arXiv subjects

Naseer Iqbal

Publications and source records attributed to Naseer Iqbal.

At least 19 recordsLinked to original sources

arXiv2026

A Possible Short-Timescale Optical Quasi-Periodic Oscillation in PKS\,0805$-$07 from High-Cadence TESS Observations

We present a timing analysis of the high-cadence optical light curve of the high-redshift flat-spectrum radio quasar PKS\,0805$-$07 obtained during \textit{TESS} Sector~34 (MJD~=~59230.90--59239.90). We search for short-timescale quasi-periodic oscillations (QPOs) using complementary time-series techniques, including the Lomb--Scargle periodogram (LSP) and the weighted wavelet $Z$-transform (WWZ), and evaluate their significance against red-noise variability using Monte Carlo simulations. The LSP reveals a dominant modulation at $f \approx 0.6\,\mathrm{d^{-1}}$ ($P \approx 1.7$\,d) exceeding the $99.99\%$ confidence level, while the WWZ independently recovers a consistent timescale at the $\sim$99.9\% level and shows that the signal is temporally localized rather than persistent. The modulation spans $\sim$5 coherent cycles, indicating a transient feature. However, the significance only marginally exceeds the highest WWZ confidence threshold, and the limited number of cycles is insufficient for a firm confirmation. In a disk-based scenario, orbital motion of a hotspot near the innermost stable circular orbit implies a black hole mass of $M_{\rm BH} \sim 7.2 \times 10^{8}\,M_\odot$, consistent with typical FSRQ values. Alternatively, magnetohydrodynamic kink instabilities in the relativistic jet can naturally produce day-scale variability for standard blazar parameters and account for its transient character. Compact SMBH binary and tidal disruption event scenarios are disfavoured by the jet-dominated nature of the source, the optical rather than X-ray character of the modulation, and a black hole mass well above that of confirmed QPE hosts. The candidate modulation is consistent with a compact, short-lived structure embedded within stochastic jet variability, and high-cadence multiwavelength monitoring will be essential to confirm its recurrence and constrain its physical origin.

astro-ph.HE↗

arXiv2026

Spectral State Switching in Mrk 421: Results from the AstroSat LAXPC/SXT Observations

We carried a detailed time and flux resolved X-ray spectral analysis of the high-synchrotron-peaked blazar Mrk\,421 using simultaneous LAXPC20 and SXT observations. The 100\,s binned LAXPC20 light curve obtained during 3--8 January 2017 reveals pronounced flux variability. The source exhibits a fractional variability amplitude of $F_{\mathrm{rms}} = 0.210 \pm 0.005$ in the SXT band and $F_{\mathrm{rms}} = 0.316 \pm 0.006$ in the LAXPC20 band. During this interval, the source reached a peak LAXPC20 count rate of 122.94\,counts\,s$^{-1}$, while the peak count rate in the SXT light curve is 26.79\,counts\,s$^{-1}$. This enabled us to carry out flux-resolved spectroscopy by dividing the 100\,s binned LAXPC20 light curve into ten flux states (S1--S10), each spanning a width of 8\,counts\,s$^{-1}$. For each flux state, simultaneous SXT and LAXPC20 spectra were extracted and fitted jointly. We find that the spectra in these states are well described by a synchrotron-convolved broken power-law, which provides a better fit than a log-parabola model. The low-energy particle index (index before the break) is found to cluster around two discrete values across flux states indicating two spectra states in the source. The break energy consistently moves to high energy with increase in flux level in these states. Time-resolved spectroscopy (10-ks segments) confirms that the flux histogram is best modelled as a double lognormal distribution and the index histogram is double normal. Inclusion of two additional long observations spanning 2017-2019 shows the same double-state behaviour on longer timescales. Together, the results indicate that Mrk\,421 routinely occupies two dominant spectral; in a leptonic synchrotron framework this can be explained by Gaussian-like fluctuations in acceleration conditions producing lognormal flux states.

