Study of TeV variability of Mrk 421 from 3 years of monitoring with the Milagro Observatory
aa r X i v : . [ a s t r o - ph . H E ] J a n August 25, 2018 19:50 WSPC - Proceedings Trim Size: 9.75in x 6.5in main Study of TeV variability of Mrk 421 from 3 years of monitoring withthe Milagro Observatory
B. PATRICELLI ∗ , M.M. GONZ ´ALEZ AND N. FRAIJA Instituto de Astronom´ıa, UNAM,M´exico D.F., 04510, M´exico ∗ E-mail: [email protected]
A. MARINELLI
Instituto de Fisica, UNAM,M´exico D.F., 04510, M´exico
FOR THE MILAGRO COLLABORATIONThe Milagro experiment was a TeV gamma-ray observatory designed to continuouslymonitor the overhead sky in the 0.1-100 TeV energy range. It operated from 2000 and2008 and was characterized by a large field of view ( ∼ ≥ Keywords : Blazars; Cherenkov detectors.
1. Introduction
Mrk 421 is one the brightest and closest (z=0.031, see Ref. 1) blazars known. Ithas been a target of many observational campaigns at very high energies (VHE, E >
100 GeV) with imaging atmospheric Cherenkov telescopes (IACTs). Its γ -ray fluxhas been found to be variable, with flaring episodes over a wide range of timescales,from months down to less than an hour (see, e.g., Ref. 2 and references therein).While IACTs have enough sensitivity to detect short duration flares, extended airshower detectors such as Milagro (see Sec. 2) are better suited to make a continuousmonitoring of the flux of the sources, thanks to their high duty cycle. In this paper,we study the TeV variability of Mrk 421 using 3 years (from September 2005 toMarch 2008) of data from Milagro.
2. The Milagro Observatory
Milagro (see Ref. 3) was a large water-Cherenkov detector located in the JemezMountains near Los Alamos, New Mexico, USA at an altitude of 2630 m abovesea level. It operated from 2000 to 2008. It was composed of a central 80 m × × ugust 25, 2018 19:50 WSPC - Proceedings Trim Size: 9.75in x 6.5in main The muon layer was used to discriminate between gamma-ray induced and hadron-induced air showers. In 2004, a sparse 200 m x 200 m array of 175 “outrigger” wasadded around the central reservoir. This array increased the area of the detector andimproved the gamma/hadron separation. The experiment was sensitive to extensiveair showers resulting from primary gamma rays at energies between 100 GeV and100 TeV (see Refs. 4 and 5).
3. Variability
We analysed Milagro data collected from September 21, 2005 to March 15, 2008, pe-riod over which Mrk 421 was detected with a statistical significance of 7.1 standarddeviations, at a median energy of 1.65 TeV. In Fig. 1 it is shown the Milagro lightcurve (LC) above 1 TeV for Mrk 421. We chose a time binning of ∼ a in orderto have high statistics, as well as to minimize the effects of the detector instabili-ties. It can be seen that the flux is consistent with being constant along the 3-yearmonitoring period, with an average value of ¯ f = (2 . ± . × − cm − s − ( χ =134 for 122 degrees of freedom, which gives a χ probability of 21.1%). There-fore, from the light curve it is clear that there is no evidence for flares in the data.To confirm this, we also calculated the significance above the average flux for eachtime bin, finding that all the Milagro measurements have a significance less than 3standard deviations (after the correction for the number of trials). -10-5 0 5 10 15 53600 53800 54000 54200 54400 54600 F l u x ( E > T e V , - c m - s - ) MJD (days)
Fig. 1. Light curve of Mrk 421 (black points); the red solid line represents the average value ofthe flux above 1 TeV: ¯ f =(2 . ± . − cm − s − . Each bin represents ∼ As we found no evidences of flares, we computed the upper limit F UL on themeasured flux considering different time scales, from one week to ∼
12 months,as for Mrk 421 outbursts lasting up to several months have been observed (seee.g. Ref. 2). Then we chose a confidence level of 99.7 % and used the method of a Milagro data are recorded in tapes and each tape contains data collected over a time intervalthat, on average, is of ∼ ugust 25, 2018 19:50 WSPC - Proceedings Trim Size: 9.75in x 6.5in main Helene. The results are shown in Fig. 2. It can be seen that F UL varies from2.26 × − cm − s − to 0.48 × − cm − s − ; its maximum value is found fora time scale of ∼ one week.The long-term average flux here reported, together with the VHE flux statedistribution of Mrk 421 (see e.g. Ref. 2), is fundamental to calculate the TeV dutycycle of the source and will be object of a future work. A further extension of thework will be also the calculation of the upper limits on the flux as a function oftime to make a comparison with observations from other VHE experiments in thesame period as Milagro. F l u x U L ( E > T e V , - c m - s - ) variability (days) Fig. 2. Upper limits on the flux as a function of flare duration.
Acknowledgments
We gratefully acknowledge Scott Delay and Michael Schneider for their dedicatedefforts in the construction and maintenance of the Milagro experiment. This workhas been supported by the Consejo Nacional de Ciencia y Tecnolog´ıa (under grantConacyt 105033), Universidad Nacional Aut´onoma de M´exico (under grant PAPIITIN105211) and DGAPA-UNAM.
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