Study of underlying particle spectrum during huge X-ray flare of Mkn 421 in April 2013

Context: In April 2013, the nearby (z=0.031) TeV blazar, Mkn 421, showed one of the largest flares in X-rays since the past decade. Aim: To study all multiwavelength data available during MJD 56392 to 56403, with special emphasis on X-ray data, and understand the underlying particle energy distribution. Methods: We study the correlations between the UV and gamma bands with the X-ray band using the z-transformed discrete correlation function. We model the underlying particle spectrum with a single population of electrons emitting synchrotron radiation, and do a statistical fitting of the simultaneous, time-resolved data from the Swift-XRT and the NuSTAR. Results: There was rapid flux variability in the X-ray band, with a minimum doubling timescale of $1.69 \pm 0.13$ hrs. There were no corresponding flares in UV and gamma bands. The variability in UV and gamma rays are relatively modest with $\sim 8 \%$ and $\sim 16 \%$ respectively, and no significant correlation was found with the X-ray light curve. The observed X-ray spectrum shows clear curvature which can be fit by a log parabolic spectral form. This is best explained to originate from a log parabolic electron spectrum. However, a broken power law or a power law with an exponentially falling electron distribution cannot be ruled out either. Moreover, the excellent broadband spectrum from $0.3-79$ keV allows us to make predictions of the UV flux. We find that this prediction is compatible with the observed flux during the low state in X-rays. However, during the X-ray flares, the predicted flux is a factor of $2-50$ smaller than the observed one. This suggests that the X-ray flares are plausibly caused by a separate population which does not contribute significantly to the radiation at lower energies. Alternatively, the underlying particle spectrum can be much more complex than the ones explored in this work.Comment: 11 pages, 7 figures, Accepted in A&

Simultaneous multi-wavelength observations of the TeV Blazar Mrk 421 during February - March 2003: X-ray and NIR correlated variability

In the present paper, we have reported the result of simultaneous multi-wavelength observations of the TeV blazar Mrk 421 during February $-$ March 2003. In this period, we have observed Mrk 421 using Pachmarhi Array of \v{C}erenkov Telescopes (PACT) of Tata Institute of Fundamental Research at Pachmarhi, India. Other simultaneous data were taken from the published literature and public data archives. We have analyzed the high quality X-ray (2-20 keV) observations from the NASA Rossi X-Ray Timing Explorer (RXTE). We have seen a possible correlated variability between X-ray and J band (1.25 $\mu$) near infrared (NIR) wavelength. This is the first case of X-ray and NIR correlated variability in Mrk 421 or any high energy peaked (HBL) blazar. The correlated variability reported here is indicating a similar origin for NIR and X-ray emission. The emission is not affected much by the environment of the surrounding medium around the central engine of the Mrk 421. The observations are consistent with the shock-in-jet model for the emission of radiations.Comment: 11 pages, 5 figures, Accepted for Publication in ChJA

Multi-waveband quasi-periodic oscillations in the light curves of blazar CTA 102 during its 2016–2017 optical outburst

Context. Quasi-periodic fluctuations in the light curves of blazars can provide insight into the underlying emission process. This type of flux modulation hints at periodic physical processes that result in emission. CTA 102, a flat spectrum radio quasar at a redshift of 1.032, has displayed significant activity since 2016. The multi-waveband light curve of CTA 102 shows signs of quasi-periodic oscillations during the 2016–2017 flare. Aims. Our goal is to rigorously quantify the presence of any possible periodicity in the emitted flux during the mentioned period and to explore the possible causes that can give rise to it. Methods. Techniques such as the Lomb-Scargle periodogram and weighted wavelet z-transform were employed to observe the power emitted at different frequencies. To quantify the significance of the dominant period, Monte-Carlo techniques were employed to consider an underlying smooth bending power-law model for the power spectrum. In addition, the light curve was modeled using an autoregressive process (AR1) to analytically obtain the significance of the dominant period. Lastly, the light curve was modeled using a generalized autoregressive integrated moving average (ARIMA) process to check whether introducing a seasonal (periodic) component results in a statistically preferable model. Results. Highly significant, simultaneous quasi-periodic oscillations (QPOs) were observed in the γ-ray and optical fluxes of blazar CTA 102 during its highest optical activity episode in 2016–2017. The periodic flux modulation had a dominant period of ∼7.6 days and lasted for ∼8 cycles (MJD 57710–57770). All of the methods used point toward significant (> 4σ) quasi-periodic modulation in both γ-ray and optical fluxes. Conclusions. Several possible models were explored while probing the origin of the periodicity, and by extension, the 2016–2017 optical flare. The best explanation for the detected QPO appears to be a region of enhanced emission (blob), moving helically inside the jet

Detection of a quasi-periodic oscillation in the optical light curve of the remarkable blazar AO 0235+164

We present a long term optical $R$ band light curve analysis of the gravitationally lensed blazar AO 0235+164 in the time span 1982 - 2019. Several methods of analysis lead to the result that there is a periodicity of ~8.13 years present in these data. In addition, each of these five major flares are apparently double-peaked, with the secondary peak following the primary one by ~2 years. Along with the well known system, OJ 287, our finding constitutes one of the most secure cases of long term quasi-periodic optical behaviour in a blazar ever found. A binary supermassive black hole system appears to provide a good explanation for these results.Comment: 7 pages, 3 figures, 1 table, Accepted for publication in MNRA