astro-ph.HE↗

arXiv2026

Spectral-Regime Overlap and Transition-like Behavior in the Blazar Population from Multi-Instrument X-ray and TeV Observations

Blazars are conventionally classified into BL Lac objects and flat-spectrum radio quasars (FSRQs), commonly associated with radiatively inefficient and efficient accretion onto supermassive black holes, respectively. Increasing evidence, however, suggests that this division is not strictly discrete, with several sources occupying intermediate or transition-like spectral states. Since the photon index traces the evolution of the non-thermal emission spectrum, it provides a useful probe of jet energetics, particle acceleration, and radiative processes across blazar populations. Using multi-mission observations from Swift-XRT, Swift-BAT, NuSTAR, ROSAT, Chandra, XMM-Newton, NICER, AstroSat, TeVcat, and archival VizieR compilations, we investigate the distribution and long-term evolution of X-ray and TeV photon indices across multiple blazar subclasses. We identify broad overlap regions in photon-index space linking EHBL, HBL, IBL, LBL, and FSRQ-like populations, suggesting a continuous rather than sharply separated spectral distribution. Across X-ray instruments, the intermediate regime is concentrated near $Γ_{\mathrm{X}} \approx 2$, typically spanning $Γ_{\mathrm{X}} \sim 1.5$--$2.2$. Multi-epoch observations reveal substantial intra-source spectral evolution, including stochastic variability in Mrk~421 and state-dependent transitions in OJ~287. Several sources exhibit spectral-index changes of $ΔΓ_{\mathrm{X}} > 0.5$, while occupancy of the intermediate regime reaches $\sim22$--$43%$ depending on instrument. Multiple objects repeatedly traverse spectral regions connecting traditionally distinct subclasses, supporting their interpretation as candidate transition-like blazars. Overall, the results favor a framework in which blazar subclasses represent overlapping and evolving spectral populations driven by long-term changes in jet emission and radiative processes.

astro-ph.HE↗

arXiv2026

Modelling the Time-variable Broadband Emission and Correlation Study of FSRQ S5 1044+71

We present a detailed temporal and spectral analysis of the blazar S5\,1044+71 using multi-wavelength data obtained from the \emph{Fermi}-LAT and Swift-XRT/UVOT telescopes. Applying the Bayesian block algorithm to the 3-day binned $γ$-ray lightcurve, we identify pronounced variability, including four major outbursts marked by significant flux enhancements. The highest flux recorded was $(1.1 \pm 0.2)\times 10^{-6}\,\text{ph}\,\text{cm}^{-2}\,\text{s}^{-1}$ on 57868.5 MJD. Each outburst comprises multiple components, and lightcurve profile analysis indicates predominantly symmetric temporal structures. The shortest variability timescale of 4.5 hours constrains the emission region to be located within 0.03 pc of the central engine, likely near the broad-line region (BLR). Additionally, two highest-energy photons were detected with energies of 46.4 GeV (on 57739.6 MJD) and 42.5 GeV (on 59161.9 MJD), observed outside the peak flaring activity. The fractional variability shows an overall increasing trend from UV/optical to $γ$-ray bands, with a noticeable dip in the X-ray range, consistent with the shape of the broadband spectral energy distribution (SED). The flux distributions during flares exhibit log-normal or double log-normal behavior, suggesting multiplicative variability processes and evolving emission zones. Cross-correlation analysis reveals a strong positive correlation between the $γ$-ray and X-ray bands, with X-rays lagging by 42.5 days. Broadband SED modeling across different flux states supports a one-zone leptonic scenario, with $γ$-ray emission produced via external Compton scattering of IR and BLR photons. High-flux states show harder electron spectra, elevated break energies, and reduced magnetic fields-features consistent with efficient particle acceleration and Compton dominance.

astro-ph.HE↗

arXiv2025

An Enhanced Thermodynamic Framework for Third-Order Galaxy Correlation Functions: A Physically Motivated Closure and Observational Test

