Constraining the synchrotron peak and estimating the VHE brightness of a sample of extreme high synchrotron peak blazars

Abstract

We present the results of a multi-wavelength study of a population of X-ray bright ($\rm log(F_{0.2-12 \ keV})>-12.5$), non-$γ$-ray detected high and extreme high synchrotron peak (HSP, EHSP; $\rm log(ν_{\rm peak,\ Hz})>16$) BL Lacs to $i$) put stronger constraints on the synchrotron peak location and shape and $ii$) model their expected behaviour in the very high-energy band. First, we performed an X-ray spectral analysis, using XMM-Newton, Chandra, Swift-XRT, and eROSITA data, and fitting the spectra using both a power law and a log parabola model. Out of 78 sources in the initial sample, 17 were best described by a log parabola model, a result that supports a scenario where the synchrotron peak falls in the X-ray band. Among these 17 sources, we further selected the 10 objects dominated by the jet emission, with no significant contamination of the host galaxy. We performed a $γ$-ray analysis of \lat\ data for these objects, obtaining upper limits providing information on their flux in the 100 MeV - 300 GeV energy range. We then modelled the broadband SED of these objects with JetSeT using two models: one assuming a log parabola for the electron distribution and the other one with a broken power law electron distribution, using parameters consistent with those describing the emission of the prototypical EHSP 1ES 0229+200. We found the models to be generally consistent with the available multi-wavelength detections and upper limits. Furthermore, they confirmed that a subsample of sources could display relevant emission in the TeV energy range, even potentially reaching the threshold for detectability by the Cherenkov Telescope Array Observatory.

Figures (17)

• Figure 1: Synchrotron peak frequency as a function of 0.2-12 keV flux for the 1007 blazars with X-ray counterpart and without a counterpart in the 4FGL catalogue (marchesi25). The subsample of sources analysed in this work is plotted in orange; the three FSRQs excluded from the analysis are instead plotted in green. Finally, the rest of the marchesi25 sample is shown in blue.
• Figure 2: From left to right, top to bottom: Results of the log parabola fitting of the blazars 5BZBJ0333-3619 ( Swift-XRT data), 5BZGJ1510+3335 (Chandra data), 5BZBJ1253+3826 ( Swift-XRT data), 5BZBJ1636--1248 ( Swift-XRT data), 5BZBJ1251-2958 (XMM- Newton data), 5BZBJ0040-2719 (XMM- Newton data), 5BZGJ1201-0011 ( Swift-XRT data), 5BZBJ1302+5056 ( Swift-XRT data) and 5BZBJ1057+2303 ( Swift-XRT data). In the inset, we report the confidence regions at 1$\sigma$ (red), 2$\sigma$ (blue) and 3$\sigma$ (pink) for the two parameters of the fit, $\alpha$ and $\beta$.
• Figure 3: From left to right, top to bottom: Results of the log parabola fitting of the blazars 5BZGJ1544+0458 (Chandra data), 5BZGJ1444+6336 ( Swift-XRT data), and 5BZGJ2310--4347 ( Swift-XRT data), 5BZGJ1616+3756 ( Swift-XRT data), 5BZGJ1552+3159 (Chandra data), 5BZBJ2217--3106 ( Swift-XRT data), 5BZBJ0124+0918 (XMM- Newton data), and 5BZBJ1258+0134 (XMM- Newton data). In the inset, we report the confidence regions at 1$\sigma$ (red), 2$\sigma$ (blue) and 3$\sigma$ (pink) for the two parameters of the fit, $\alpha$ and $\beta$.
• Figure 4: Distribution of the $\Delta$Cstat of the analysed sources plotted against their synchrotron peak. The blue triangles represent the results for all the analysed sources, whereas the orange circles represent the sources favouring a log parabola fit, having $\Delta$Cstat $>$ 2.7 and -1 $< \beta <$ 1.5. The dotted lines represent the quartiles and median distribution for the two populations. As it can be seen, the sources best fitted with a log parabola model tend to have slightly larger synchrotron peak frequency.
• Figure 5: Models for different broadband SEDs of the blazar 1ES0229+200, assuming a log parabola distribution for electrons, $z$=0.139, and $\gamma_0$ of the order of $10^5$, varying in the range 2.8$\times 10^5$-3.9$\times10^5$, due to the different values of B used in each model, as reported in Table \ref{['tab:logpar_par']}.