Not so Swift: 20 years of multiwavelength observations of Mrk 421 and Mrk 501

Abstract

Aims. The blazars Mrk 421 and Mrk 501 have shown multiwavelength variability on all observed timescales, and have been well studied at high energies on short timescales. We aim to characterise the long-term temporal behaviour of these blazars at synchrotron energies, namely optical, UV, and X-ray, in order to assess current models of these objects and their processes. Methods. Amongst the longest light curves ever studied for these sources, we investigated 20 years of data (2005-2025) from the Swift-UVOT and Swift-XRT telescopes. We examined spectral models, fractional variabilities, flux distributions, and X-ray photon index vs flux relations, as well as carrying out in-depth time series analysis using structure functions, Lomb-Scargle periodograms, and discrete correlation functions. Results. Mrk 421 and Mrk 501 both showed intriguing variability in all studied wavelengths; this variability has been found to be energy dependent, as has the trend of lognormality in flux distributions. X-ray photon indices fluctuated greatly throughout the entire period, showing an overall harder-when-brighter trend. Hints of a quasi-periodicity have been found in the X-ray of Mrk 501 (host frame time scale $\sim 390$ days, >3$\sigma$) but not in the UV or X-ray of Mrk 421, or in the UV of Mrk 501. No correlation at any time lag was found between the optical/UV and X-ray bands in either source.

Accepted by Astronomy and Astrophysics (A&A)

References in corpus (21)

• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• HI4PI: A full-sky HI survey based on EBHIS and GASS
• A massive binary black-hole system in OJ287 and a test of general relativity
• Assessing statistical significance of periodogram peaks
• Polarized Blazar X-rays imply particle acceleration in shocks
• Swift and infra-red observations of the blazar 3C~454.3 during the giant X-ray flare of May 2005
• Leptonic and Hadronic Radiative Processes in Supermassive-Black-Hole Jets
• The minijets-in-a-jet statistical model and the RMS-flux correlation
• A multi-zone model for simulating the high energy variability of TeV blazars
• A comprehensive power spectral density analysis of astronomical time series I: the Fermi-LAT gamma-ray light curves of selected blazars
• Lognormal variability in BL Lacertae
• Quasi-periodic oscillations in the $\gamma$-ray light curves of bright active galactic nuclei
• Characterizing the gamma-ray long-term variability of PKS 2155-304 with H.E.S.S. and Fermi-LAT
• Swift observations of Mrk\,421 in selected epochs. iii. Extreme x-ray timing/spectral properties and multiwavelength lognormality in 2015\,December--2018\,April
• Multiwavelength periodicity search in a sample of $\gamma$-ray bright blazars
• The vanishing of the primary emission region in PKS 1510-089
• The relentless variability of Mrk 421 from the TeV to the radio
• Insights into the broad-band emission of the TeV blazar Mrk 501 during the first X-ray polarization measurements
• X-ray Intraday Variability of HBL Blazars with XMM-Newton
• A possible disk mechanism for the 23d QPO in Mkn~501
• Long-term multi-band photometric monitoring of Mrk 501

Cited by in corpus (2)

• 20 years of monitoring: PKS 2155-304 and PKS 1510-089 in the eyes of Swift and Fermi. I. The case of PKS 2155-304
• 20 years of monitoring: PKS 2155-304 and PKS 1510-089 in the eyes of Swift and Fermi. II. PKS 1510-089 and comparison