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Extremely High energy peaked BL Lac (EHBL) objects are a special class of blazars with peculiar observational properties at X-ray and γ-ray energies. The observations of these sources indicate hard X-ray and γ-ray spectra and absence of rapid flux variations in the multi-wavelength light curves. These observational features challenge the leptonic models for blazars due to unusually hard particle spectrum in the emission region of the blazar jet and provide a strong motivation for exploring alternative scenarios to interpret the broad-band emission from blazars. At present, only few TeV blazars have been observed as EHBL objects in the extragalactic Universe. Due to their hard γ-ray spectra and long term variability, the observations of EHBL type of blazars at different redshifts help in probing the cosmic magnetic field and extragalactic background light in the Universe. Such objects also provide astrophysical sites to explore the particle acceleration mechanisms like magnetic reconnection and second order Fermi acceleration. Therefore, it has become important to identify more objects as EHBL using the observations available in the literature. Recent studies on the blazar Mrk 501 indicate that this source may exhibit an EHBL behaviour. In this paper, we use long term observations of Mrk 501 to explore its nature. Two sets of data, related to low and high/flaring activity states of Mrk 501, have been presented and compared with the observed features of a few well known EHBL type of blazars.We find that the spectral features of the blazar Mrk 501 indicate an EHBL nature of the source. Whereas, the temporal characteristics with fast variability during the high activity state of the source in X-ray and γ-ray energy bands are not compatible with the behaviour of EHBL type of blazars. However, Mrk 501 can be considered as an EHBL candidate in its low emission state. We also discuss the implications of identifying more EHBL objects using present and future groundbased γ-ray observatories.

In this paper, we explore the behavior of optical polarization during the multiwavelength outburst of the blazar Mrk 421 detected in February 2010. We use optical polarization measurements in the wavelength range 500-700 nm from SPOL observations available between January 1, 2010 and March 31, 2010 (MJD 55197-55286) including the period of multi-wavelength flaring activity detected from the source around February 16-17, 2010 (MJD 55243-55244). We also use near simultaneous optical and radio flux measurements from SPOL in V and R bands and OVRO at 15 GHz respectively. We find that the emissions in the optical and radio bands do not show any significant change in the source activity unlike at X-ray and γ-ray energies during the outburst. The optical and radio flux measurements are found to be consistent with the long term quiescent state emission of the source. Moreover, the linear polarization in the wavelength range 500-700 nm decreases to a minimum value of 1.6% during the X-ray and γ-ray outburst which is significantly lower than the long term average value of ∼ 4.2%.The angle of polarization varies between 114 • -163 • with a preferred average value of ∼ 137 • during this period. We estimate the degree of polarization intrinsic to the jet taking into account the host galaxy contamination in R band and compare this with the theoretical synchrotron polarization estimated for a power law distribution of relativistic electrons gyrating in an emission region filled with ordered and chaotic magnetic fields. The intrinsic linear polarization estimated for different epochs during the above period is found to be consistent with the theoretical synchrotron polarization produced by the relativistic electrons with power law spectral index ∼ 2.2. We find that the behavior of optical polarization possibly supports the two emission zone hypothesis for blazars in which X-ray and γ-rays are produced in one region whereas the optical emission takes place from -3another region permeated with ordered and chaotic magnetic fields. The decrease in linear polarization during the X-ray and γ-ray outburst can be attributed to the sudden dominance of chaotic magnetic field over the ordered field in the optical emission region in the relativistic jet of the blazar Mrk 421.

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A very bright and fast varying gamma-ray flare has been detected from the blazar 3C 279 on June 16, 2015. We have studied the broadband spectral energy distribution of the source during the flaring episode and in the low activity state using a simple one zone leptonic model. We find that an electron energy distribution described by a broken power law can be used to reproduce the broadband emissions during the high and low activity states. The flux measurements at radio, infrared and optical frequencies are reproduced by the synchrotron emission resulting from the relativistic electrons in a jet magnetic field strength of 0.37 G. The gamma-ray emission from the blazar 3C 279 is attributed to the Comptonization of the IR seed photons from the dusty torus with a temperature of 870 K. The outburst from the source observed on June 16, 2015 can be ascribed to an efficient acceleration process associated with a sudden enhancement in the electron energy density in the emitting region with respect to the low activity state. The fast gamma-ray variability at a minute timescale implies that the emission during the flare originates from a more compact region and the size of the emission zone in the low activity state is found to be four times larger than that during the flare. We have also used the model parameters derived from the broadband spectral energy distribution modelling to investigate a few physical properties of the jet during the outburst.