The three-point correlation function (3PCF) is a crucial probe of non-Gaussianity and nonlinear structure formation. We develop a thermodynamic framework for the galaxy 3PCF by closing the BBGKY hierarchy with a physically motivated hierarchical ansatz, yielding a separable, analytic solution for the equilateral 3PCF. Our framework addresses the apparent discrepancy between the perturbation theory prediction for dark matter ($Q_{dm} \approx 1.6$) and observed galaxy measurements ($Q_{gal} \approx 0.5$) by incorporating thermodynamic virial effects and velocity dispersion. We validate this model with SDSS/BOSS CMASS measurements, obtaining an excellent fit ($χ^2/\mathrm{dof} = 1.27$) across $1$-$50,h^{-1}\mathrm{Mpc}$. The analysis utilizes the Szapudi-Szalay estimator with robust covariance estimation from the SLICS simulation suite. By linking the thermodynamic temperature $T$ to the small-scale velocity dispersion (Fingers-of-God), we establish the thermodynamic approach as a predictive, complementary description of higher-order galaxy clustering on quasi-linear scales.

astro-ph.CO↗

arXiv2025

Indication for Dual Periodic Signatures in PKS 0805-07 from Multi-technique Time Series Analysis

We report the identification of two statistically significant quasi-periodic oscillations in the weekly binned $γ$-ray light curve of the flat-spectrum radio quasar PKS 0805$-$07, observed by Fermi-LAT over the period MJD 59047.5-59740.5. By applying a suite of complementary time-series analysis techniques, we identify periodic signatures at approximately 255 and 112 days. These techniques include the Lomb-Scargle periodogram (LSP), Weighted Wavelet Z-transform (WWZ), REDFIT, Date-Compensated Discrete Fourier Transform (DCDFT), Phase Dispersion Minimization (PDM), and the String-length method. The reliability of these signals is supported by high local significance (> 99) in all methods and reinforced through phase-folding. Model selection using the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) strongly supports a two-component periodic model. The detection of dual QPOs is rare among blazars and suggests complex variability mechanisms. Although a binary supermassive black hole (SMBH) scenario could be considered given the sources high redshift (z = 1.837), the short periodicities are difficult to reconcile with orbital motion unless invoking extreme parameters. Double-sine model fitting reveals that the oscillatory components have comparable amplitudes but are out of phase, suggesting a potential beating phenomenon due to interference. This amplitude-modulated variability is consistent with a geometric origin, most plausibly jet precession driven by Lense-Thirring torques, superimposed with a secondary process such as polar jet oscillation. Doppler factor modulation arising from these effects can account for the observed flux variations without requiring an unrealistically compact binary.

astro-ph.HE↗

arXiv2025

Multi-wavelength analysis of FSRQ B2 1348+30B: Constraints on the jet power

We present 14.5-year multi-wavelength analysis of flat-spectrum radio quasar B2 1348+30B using Swift-UVOT, Swift-XRT, and Fermi-LAT observations. In the gamma-ray band, the 3 day bin lightcurve reveals two major flaring events on 2010-09-19 (55458 MJD) and 2022-05-26 (59725 MJD) detected at flux levels $(2.5\pm 0.5) \times 10^{-7}\,\rm{ph\,cm^{-2}\,s^{-1}}$ and $(5.2\pm 0.6) \times 10^{-7}\,\rm{ph\,cm^{-2}\,s^{-1}}$. The Bayesian block analysis of the flares suggested the variability timescale to be $\leq$ 3\,day. To study the dynamic nature of the source, multi-wavelength spectrum was obtained for three flux states which includes the two flaring state and a relative low state. The $γ$-ray spectra of the source in all the states are well fitted by a power-law model with maximum photon energy < 20 GeV. In X-ray, a power-law model can explain the flaring state spectra while a broken power-law with extremely hard high energy component was required to model the low flux state. This indicates the presence of the low energy cutoff in the Compton spectral component. A simple one-zone leptonic model involving synchrotron, synchrotron self Compton and external Compton mechanism can successfully reproduce the broad-band spectral energy distribution of all the flux states. The model parameters suggest significant increase in the jet Lorentz factor during the high flux states. Further, the best fit parameters are used to constrain the minimum energy of the emitting electron distribution from the hard high energy spectrum of the low flux state. This analysis was extended to draw limits on the kinetic power of the blazar jet and was compared with the Eddington luminosity of the central black hole.

