Variations in the very-high-energy gamma-ray radiation from the blazar H 1426+428

Search for TeV γ-rays from H1426+428 during 2004–2007 with the TACTIC telescope

“…It may be noted here that the TACTIC 2004 results discussed above on this source are in agreement with those obtained with the CELESTE system [27] around 100 GeV during the same period. However, our results are in conflict with those of GT-48 telescope above 1 TeV during the period from 15 April to 25 April 2004, wherein a γ -ray signal at 5.8 σ level was reported [29].…”

“…In addition, the available TeV light curves reported by different groups upto the year 2004, are also depicted in Figure 4 starting from the Whipple 1999 observations. As is clear from this figure that the GT-48 telescope has recorded the source at more than 200% of the GT-48 detected Crab level on April 19, 2004 (MJD 53115) at 3.9 σ level [29,41]. It may be noted here that the TACTIC 2004 results discussed above on this are in agreement with those obtained with the CELESTE system [27] around 100GeV during the same period.…”

“…Further these data were also analysed on a nightly basis and the corresponding results obtained are depicted in figures 2(b) and (c), respectively, again showing the day-to-day variations of the hourly γ -ray rate with time. Light curves are characterized with the reduced χ 2 values of 12.66/13 and 31.31/34, respectively, with respect to the zero-degree polynomial fitted constant value of 2.22 ± 1.79 and −0.49 ± 0.74 photon events with corresponding probabilities of 0.47 and 0.69 respectively which are consistent , CAT [24], CELESTE [27] and GT-48 [29]. with the no-variability hypothesis.…”

“…We have placed an upper limit of ≤ 1.18 × 10 −12 photons cm −2 s −1 at 3σ level (13.1 % of the TACTIC detected Crab Nebula integrated γ-ray flux) on the integrated TeV γ-ray flux from the source direction, using the method of Helene [42]. In Table 3, we have compiled all results obtained on a yearly spell basis as well as for the case when all the data are considered H1426+428 TeV light curves obtained by various groups, namely Whipple [23], CAT [24], CELESTE [27] and GT-48 [29].…”

“…The CELESTE and STACEE groups, both with non-imaging detectors have also observed the source and reported upper limits during the periods 2002-2004 [27] and 2001-2002, respectively. However, the Crimean group has reported the detection of VHE γ -rays from this source using their GT-48 system in April 2004 with a statistical significance of 5.8 σ [29]. More recently, the VERITAS group observed this source for 12.5 h using their stereoscopic telescope system at Mt Hopkins.…”