The flat spectrum radio quasar 3C 279 was observed in an extremely high activity state on June 16, 2015 (MJD 57189). In this paper, we investigate the properties of this flaring episode in the high energy γ-ray and optical bands using data from the Fermi-LAT, SMARTS, and SPOL observations during the period June 1-30, 2015 (MJD 57174-57203). The highest emission state in the γ-ray band detected by the Fermi-LAT exhibits a peak flux of ∼ 2×10 −7 erg cm −2 s −1 which is more than 25 times the flux level measured in the low activity state of the source. The temporal analysis of the daily Fermi-LAT light curve suggests that the giant flaring episode has characteristic rise and decay times less than one day. The optical daily light curves in B, V, R, and J bands also indicate the flaring activity from 3C 279 with flux levels peaking for two days on June 16-17, 2015 (MJD 57189-57190). The discrete correlation function analysis indicates a time lag of 1 day or longer between the γ-ray and optical peaks during the flaring episode. The γ-ray emission is also observed to show a harder-when-brighter behaviour whereas optical emission exhibits an opposite behaviour. The γ-ray emission region during the flare is observed to be very compact and is located close to the base of the jet. The degree of linear polarization in the wavelength range 500-700 nm measured using SPOL during this period is also highly variable with a peak of ∼ 30% one day after the γ-ray flare. Near simultaneous γ-ray flux points show a linear anti-correlation with the degree of polarization during the period of γ-ray flare. The significant drop in the degree of linear polarization suggests a sudden increase in the tangled magnetic field strength in the emission region.

The discovery of the white dwarf binary system AR Scorpii (AR Sco) with its fascinating nonthermal dominated multi-frequency emission has sparked renewed interest in potential high energy gamma-ray emission from white dwarf pulsars. The Spectral Energy Distribution (SED) below and above optical shows evidence of non-thermal synchrotron emission, with pulsed emission in optical and X-ray bands at the white dwarf spin period (P * = 117 s) as well as a beat period (P b = 118.1 s) with the binary period. From an energy perspective, the highly magnetic rotating white dwarf can accelerate particles to TeV energies. In this study, a search for high energy gamma-ray emission was conducted between 100 MeV -500 GeV by analysing the newly available Fermi-LAT Pass 8 data with the new Fermi 1.0.1 Science Tools. Binned likelihood analysis was done using power law, broken power law and log parabola models. From the selected Region of Interest (ROI) centred on AR Sco's position, we calculated a significance of √ T S ≤ 3.87σ for the integrated gamma-ray activity between 100 MeV -500 GeV at a photon flux level of 0.486 ± 0.261 x 10 −8 photons cm −2 s −1 using the broken power law model. This resulted in a 3σ upper-limit detection from the position of AR Sco. The location of AR Sco inside the Rho Ophiuchi (Rho Oph) molecular cloud complex combined with the poor spatial resolution of Fermi-LAT, complicates any positive identification of low-level gamma-ray activity at the location that coincides with the position of AR Sco.

The newly discovered white dwarf binary system AR Scorpii (AR Sco) with its fascinating nonthermal dominated multi-frequency emission has sparked renewed interest in potential high energy gamma-ray emission from white dwarf pulsars. Recent studies revealed that the optical and X-ray components of the emission in AR Sco can be powered entirely by the spin-down luminosity of the white dwarf in the absence of any mass accretion in this system. The Spectral Energy Distribution (SED) below and above optical shows a clear indication of a non-thermal synchrotron component, with pulsed emission in optical and X-ray bands at the white dwarf spin period as well as a beat with the binary period. From an energy perspective the highly magnetic rotating white dwarf can accelerate particles to TeV energies. In this study, a search for high energy gamma-