Multiwavelength study of VHE emission from Markarian 501 using TACTIC observations during April-May, 2012

TL;DR

This study reports the detection of VHE gamma-ray emission from Markarian 501 during 2012, analyzes its spectral properties, and supports a leptonic emission model based on multiwavelength observations.

Contribution

First detailed multiwavelength analysis of Markarian 501's VHE emission during a high state in 2012, confirming leptonic processes as the emission mechanism.

Findings

Detected VHE gamma-ray signal with 8.89σ significance above 850 GeV.

Observed a high emission state with 8.05σ significance in May 2012.

Derived a power-law energy spectrum with index 2.57 ± 0.15.

Abstract

We have observed Markarian 501 in Very High Energy (VHE) gamma-ray wavelength band for 70.6 hours from 15 April to 30 May, 2012 using TACTIC telescope. Detailed analysis of $\sim$66.3 hours of clean data revealed the presence of a TeV $\gamma$-ray signal (686$\pm$77 $\gamma$-ray events) from the source direction with a statistical significance of 8.89$\sigma$ above 850 GeV. Further, a total of 375 $\pm$ 47 $\gamma$-ray like events were detected in 25.2 hours of observation from 22 - 27 May, 2012 with a statistical significance of 8.05$\sigma$ indicating that the source has possibly switched over to a relatively high gamma-ray emission state. We have derived time-averaged differential energy spectrum of the state in the energy range 850 GeV - 17.24 TeV which fits well with a power law function of the form $dF/dE=f_{0}E^{-\Gamma}$ with $f_{0}= (2.27 \pm 0.38) \times 10 ^{-11} $ photons cm$^{-2}$ s$^{-1}$ TeV$^{-1}$ and $\Gamma=2.57 \pm 0.15$. In order to investigate the source state, we have also used almost simultaneous multiwavelength observations viz: high energy data collected by $\it{Fermi}$-LAT, X-ray data collected by $\it{Swift}$-XRT and MAXI, optical and UV data collected by $\it{Swift}$-UVOT, and radio data collected by OVRO, and reconstructed broad-band Spectral Energy Distribution (SED). The obtained SED supports leptonic model (homogeneous single zone) for VHE gamma-ray emission involving synchrotron and synchrotron self Compton (SSC) processes.

Tables

Table 1. Table 1: Details of Mrk 501 observations using TACTIC during April-May 2012.

Month, Year	Observation Dates	Total Data	Data Selected
		(hours)	(hours)
April, 2012	15,16,20,22,23,25,26	13.4	11.6
May, 2012	11,14-20,22-30	57.2	54.7
April-May, 2012		70.6	66.3

Table 2. Table 2: Dynamic supercuts selection criteria used in the present work.

BT=2.9$\sigma$ and PT=6.5$\sigma$

1pe $\approx$ 6.2 CDC Counts

S$>$50pe

0.53 $\times$ $Co{s}^{0.47}\theta$ $\le$ D $\le$ 1.14 $\times$ $Co{s}^{-0.41}\theta$; $\theta$=Zenith Angle

Minimum number of pixels $\ge 4$

F2 $>$ 0.29 + 0.1146 $\times \theta$

0.16$\le$L$\le$[0.1446+0.07179$\times \theta$]+[0.0683-0.0375$\times \theta \times$log(S)]

0.04$\le$W$\le$[-0.1436+0.3217$\times \theta$]+[0.0738-0.0790$\times \theta \times$log(S)]

L/W $>$ 1.55, Asymmetry $>$ 0.0

Table 3. Table 3: Monthly spell wise analysis of Mrk 501 data recorded during April - May, 2012.

Spell	Observation Period	Time	Excess events	Sig.
		(hours)		($\sigma$)
1	15-26 April, 2012	11.6	110 $\pm$ 35	3.19
2	11-30 May, 2012	54.7	576 $\pm$ 69	8.35
		66.3	686 $\pm$ 77	8.89

Table 4. Table 4: Details of Mrk 501 data analysis results of TACTIC observations during 22 - 27 May, 2012.

Obs. date
(May, 2012)	Start MJD	End MJD	$\gamma$-rays	Sig.($\sigma$)	Obs.
					Time (hours)
22	56069.75	56069.95	119 $\pm$ 22	5.54	4.7
23	56070.74	56070.95	29 $\pm$ 20	1.42	4.5
24	56071.77	56072.00	84 $\pm$ 19	4.44	4.3
25	56072.77	56072.95	30 $\pm$ 20	1.53	4.1
26	56073.77	56073.96	52 $\pm$ 17	3.13	4.1
27	56074.78	56074.95	61 $\pm$ 16	3.74	3.5
	56069.75	56074.95	375 $\pm$ 47	8.05	25.2

Table 5. Table 5: Details of Mrk 501 divided into four spell using TACTIC during period April-May 2012.

