Long-term Spectra of the Blazars Mrk 421 and Mrk 501 at TeV Energies Seen by HAWC

Abstract

The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory surveys the very high-energy sky in the 300 GeV to >100 TeV energy range. HAWC has detected two blazars above 11σ, Markarian 421 (Mrk 421) and Markarian 501 (Mrk 501). The observations are comprised of data taken in the period between 2015 June and 2018 July, resulting in ∼1038 days of exposure. In this work, we report the time-averaged spectral analyses for both sources, above 0.5 TeV. Taking into account the flux attenuation due to the extragalactic background light, the intrinsic spectrum of Mrk 421 is described by a power law with an exponential energy cutoff with index α = 2.26 ± 0.12 stat − 0.2 + 0.17 sys and energy cutoff E c = 5.1 ± 1.6 stat − 2.5 + 1.4 sys TeV, while the intrinsic spectrum of Mrk 501 is better described by a simple power law with index α = 2.61 ± 0.11 stat − 0.07 + 0.01 sys . The maximum energies at which the Mrk 421 and Mrk 501 signals are detected are 9 and 12 TeV, respectively. This makes these some of the highest energy detections to date for spectra averaged over years-long timescales. Since the observation of gamma radiation from blazars provides information about the physical processes that take place in their relativistic jets, it is important to study the broadband spectral energy distributions (SEDs) of these objects. For this purpose, contemporaneous data in the gamma-ray band to the X-ray range, and literature data in the radio to UV range, were used to build time-averaged SEDs that were modeled within a synchrotron-self Compton leptonic scenario.

Probing the Extragalactic Mid-infrared Background with HAWC

“…These are two extensively studied extragalactic sources and the brightest in the teraelectronvolt band. Both sources have been significantly detected by HAWC above 300 GeV up to 10 TeV (Albert et al 2022) and by Fermi-LAT between 100 MeV and 1 TeV . For the HAWC data set, 1343 days of data were used, taken between 2015 June and 2019 June ).…”

“…To avoid an overestimation of the intrinsic flux, each source's lowest energy flux point observed by HAWC (Albert et al 2022) is used as a guide; observed Mrk 421 and Mrk 501 flux points at an energy of 830 and 750 GeV, respectively, are de-absorbed according to Franceschini et al (2008) model. These de-absorbed flux points are included in the set of flux points obtained from the Fermi-LAT analysis described above, and altogether are used to fit a physically motivated emission model with Naima: a python software that calculates the nonthermal emission from a leptonic or hadronic population of particles, Zabalza (2015).…”

“…where A is a normalization factor, E 0 is a reference energy set at 1 TeV, α is the power-law index, and E c is the cutoff energy. The decision for this particular leptonic distribution is motivated by other studies performed on the Markarians by HAWC (Albert et al 2022) and others (Zhu et al 2016;. Based on the results reported in Albert et al (2022) the magnetic field and emission radius values are set at 0.03 G and 4 × 10 16 cm for Mrk 421 and 0.02 G and 1 × 10 17 cm for Mrk 501, respectively.…”

“…The decision for this particular leptonic distribution is motivated by other studies performed on the Markarians by HAWC (Albert et al 2022) and others (Zhu et al 2016;. Based on the results reported in Albert et al (2022) the magnetic field and emission radius values are set at 0.03 G and 4 × 10 16 cm for Mrk 421 and 0.02 G and 1 × 10 17 cm for Mrk 501, respectively. Table 1 shows the bestfit parameters for the electron distributions of each source.…”

“…The High-Altitude Water Cherenkov Gamma Ray Observatory (HAWC) is a water Cherenkov detector that has been operational since 2015 (further technical details can be found in Section 2) and has detected extragalactic sources significantly up to 10 TeV (Albert et al 2022). This energy is close to the one established by the gamma-ray horizon for sources like the two blazars Markarian 501 (Mrk 501) and Markarian 421 (Mrk 421; hereafter collectively referred to as Markarians), putting HAWC in an advantageous position to potentially probe the mid-IR region of EBL using observations from these sources.…”