tokens: study. Dashed lines show the previous (grey) and improved (blue) sensitivity limits. The cut at the bottom left corner seems to represent the increasing difficulty for detecting sources with the lowest synchrotron log() values. A possible explanation is that for low-synchrotron peak values, the IC peak may move to lower energies, hindering the detections; but also this could be connected to incompleteness of the 2WHSP sample close to the synchrotron peak cut, at ν ∼ 10 15 Hz. As reported in Chang et al. ( 2017 ) the incompleteness might be induced by the multifrequency selection criteria that was used to build the 2WHSP sample. 4.3 Notable sources The stacked γ-ray SEDs displayed in Fig. 2 present two distinct outliers, namely the sources 1BIGB J224910.6 − 130002 and 1BIGB J194356.2+211821. The source 1BIGB J224910.6 − 130002, also known as RBS 1899, is a blazar of unknown redshift classified as an EHSP due to its synchrotron peak position estimated at 10 17.5 Hz. Interestingly, the Fermi -LAT spectrum obtained with over 9.3 yr of data shows a steep decaying behaviour in log (ν) versus log (ν F ν ) space, with photon spectral index Γ ∼ 2.33 and relatively bright in γ-rays, as seen in Fig. 5(a) . However, this unusual behaviour is probably due to the averaging of the quiescent state with a series of γ-ray flares that happened along 2016 to 2017, as can be seen in the light curve reported in Fig. 5(b) . During these flaring episodes, the flux increased of ∼40 times with respect to the quiescent phase. This may also explain why the source was undetected in previous FGL and FHL catalogues. This is an important example of a γ-ray blazar that could represent a major target for the future CTA during periods of extreme activity (as detected during MJD 57662–57687, Fig. 5a in black). It also illustrates the strong impact of flaring activity in attempting a population study of extreme or faint blazars, since the most extreme γ-ray activity is smoothed over time when integrating over years of Fermi -LAT observations. Figure 5. Left plot: SED of 1BIGBJ224910.6 − 130002, showing the mean γ-ray SED obtained by integrating over 9.3yr in blue, and during the 25 d flare peak (MJD 57662.65–57687.65) in black. Right plot: Light curve of 1BIGBJ224910.6 − 130002 considering 1 month time bins, integrating over the broad-band of 900 MeV up to 500 GeV. Flux calculated only for > 3σ bins (red points), error bars, and upper limits (black cross) are calculated using Integral method considering 95 per cent of confidence level. The second peculiar source, 1BIGB J194356.2+211821 also known as HESS J1943+213, is the most extreme blazar of the 1BIGB catalogue, with log (ν p ) = 18.1. It was serendipitously discovered by the H.E.S.S. Collaboration (H.E.S.S. Collaboration et al. 2011 ) at VHE γ-rays during a galactic plane survey and later confirmed with VERITAS observations (VERITAS Collaboration et al. 2015 ). Due to the source’s position on the galactic plane and its
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