Spell	Start	End	Time	Dates	$\gamma$-rays	$\gamma$-rays/hr	Sig.
No.	MJD	MJD	(hours)				($\sigma$)
1	56033	56043	11.6	16-26 April 2012	110 $\pm$ 35	9.48 $\pm$ 3.02	3.19
2	56058	56066	23.4	11-19 May 2012	175 $\pm$ 47	7.48 $\pm$ 2.01	3.74
3	56069	56074	25.2	22-27 May 2012	375 $\pm$ 47	14.88 $\pm$ 1.87	8.05
4	56075	56077	5.9	28-30 May 2012	26 $\pm$ 20	4.41 $\pm$ 3.39	1.29

Table 6. Table 6: Differential energy spectrum data for Mrk 501 during 22-27 May, 2012 with the TACTIC telescope.

Energy	Differential flux	EBL corrected flux
(TeV)	(photon cm-2 s-1 TeV-1)	(photon cm-2 s-1 TeV-1)
0.85	(2.92 $\pm$ 1.01)$\times {10}^{-11}$	(3.71 $\pm$ 1.28)$\times {10}^{-11}$
1.31	(1.43 $\pm$ 0.27)$\times {10}^{-11}$	(1.99 $\pm$ 0.37)$\times {10}^{-11}$
2.01	(2.05 $\pm$ 0.98)$\times {10}^{-12}$	(3.07 $\pm$ 1.46)$\times {10}^{-12}$
3.09	(1.65 $\pm$ 0.46)$\times {10}^{-12}$	(2.62 $\pm$ 0.73)$\times {10}^{-12}$
4.75	(7.91 $\pm$ 2.11)$\times {10}^{-13}$	(1.40 $\pm$ 0.37)$\times {10}^{-12}$
7.30	(1.11 $\pm$ 0.69)$\times {10}^{-13}$	(2.42 $\pm$ 1.52)$\times {10}^{-13}$
11.22	(3.63 $\pm$ 2.72)$\times {10}^{-14}$	(1.31 $\pm$ 0.98)$\times {10}^{-13}$
17.24	(1.04 $\pm$ 0.91)$\times {10}^{-14}$	(1.26 $\pm$ 1.10)$\times {10}^{-13}$

Table 7. Table 7: Details of energy dependence of fractional and amplitude variability in the multi-wavelength observation of Mrk 501 during April - May, 2012.

Instrument	Energy Band	Fvar	Fmax/Fmin
TACTIC	0.850 - 17 TeV	0.67 $\pm$ 0.04	3.42
$\mathrm{𝐹𝑒𝑟𝑚𝑖}$-LAT	0.1 - 300 GeV	0.58 $\pm$ 0.12	2.60
$\mathrm{𝑆𝑤𝑖𝑓𝑡}$-XRT	0.3 - 10 KeV	0.58 $\pm$ 0.08	1.80
$\mathrm{𝑆𝑤𝑖𝑓𝑡}$-UVOT	2.29 - 6.11 eV	0.61 $\pm$ 0.01	1.17
OVRO	15 GHz	0.51 $\pm$ 0.01	1.05

Table 8. Table 8: The model parameters of Mrk 501 using one zone homogeneous SSC model.

Parameter	Value
$\delta$	14.2
B	0.12 G
R	6.1$\times$1015cm
n1	2.12
n2	4.90
${\gamma }_{min}$	50
${\gamma }_{max}$	1.0$\times {10}^8$
ue	3.1$\times {10}^{-2}erg/c{m}^{-3}$

Equations

$$F_{var}=\frac{{\left[S^2-⟨{\sigma }_{err}^2⟩\right]}^{\frac{1}{2}}}{⟨F⟩}$$

$$\Delta F_{var}=\frac{1}{F_{var}}\sqrt{\frac{1}{2N}}\frac{⟨{\sigma }_{err}^2⟩}{{⟨F⟩}^2}$$