astro-ph.HE↗

arXiv2025

Statistical Insights into Flux and Photon Index Distributions of VHE FSRQs from Fermi-LAT Observations

This study examines the flux and photon index distributions of 11 Very High Energy (VHE) Flat Spectrum Radio Quasars (FSRQs) using over 16 years of Fermi-LAT $γ$-ray data. The distributions reveal double lognormal profiles in both flux and index, primarily in the 3-day and 7-day binnings, supporting the ``two-flux-state hypothesis" for blazars. These profiles, which become insignificant at 30-day binning, suggest that shorter timescales are better at capturing distinct states, while longer timescales smooth out shorter variations. Most VHE FSRQs exhibit a ``harder-when-brighter" trend, where the photon index decreases during high-flux states, suggesting efficient particle acceleration and possibly reduced radiative cooling. In contrast, two sources display a ``softer-when-brighter" behavior, likely due to enhanced radiative cooling in high photon density environments. Additionally, we observe that the Spearman rank correlation between flux and photon index strengthens with increasing time bin sizes, indicating more pronounced correlations over longer timescales. This possibly indicates that, on shorter timescales, flux variations are driven by a combination of photon index changes and normalization effects. Averaging flux over longer durations minimizes the effect of normalization variation, thereby enhancing the observed correlation. We also compare the flux and index distributions of VHE and non-VHE FSRQs, emphasizing the differences in their variability and emission patterns.

astro-ph.HE↗

arXiv2025

Study of Multi-Wavelength Variability, Emission Mechanism and Quasi-Periodic Oscillation for Transition Blazar S5 1803+784

This work present the results of a multi-epoch observational study of the blazar S5\,1803+784, carried out from 2019 to 2023. The analysis is based on simultaneous data obtained from the Swift/UVOT/XRT, ASAS-SN, and Fermi-LAT instruments. A historically high $γ$-ray flux observed for this source on march 2022 ($\mathrm{2.26\pm0.062)\times10^{-6}~phcm^{-2}s^{-1}}$. This study investigates the $γ$-ray emission from a blazar, revealing a dynamic light curve with four distinct flux states: quiescent and high-flux by using the Bayesian Blocks (BB) algorithm. A potential transient quasi-periodic signal with an oscillation timescale of $\sim$411 days was identified, showing a local significance level surpassing 99.7$\%$ from the Lomb-Scargle Periodogram (LSP) and Damped Random Walk (DRW) analysis and exceeds 99.5$\%$ from the Weighted Wavelet Z-Transform (WWZ) analysis. The observed QPO was confirmed through an autoregressive process (AR(1)), with a significance level exceeding 99$\%$, suggesting a potential physical mechanism for such oscillations involves a helical motion of a magnetic plasma blob within the relativistic jet. Log parabola modeling of the $γ$-ray spectrum revealed a photon index ($α_γ$) variation of 1.65$\pm$0.41 to 2.48$\pm$0.09 with a steepening slope, potentially indicative of particle cooling, changes in radiative processes, or modifications in the physical parameters. The $α_γ$ of 2.48$\pm$0.09 may hint at an evolutionary transition state from BL\,Lac to FSRQ. A comparative analysis of variability across different energy bands reveals that Optical/UV and GeV emissions display greater variability compared to X-rays. Broadband SED modeling shows that within a one-zone leptonic framework, the SSC model accurately reproduces flux states without external Compton contributions, highlighting magnetic fields crucial role.

