Multimessenger Characterization of Markarian 501 during Historically Low X-Ray and γ-Ray Activity

Abstract

We study the broadband emission of Mrk 501 using multiwavelength observations from 2017 to 2020 performed with a multitude of instruments, involving, among others, MAGIC, Fermi's Large Area Telescope (LAT), NuSTAR, Swift, GASP-WEBT, and the Owens Valley Radio Observatory. Mrk 501 showed an extremely low broadband activity, which may help to unravel its baseline emission. Nonetheless, significant flux variations are detected at all wave bands, with the highest occurring at X-rays and very-high-energy (VHE) γ-rays. A significant correlation (>3σ) between X-rays and VHE γ-rays is measured, supporting leptonic scenarios to explain the variable parts of the emission, also during low activity. This is further supported when we extend our data from 2008 to 2020, and identify, for the first time, significant correlations between the Swift X-Ray Telescope and Fermi-LAT. We additionally find correlations between high-energy γ-rays and radio, with the radio lagging by more than 100 days, placing the γ-ray emission zone upstream of the radio-bright regions in the jet. Furthermore, Mrk 501 showed a historically low activity in X-rays and VHE γ-rays from mid-2017 to mid-2019 with a stable VHE flux (>0.2 TeV) of 5% the emission of the Crab Nebula. The broadband spectral energy distribution (SED) of this 2 yr long low state, the potential baseline emission of Mrk 501, can be characterized with one-zone leptonic models, and with (lepto)-hadronic models fulfilling neutrino flux constraints from IceCube. We explore the time evolution of the SED toward the low state, revealing that the stable baseline emission may be ascribed to a standing shock, and the variable emission to an additional expanding or traveling shock.

Broadband Multiwavelength Study of LHAASO-detected Active Galactic Nuclei

“…Aleksić et al (2013) found that both one-zone and two-zone SSC models can well reproduce the SED observed by 1ES 2344 + 514 in late 2008. Abe et al (2023) SED of Mrk 421 with a VHE flare observed on 2017 February 4 is better reproduced by a two-zone leptonic model than by a one-zone leptonic model. Sahu et al (2020Sahu et al ( , 2021b, Sahu & Valadez Polanco (2022) found that the one-zone photohadronic model is inadequate to explain the multi-TeV flaring events from the transient extreme HSP-like sources of Mrk 501, Mrk 421, and 1ES 2344 + 514, and they proposed a two-zone photohadronic model as an effective methodology.…”

“…It should be noted that neutrino events from Mrk 421 and Mrk 501 have not yet been detected by IceCube. These best-fitting numbers of astrophysical neutrino events both correspond to very large p-values, which means that this result can only be treated as a rough upper limit (e.g., Abe et al 2023). The neutrino event rate can be obtained by ( ) ( )…”

Long-term X-Ray Outburst in the TeV-detected Blazar Mrk 501 in 2021–2022: Further Clues for the Emission and Unstable Processes

“…This mechanism also could yield a log-parabolic flux distribution (Rieger 2019), although this may break the correlation between the γ-ray and synchrotron (e.g., optical-to-X-ray) emissions expected in the framework of the one-zone SSC model (Böttcher & Dermer 2010). Note also that a significant X-ray and LAT-band correlation was found by Abe et al (2023) for the entire period 2008-2020, while this correlation was absent in the separate interval 2017-2020. Similarly, the correlation F XRT − F LAT was also absent during 2014 March-October (Paper I), and the data sets not belonging to these two intervals should be responsible for the positive correlation between these emissions in the entire period 2008-2020.…”

Very High-energy (>50 GeV) Gamma-Ray Flux Variability of Bright Fermi Blazars

“…Mkn 501: This well-studied, bright TeV blazar has exhibited violent flux variability across the electromagnetic spectrum (e.g., Abdalla et al 2019). The TeV band light curve of this source has revealed several episodes of VHE outbursts, with some of them also being detected with various Cherenkov telescopes (see Ahnen et al 2018;Abdalla et al 2019;Abe et al 2023). The estimated TS var parameter suggests the source to be variable in the TeV band.…”