Full text

††journal: New Astronomy

1 Introduction

Blazars, a class of active galactic nuclei (AGN), are well known variable sources of radiation and currently represent a dominant class of the extragalactic gamma-ray sky. These objects are further classified as BL Lacertae (BL Lac) and Flat Spectrum Radio Quasar (FSRQ) with the mere difference of the presence of strong optical emission lines in the later. These objects are believed to be powered by accretion onto super massive black holes and have relativistic jets which are connected to the accretion disk and aligned very close to the observer’s line of sight [1]. The broad-band emission from radio to $\gamma$-rays from these objects is dominated by strongly Doppler-boosted non thermal radiation which is produced in the jets consisting of magnetised plasma. The observed broad-band SED of blazars consists of two peaks with first peak usually in the IR to X-ray, while second one at $\gamma$-ray energies when plotted in the $\nu F_{\nu}$ versus $\nu$ representation [2, 3]. The non-thermal emission in the first peak is relatively well understood and ascribed as due to synchrotron radiation of relativistic electrons accelerated in the jets, while the origin of the second peak is still unclear. In the leptonic scenario, the high energy SED peak is attributed to Inverse Compton (IC) scattering of either the synchrotron photons themselves called Synchrotron Self-Compton (SSC) emission or external photons called External Compton (EC) emission or both by the same population of relativistic electrons [4, 5, 6]. Alternatively, the hadronic models explain the high energy peak of the blazar emission via pion decay with subsequent synchrotron and/or IC emission from the decay products, or by synchrotron radiation from ultra-relativistic protons [7, 8, 9, 10, 11].

Mrk 501 (z = 0.034) is a high-energy peaked blazar (HBL) discovered first in the energy range above 500 GeV by Whipple collaboration [12]. Several aspects of the Very High Energy (VHE) $\gamma$-ray emission from this blazar including the correlations between intensity of X-rays, optical and TeV gamma-rays have been studied [13]. These observations have reflected a variable nature of the source in terms of both flux as well as spectrum, with a time scale as short as few minutes in some flares [14]. The variability time scales and flaring timescales can be the most direct way to probe the dynamics operating in jet plasma, in particular compact regions of shock acceleration [15].

Mrk 501 exhibited a strong temporal and spectral variability in $\gamma$-ray flux during 1997 when it underwent through a major VHE $\gamma$-ray flare at 10 times of the Crab nebula flux above 1TeV [16, 17, 18]. A fastest $\gamma$-ray flux variability on a timescale of a few minutes was detected from it in 2005 by Whipple collaboration [19]. The source spectra have been found to be harder at brighter states as compared to those during the source low-activity states [19, 20, 21]. In May 2009, VERITAS collaboration detected a variable TeV $\gamma$-ray emission at the flux level exceeding 2 Crab [22] from this source. In October 2011, Mrk 501 underwent a strong flare detected by the MAXI satellite in X-rays [23] and by the ARGO-YBJ detector in VHE $\gamma$-rays [24]. During a recent simultaneous radio-to-TeV observing campaign, Mrk 501 was observed both in quiescent and elevated state wherein the observed broad band states were reproduced by SSC model through a decrease in magnetic field and increased luminosity and hardness of relativistic particles [25]. H.E.S.S. observations of this source during 2012 and 2014 show rapid variability down to a time scale of four minutes in the 2-20 TeV energy range, particularly 2014 observations have reported a flux level comparable to the 1997 historical flaring state [26]. A harder-when-brighter behaviour has been again reported in these observations.

In this paper, we present results of TeV $\gamma$-ray observations conducted during 2012 using TACTIC $\gamma$-ray telescope along with results in other wavelength bands. In Section 2, we briefly describe the TACTIC telescope and provide the details of Mrk 501 observations, data analysis procedure and results. In Section 3, we discuss multiwavelenght studies of the source including light curves, variability and spectral energy distribution during a possible high state period. Finally in Section 4, we present discussion and conclusions.

2 TACTIC telescope

TACTIC (TeV Atmospheric Cherenkov Telescope with Imaging Camera) $\gamma$-ray telescope is located at Mt Abu (24.6∘N, 72.7∘E, 1300m asl) Rajasthan, India. It deploys a F/1 type tessellated parabolic tracking light collector of $\sim$9.5 m2 area consisting of 34 front-coated spherical glass facets of 0.6m diameter. The mirror facets have been pre-aligned to produce an on-axis spot of $\sim$0.3∘ diameter at the focal plane. The focal plane instrumentation of the telescope is a 349 pixel (ETL 9083UVB) Photo-Multiplier Tube (PMT) based imaging camera which has an uniform pixel resolution of $\sim$0.3∘. The camera records images of the atmospheric Cherenkov showers with a total field of view of $\sim$6∘ $\times$ 6∘.