astro-ph.HE↗

arXiv2025

Signature of hadronic emission in gamma-ray spectrum of B2 1308+326

The Flat Spectrum Radio Quasar (FSRQ) B2\,1308+326 was in its highest $γ$-ray flaring state during 60260-60310\,MJD. During this period, the source was detected in very high energy (VHE) by the large-sized telescope (LST-1). We conducted a detailed broadband spectral study of this source using the simultaneous data available in optical/UV, X-ray, and $γ$-ray bands. For the broadband spectral study, we select two gamma-ray high flux states (59750-59800\,MJD, 60260-60310\,MJD) and one low flux state (59250-59320\,MJD). During the epochs, 59750-59800\,MJD (high flux state) and 59250-59320\,MJD (low flux state), the broadband spectral energy distribution (SED) is well fitted using one zone leptonic emission model involving synchrotron, synchrotron self Compton (SSC) and external Compton (EC) processes. However, the flaring state (60260-60310\,MJD) during which the source showed VHE emission requires an additional component. We show that the inclusion of the photo-meson process can successfully explain this excess $γ$-ray emission. Further the estimated parameters, also suggest the source is transparent to VHE gamma-rays against pair production process.

astro-ph.HE↗

arXiv2025

Probing Spectral Evolution and Intrinsic Variability of Mkn\,421: A Multi-Epoch AstroSat Study of X-ray Spectra

Our study presents a time-resolved X-ray spectral analysis of Mkn 421, using AstroSat observations taken during different epochs between 2016 and 2019. The variability of the source in X-rays is utilized to investigate the evolution of its spectral properties. Each observation period was divided into segments of about 10 ks, and we employed three forms of particle distributions: broken-power law (BPL), log-parabola (LP), and power-law with maximum electron energy (xi-max model) undergoing synchrotron losses to fit the broad X-ray spectrum in each segment. We observed that all of these models provided good fits to the spectra. In the case of the broken-power law model, we investigated the relationship between normalized particle density at an energy less than the break energy and the index before the break. The results revealed an inverse correlation between the index and particle density with no time delay. Additionally, correlations between spectral parameters were used to determine the pivot energy. We observed that the pivot energy remained the same across the observations. For xi-max and LP models, we define analogous pivot energies and show that they also do not vary, indicating the model-independent nature of the result. The constant pivot energy suggests that the source's variability arises from index variations and not due to changes in the normalization. Consequently, parameters such as magnetic field strength, Doppler factor, etc., do not contribute to the source's variability. Instead, variations are primarily associated with the acceleration or escape timescales of emitted particles within the source.

astro-ph.HE↗

arXiv2024

Insights into the Long-Term Flaring Events of Blazar PKS 0805-07: A Multi-Wavelength Analysis over the 2009-2023 Period

We conducted a comprehensive temporal and spectral study of the FSRQ PKS 0805-07 by using the broadband observations from the Fermi-LAT and Swift-XRT/UVOT instruments over the period MJD 54684-60264. The 3-day binned $γ$-ray light curve during the active state, revealed eleven distinct peak structures with the maximum integral flux (E $>$ 100 MeV) reached $(1.56\pm 0.16)\times10^{-6}\, \text{photons cm}^{-2}\, \text{s}^{-1}$ on MJD 59904.5. The shortest observed $γ$-ray variability was $2.80\pm 0.77$ days. A correlation analysis between the $γ$-ray spectral index and flux indicated the typical trend of hardening when the source is brighter, commonly observed in blazars. We identified a lag of 121 (+27.21, -3.51) days in the spectral index relative to the flux, within the time interval MJD 59582 to 60112. The Anderson-Darling test and histogram-fit rejected the normality of the $γ$-ray flux distribution, and instead suggest a log-normal distribution. To gain insight into the underlying physical processes, we extracted broadband spectra from different time periods in the light curve. The spectral energy distribution during various flux states were well-reproduced using synchrotron, synchrotron-self-Compton, and external-Compton emissions from a broken power-law electron distribution. The seed photons required for the external Compton process are from IR region. A comparison of the best-fit physical parameters indicated that the variations in different flux states were primarily associated with an increase in the bulk Lorentz factor and magnetic field from low to high flux states.

