NICER, NuSTAR, and Swift follow-up observations of the γ-ray flaring blazar BL Lacertae in 2020 August–October

Abstract

ABSTRACT During a period of strong γ-ray flaring activity from BL Lacertae, we organized Swift, Neutron star Interior Composition Explorer (NICER), and Nuclear Spectroscopic Telescope Array (NuSTAR) follow-up observations. The source has been monitored by Swift-XRT (X-ray Telescope) between 2020 August 11 and October 16, showing a variability amplitude of 65, with a flux varying between 1.0 × 10−11 and 65.3 × 10−11 erg cm−2 s−1. On 2020 October 6, Swift-XRT has observed the source during its historical maximum X-ray flux. A softer-when-brighter behaviour has been observed by XRT, suggesting an increasing importance of the synchrotron emission in the X-ray part of the spectrum covered by XRT during this bright state. Rapid variability in soft X-rays has been observed with both the Swift-XRT and NICER observations with a minimum variability time-scale of 60 and 240 s, and a doubling time-scale of 274 and 1008 s, respectively, suggesting very compact emitting regions (1.1 × 1014 and 4.0 × 1014 cm). At hard X-rays, a minimum variability time-scale of ∼5.5 ks has been observed by NuSTAR. We report the first simultaneous NICER and NuSTAR observations of BL Lacertae during 2020 October 11–12. The joint NICER and NuSTAR spectra are well fitted by a broken power law with a significant difference of the photon index below (2.10) and above (1.60) an energy break at ∼2.7 keV, indicating the presence of two different emission components (i.e. synchrotron and inverse Compton) in the broad-band X-ray spectrum. Leaving the total hydrogen column density towards BL Lacertae free to vary, a value of NH,tot = (2.58 ± 0.09) × 1021 cm−2 has been estimated.