTAR PI, Fiona Harrison, for approving the DDT request, and the NuSTAR SOC for carrying out the observation and the excellent support.
We thank the NICER PI, Keith Gendreau, and the operation team for the rapid approval and execution of the DDT 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.
