Spectral analysis of the blazars Markarian 421 and Markarian 501 with the HAWC Gamma-Ray Observatory
Sara Coutiño de León, Alberto Carramiñana Alonso, Daniel Rosa-González, Anna Lia Longinotti
SSpectral analysis of the blazars Markarian 421 andMarkarian 501 with the HAWC Gamma-RayObservatory
Sara Coutiño de León, Alberto Carramiñana Alonso ∗ , Daniel Rosa-González andAnna Lia Longinotti for the HAWC collaboration † Instituto Nacional de Astrofísica, Óptica y Electrónica, Puebla, Mexico.E-mail: [email protected] , [email protected] , [email protected] , [email protected] The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory surveys the very high en-ergy sky in the ∼
300 to 100 TeV energy range and has detected two high-synchrotron-peaked BLLacertae objects, Markarian 421 and Markarian 501 in a period of time of 837 days between June2015 and December 2017. In this work we present the detailed time-average spectral analysis.Using an extragalactic background light model, we address the difference in the intrinsic spectralproperties between the two blazars above 1 TeV, preliminary results show that the intrinsic spec-trum of Mrk 421 is better described by a power-law with an exponential energy cut-off functionwith photon index α = . ± .
10 and energy cut-off Ec = . ± .
02 TeV, and for Mrk501 the intrinsic spectrum is well described by a power law with spectral index α = . ± . ∗ Speaker. † c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ a r X i v : . [ a s t r o - ph . H E ] S e p pectral analysis of Markarian 421 and Markarian 501 with HAWC Alberto Carramiñana Alonso
1. Introduction
Most extragalactic sources of very high-energy (VHE) radiation are associated to active galac-tic nuclei (AGN), characterized by ultra-relativistic jets escaping from a super massive black hole(SMBH) in the central region of the galaxies. Two of the most studied and brightest sources inthe extragalactic TeV sky are Markarian 421 (Mrk 421) and Markarian 501 (Mrk 501) with a red-shift of z = .
031 and z = . Fermi -LAT whose latest report is in
Fermi
Large Area Telescope Third Source Catalog [4].The spectral energy distribution of HBL has two components, one at low energies is origi-nated via synchrotron radiation of relativistic electrons, whereas the high-energy that is commonlyattributed to inverse Compton scattering (IC); however the nature of the high energy component isstill under debate [5, 6, 7]. For this reason, Mrk 421 and Mrk 501 are often studied to constrainemission blazar models by separating their intrinsic properties and the attenuation effects due to theextragalactic background light (EBL). In this work we present the preliminary results of the VHEobservations of both sources as a result of 837 days of observations with the High Altitude WaterCherenkov (HAWC) Gamma-Ray Observatory in order to address the difference in their intrinsicspectral properties.
2. Data
The HAWC Observatory is a ground-based TeV gamma-ray detector in the state of Puebla,Mexico at an altitude of 4100 m a.s.l. The detector continuously measures the arrival time anddirection of cosmic and gamma-ray primaries within its 2 sr field of view. It is most sensitive togamma-ray energies ranging from 300 GeV - 100 TeV. Cuts on the data can be applied to differ-entiate gamma-ray air showers from the large cosmic-ray background. A reconstruction of the airshower in the detector is performed to obtain the event’s size and the direction of the primary γ -ray.For more details about the detector performance read [8, 9].The data used for this analysis goes from June 2015 to December 2017 comprising 837 daysof effective exposure. The data is divided into bins which definition depends on the fraction ofphotomultiplier tubes that are triggered in each event; however this method does not take intoaccount energy assignment variables so the energy of the gamma-ray events is estimated througha ground parameter method that measures the charge 40 meters from the air shower axis of eachevent and the estimated energy is found performing a fit using an empirical function that matchesthe simulated events, this way the bins are sub-divided into quarter-decade energy bins in the 0.316-100 TeV range. Due to the poor energy resolution below 1 TeV, we performed the fits above thisenergy. For more details about the energy estimator performance see [10].