Data used in this work have been collected with the inner 225 pixels and the innermost 121 pixels were used for generating the event trigger. The trigger is based on the three nearest neighbour triplets logic with single pixel threshold set to 14 photo-electrons (pe) [27]. The triggered events are digitized by CAMAC based 12-bit charge to digital converters (CDC) which have a full scale range of 600pC. The safe anode current (3$\mu$A) operation of the PMT has been ensured by implementing a gain control algorithm [28]. The detailed description of the telescope related hardware and software can be found in [29, 30].

Major upgradation work was carried out for TACTIC during the fall of 2011 involving replacement of high voltage and signal cables and installing new compound parabolic concentrators (CPC) on the imaging camera of the telescope in order to increase the photon collection efficiency. These CPCs have square entry and circular exit aperture with $\sim$85% reflectivity in the wavelength range 400 - 500 nm. A dedicated CCD camera was also installed for detailed pointing runs which gives source position with an accuracy of $\sim$3 arc-min on the camera plane. The telescope is sensitive to $\gamma$-rays above 850 GeV and can detect the Crab Nebula at 5$\sigma$ significance level in 12 hours of observation. More detail about the up-gradation of TACTIC can be found in [27].

2.1 TeV Observations of Mrk 501 and data analysis

TeV observations on Mrk 501 were taken during 15 April - 30 May, 2012. Total duration of observations on the source during above mentioned observation period was $\sim$70.6 hours. Several standard data quality checks have been used to evaluate the overall system behavior and the general quality of the recorded data. These include conformity of the prompt coincidence rates with the expected zenith angle dependence, compatibility of the arrival times of prompt coincidence events with Poissonian statistics and the behavior of the chance coincidence rate with time. After applying these data quality checks, we have selected good quality data sets of 66.3 hours as reported in detail in Table 1.

Detailed analysis of an imaging atmospheric Cherenkov telescope data, involves a number of steps including image cleaning, accounting for the differences in the relative gains of the PMTs, image parameterization, event classification and energy determination. In the first step, the CDC pedestals are subtracted from the CDC counts of each pixel of the camera. In order to select pixels containing Cherenkov image and to reduce noise contribution, we have used picture threshold (PT) of 6.5$\sigma$ and boundary threshold (BT) of 2.9$\sigma$. Here $\sigma$ is standard deviation of CDC distribution derived using 2000 events recorded for night sky background. In the second step, relative gain calibration is performed by recording 2000 events from a pulsed light source (LED) in front of the camera surface during the data taking. The calibration data collected are then used to determine the relative gains of each pixel by comparing their mean signals with respect to a reference pixel. The next step in the data analysis is the image parameterization and event selection. Characterization of the Cherenkov images formed at the focal plane is performed using the moment analysis [31, 32] and various image parameters, viz., Length (L), Width (W), Distance (D), Frac2 (F2), Asymmetry, Alpha ($\alpha$), Size (S) etc [33] are calculated. The images from gamma-ray initiated showers are roughly elliptical in shape and described by the L and W parameters and its location and orientation within the telescope field of view are given by the D and $\alpha$ parameters, respectively.

The standard dynamic supercut procedure is then used to separate $\gamma$-ray-like images from the overwhelming background of cosmic rays. This procedure uses the image shape parameters L and W as a function of the image size and zenith angle so that energy and zenith angle dependence of these parameters can be taken into account. The $\gamma$-ray selection criteria used in this analysis are given in Table 2. The shape selected images are used to generate $\alpha$-distribution with its gamma-domain range of $\leq$ 180 for the TACTIC telescope. The contribution of the background events is estimated from a reasonably flat $\alpha$ region of 270 $\leq$ $\alpha$ $\leq$ 810. The number of $\gamma$-ray events is then calculated by subtracting the expected number of background events, calculated on the basis of the background region, from the $\gamma$-ray domain events. The significance of the excess events is calculated using the maximum-likelihood-ratio method of Li and Ma [34].

2.2 TACTIC results

The data collected on Mrk 501 during 15 April - 30 May, 2012 were divided into two monthly spells and analysed using the analysis procedure mentioned above. A total of 686 $\pm$ 77 $\gamma$-ray like events were detected with statistical significance of 8.89$\sigma$. As evident from Table 3, the most of the $\gamma$-ray like events (576 $\pm$ 69) were detected during the spell 2 with statistical significance of 8.35$\sigma$.

Further, daily TACTIC light curve as shown in Figure 1(a) indicates, the source flux levels were more than 1 Crab Unit (CU) on few days during 22 - 27 May, 2012. This relatively high emission state of Mrk 501 is also clearly visible from Figure 1(b).