Swift follow-up observations of BL Lacertae
ATel #16855; Seiya Nozaki (Max Planck Institute for Physics), David Sanchez (Laboratoire d'Annecy de Physique des Particules, CNRS/IN2P3)
on 8 Oct 2024; 19:42 UT
Credential Certification: Seiya Nozaki (nozaki@mpp.mpg.de)
Subjects: Optical, Ultra-Violet, X-ray, AGN, Blazar
After the recent report of the optical flare of the blazar BL Lacertae (Atel #16806, #16810, #16830), we have been monitoring the gamma-ray emission of the BL Lacertae measured with the Fermi-LAT using the FlaapLUC pipeline (J.-P. Lenain (2018)). The gamma-ray flux (>100 MeV) exceeded the pre-defined threshold on Sep 20 and Oct 4, 2024. Fermi-LAT also detected several high-energy photons (>10 GeV), which indicated a sign of gamma-ray flares. Therefore, we requested Swift-ToO observations (ToO ID 21169, 21248), which were performed on Sep 24, 25, 26, and Oct 5, 6, 7, 2024. We report the preliminary analysis results of the Swift-XRT and Swift-UVOT observations here.
Swift-XRT data were taken in a Photon Counting mode. The observed X-ray flux (0.3-10 keV) based on the preliminary analysis using the automatic online pipeline (Evans et al. (2009)) is as below. Swift-XRT measured the highest flux on Oct 5, 2024 during this period. Fermi-LAT and LHAASO collaboration also reported the GeV/TeV gamma-ray flare on Oct 5, 2024 (Atel #16849, #16850). After Oct 5, the X-ray flux was decreased and the X-ray flux on Oct 7 was almost back to the one measured at the end of September 2024.
Date (MJD, exposure): Flux (0.3-10 keV)
Sep 24 (MJD 60577.96, 0.40 ks): 3.1 (+0.5, -0.5) x 10^-11 erg cm^-2 s^-1
Sep 25 (MJD 60578.55, 0.60 ks): 3.3 (+0.6, -0.5) x 10^-11 erg cm^-2 s^-1
Sep 26 (MJD 60579.86, 0.69 ks): 3.0 (+0.4, -0.5) x 10^-11 erg cm^-2 s^-1
Oct 5 (MJD 60588.58, 1.00 ks): 7.9 (+0.8, -0.6) x 10^-11 erg cm^-2 s^-1
Oct 6 (MJD 60589.76, 0.97 ks): 4.4 (+0.4, -0.5) x 10^-11 erg cm^-2 s^-1
Oct 7 (MJD 60590.63, 0.97 ks): 3.6 (+0.4, -0.4) x 10^-11 erg cm^-2 s^-1
We also detected a decrease in the observed magnitude measured with Swift-UVOT on Oct 5, 2024. The observed magnitude based on the preliminary Swift-UVOT analysis is as below. If the source was detected in areas of low sensitivity on the detector, those data were discarded here.
Date, V, B, U, W1, M2, W2
Sep 24, 14.18 (+/-0.05), 15.07 (+/-0.04), 14.79 (+/-0.05), - , -, -
Sep 25, 13.84 (+/-0.04), 14.59 (+/-0.03), 14.19 (+/-0.04), 14.87 (+/-0.05), 15.62 (+/-0.07), 15.63 (+/-0.05)
Sep 26, 14.42 (+/-0.04), 15.27 (+/-0.04), - , - , - , -
Oct 5, 12.80 (+/-0.03), 13.55 (+/-0.03), 13.04 (+/-0.03), - , -, -
Oct 6, 13.79 (+/-0.03), 14.54 (+/-0.03), 14.08 (+/-0.03), - , - , -
Oct 7, 13.87 (+/-0.03), 14.71 (+/-0.03), 14.26 (+/-0.03), 14.89 (+/-0.04), 15.51 (+/-0.05), 15.67 (+/-0.04)
We would like to thank the Swift Team for accepting and scheduling those ToO observations, in particular M. H. Siegel and B. Sbarufatti, as the Swift Observatory Duty Scientist.