3. Analysis
A forward-folding method is performed to fit the spectral shape of the sources using the HAWCmaximum-likelihood framework (LiFF) described in [11]. Given a source spectral model, the1 pectral analysis of Markarian 421 and Markarian 501 with HAWC
Alberto Carramiñana Alonso data and background maps are convolved with the detector response and LiFF computes a binnedPoisson log-likelihood value. The test statistic (
T S ) is computed using the likelihood of the ob-servations, estimated using a background-only model (null hypothesis, H ), and the signal-plus-background model (alternative hypothesis, H ). T S is defined as
T S = L ( H ) L ( H ) , (3.1)where L is the likelihood function. To determine a source spectrum, the T S is numerically max-imized by iteratively changing the input parameters, yielding those values that have the highestlikelihood of describing the observed data for the point source model assumption. Using Wilks’Theorem [12] the significance is σ = ±√ T S and according to [9], we consider a source detectedwhen σ > dNdE = N (cid:18) EE (cid:19) − α × exp ( − τ ) , (3.2) dNdE = N (cid:18) EE (cid:19) − α × exp (cid:18) − EE c (cid:19) × exp ( − τ ) , (3.3)where N is the flux normalization [ TeV − cm − s − ] , E is the pivot energy fixed at 1 TeV, α is thespectral index, E c is the energy cut-off [ TeV ] , and τ is the opacity value given by EBL models, andwhich is an increasing function of E and the source redshift, z .Depending on the T S values in the global fit using all the available energy bins, a preferredspectral shape is chosen. As described in [10], the process to estimate flux points is performingindividual fits in each energy bin where only N is free to vary and α and E c are fixed using theresulting values from the global fit. If a fit from an individual energy bin has a T S <
25, an upperlimit at a 95% confidence interval is set following [14].
4. Results
The model that best describes the intrinsic spectrum of the sources is a PL+CO for Mrk 421;however for Mrk 501 the spectral fit has very similar values of
T S for both a PL and a PL+COspectral models but the fitted energy cut-off is larger than 700 TeV, so that within the HAWC energyrange the intrinsic spectrum can be modeled with a single PL. The resulting spectral parameters aregiven in Table 1 where the corresponding uncertainties are statistical only. Figure 1 show the bestfits for the observed (black) and intrinsic (blue) spectra of both sources.2 pectral analysis of Markarian 421 and Markarian 501 with HAWC
Alberto Carramiñana Alonso
Table 1:
Fitted spectral parameters for Mrk 421 and Mrk 501 following the method described in section 3. √ T S N α E c [ TeV − cm − s − ] [TeV] Mrk 421 48 ( . ± . ) × − . ± .
10 5 . ± . ( . ± . ) × − . ± . ∞ Figure 1:
Differential energy spectra of Mrk 421 (top) and Mrk 501 (bottom). The observed (black line anddots) spectra is the best fit to the data and the intrinsic spectra (blue dashed line) is obtained using the EBLmodel from [13]. The black points are the result of individual fits in each energy bin as described in section3.
5. Summary
We present the VHE spectra of Mrk 421 and Mrk 501, the nearest BL Lacs, with HAWC. Thedata set comprises 837 transits and the spectral analysis was performed using an energy estimatorbased on the measurement of the charge at the ground to divide the data into energy bins [10], andthe HAWC maximum-likelihood framework [11].The differential energy spectra of each source is also EBL-corrected using the model from[13], and the preliminary results show that Mrk 421 has a curved shape which may indicate photon-photon attenuation inside the source, while Mrk 501 is well described by a simple power law.3 pectral analysis of Markarian 421 and Markarian 501 with HAWC
Alberto Carramiñana Alonso
These differences prompt a deeper exploration of the intrinsic nature of both sources through blazaremission models in order to determine the physical processes and the nature of the particles thatproduce radiation at very high energies.
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