astro-ph.HE↗

arXiv2024

Probing Broadband Spectral Energy Distribution and Variability of Mrk\,501 in the low flux state

We conducted a multi-wavelength analysis of the blazar Mrk\,501, utilizing observations from \emph{Astro}Sat (SXT, LAXPC), \emph{Swift-UVOT}, and \emph{Fermi-LAT} during the period August 15, 2016 to March 27, 2022. The resulting multi-wavelength light curve revealed relatively low activity of the source across the electromagnetic spectrum. Notably, logparabola and broken power-law models provided a better fit to the joint X-ray spectra from \emph{Astro}Sat-SXT/LAXPC instruments compared to the power-law model. During the low activity state, the source showed the characteristic harder when brighter trend at the X-ray energies. To gain insights into underlying physical processes responsible for the broadband emission, we performed a detailed broadband spectral analysis using the convolved one-zone leptonic model with different forms of particle distributions such as logparabola (LP), broken power-law (BPL), power-law model with maximum energy ($ξ_{max}$), and energy-dependent acceleration (EDA) models. Our analysis revealed similar reduced-$χ^2$ values for the four particle distributions. The LP and EDA models exhibited the lowest jet powers. The correlation analyses conducted for the LP and BPL models revealed that there is a positive correlation between jet power and bulk Lorentz factor. Specifically, in the LP model, jet power proved independent of $γ_{min}$, whereas in the broken power-law model, jet power decreased with an increase in $γ_{min}$. The jet power in the LP/EDA particle distribution is nearly 10 percent of the Eddington luminosity of a $10^7$ M$_\odot$ black hole. This result suggests that the jet could potentially be fueled by accretion processes.

astro-ph.HE↗

arXiv2024

Broadband spectral and temporal study of Ton 599 during the brightest January 2023 flare

In this work, we provide a detailed analysis of the broadband temporal and spectral properties of the blazar Ton\,599 by using the observations from \emph{Fermi}-LAT and \emph{Swift}-XRT/UVOT telescopes, during its brightest $γ$-ray flaring. The one-day bin $γ$-ray light curve exhibits multiple substructures with asymmetric and symmetric profiles. Notably, the $γ$-ray light curve shows a maximum flux of $\rm 3.63 \times 10^{-6}\, ph \,cm^{-2}\,s^{-1}$ on MJD\,59954.50, which is the highest flux ever observed from this source. The correlation between the $γ$-ray flux and $γ$-ray spectral indices suggests a moderate harder when the brighter trend. Taking $γ$-ray light curve as the reference, a strong correlation is observed with X-ray, optical, and UV energies. Additionally, the $γ$-rays and optical/UV emissions exhibit higher variability compared to X-rays. To understand the parameter variation during the active state of the source, we conducted a statistical broadband spectral modelling of the source in 10 flux intervals of equal duration. A one-zone leptonic model involving synchrotron, synchrotron-self-Compton, and external-Compton processes successfully reproduces the broadband SED in each of these flux intervals. We observed that the flux variation during the active state is mainly associated with the variation in the magnetic field and the particle spectral indices.

astro-ph.HE↗

arXiv2024

Time resolved spectroscopy of a GRS 1915+105 flare during its unusual low state using AstroSat

Since its discovery in 1992, GRS 1915+105 has been among the brightest sources in the X-ray sky. However, in early 2018, it dimmed significantly and has stayed in this faint state ever since. We report on AstroSat and NuSTAR observation of GRS 1915+105 in its unusual low/hard state during 2019 May. We performed time-resolved spectroscopy of the X-ray flares observed in this state and found that the spectra can be fitted well using highly ionised absorption models. We further show that the spectra can also be fitted using a highly relativistic reflection dominated model, where for the lamp post geometry, the X-ray emitting source is always very close to the central black hole. For both interpretations, the flare can be attributed to a change in the intrinsic flux, rather than dramatic variation in the absorption or geometry. These reflection dominated spectra are very similar to the reflection dominated spectra reported for Active Galactic Nuclei in their low flux states.

astro-ph.HE↗

arXiv2023

Can FSRQ 3C 345 be a Very High Energy Blazar Candidate?

