Determine whether H 2356-309 defines a new EHBL subclass

Determine whether H 2356-309 constitutes a distinct subclass of extreme high-frequency peaked BL Lac objects characterized by hard and very soft intrinsic spectra during different flaring epochs, rather than a transitional EHBL.

Background

H 2356-309 displays the characteristic synchrotron peak and lack of short-timescale variability associated with EHBLs, but its VHE spectra exhibit anomalous behavior: the 2004 and 2005 spectra have positive TeV slopes and hard intrinsic spectra, whereas the 2006 spectrum requires a very soft intrinsic spectrum with a negative TeV slope. The authors distinguish this behavior from transitional EHBLs, which are typically HBLs that occasionally enter an EHBL-like state and show variable spectra. They therefore raise the unresolved possibility that H 2356-309 represents a new EHBL subclass, but do not establish that classification.

References

H 2356-309 is having synchrotron peak always above 1 keV and short scale variability is not observed from it. Thus it cannot be a transitional type EHBL, like Mrk 501 or 1ES 2344+514. So, it is possible that this can be a new subclass of EHBL with both hard and very soft intrinsic spectra in different flaring epochs, a feature not seen before in other EHBLs.

— Deciphering the VHE gamma-rays spectra of Extreme High-Frequency peaked BL Lac Objects through a Photohadronic Model  (2609.30641 - Sahu et al., 25 Sep 2026) in Section 4, “VHE Spectra of H 2356-309”