The recent detection of very high energy (VHE) emissions from flat spectrum radio quasars (FSRQs) at high redshifts has revealed that the universe is more transparent to VHE $γ$-rays than it was expected. It has also questioned the plausible VHE emission mechanism responsible for these objects. Particularly for FSRQs, the $γ$-ray emission is attributed to the external Compton process (EC). We perform a detailed spectral study of \emph{Fermi}-detected FSRQ 3C 345 using synchrotron, synchrotron self Compton (SSC) and EC emission mechanisms. The simultaneous data available in optical, UV, X-ray, and $γ$-ray energy bands is statistically fitted under these emission mechanisms using the $χ^2$-minimization technique. Three high flux states and one low flux state are chosen for spectral fitting. The broadband spectral energy distribution (SED) during these flux states is fitted under different target photon temperatures, and the model VHE flux is compared with the 50\hspace{0.05cm}hr CTA sensitivity. Our results indicate a significant VHE emission could be attained during the high flux state from MJD 59635-59715 when the target photon temperature is within 900K to 1200K. Furthermore, our study shows a clear trend of variation in the bulk Lorentz factor of the emission region as the source transits through different flux states. We also note that during high $γ$-ray flux states, an increase in external photon temperature demands high bulk Lorentz factors, while this behaviour reverses in case of low $γ$-ray flux state.

astro-ph.HE↗

arXiv2023

Understanding the Very High Energy γ-ray excess in nearby blazars using leptonic model

The availability of simultaneous X-ray and Very High Energy (VHE) observations of blazars helps to identify the plausible radiative contributors to the VHE emission. Under leptonic scenario, the VHE emission from BL Lacs are attributed to the synchrotron self Compton (SSC) emission. However, many BL Lacerate (BL Lacs) have shown significant hardening at VHE after correction for the Extra Galactic Background Light (EBL) attenuation. We study the spectral hardening of two nearby BL Lac objects, Mkn 421 and Mkn 501 having most number of simultaneous X-ray and VHE observations available among all the blazars. These BL Lacs are relatively close and the effect of EBL attenuation is relatively minimal/negligible. We study the scatter plot between the X-ray spectral indices and intrinsic VHE indices to identify the plausible origin of the VHE emission. For Mkn 501, the VHE spectral indices are steeper than X-ray spectra, suggesting the scattering process happening at extreme Klein-Nishina regime. On the other hand, for Mkn 421, the VHE spectra is remarkably harder than the X-ray spectra, which suggests an additional emission mechanism other than the SSC process. We show this hard VHE spectrum of Mkn 421 can be explained by considering the inverse Compton (IC) emission from a broken power law electron distribution with Maxwellian pileup. The possibility of the hadronic contribution at VHE γ-rays is also explored by modeling the hard spectrum under photomeson process.

astro-ph.HE↗

arXiv2023

Statistical description of galaxy clusters in Finzi model of gravity

We exploit a new theory of gravity proposed by Finzi, which gives stronger interaction at large scales, to study the thermodynamic description of galaxy clusters. We employ a statistical model to deduce various thermodynamics equations of state. In addition, we analyze the behavior of clustering parameter in comparison to its standard (Newtonian) counterpart. The general distribution function and its behavior with varying strength of clustering parameter are also studied. The possibility of phase transition is investigated and it is observed that a phase transition is possible though hierarchically. We also analyze the model by comparing the results with data available through SDSS-III, and obtain the parameters involved.

gr-qc↗