Extragalactic Background Light Photons Research Articles

First constraints on the strength of the extragalactic magnetic field from γ-ray observations of GRB 221009A

ABSTRACT The extragalactic magnetic field (EGMF) could be probed with γ-ray observations of distant sources. Primary very high-energy γ-rays from these sources absorb on extragalactic background light photons, and secondary electrons/positrons from the pair production acts create cascade γ-rays. These cascade γ-rays could be detected with space γ-ray telescopes such as Fermi-LAT. The γ-ray burst GRB 221009A was an exceptionally bright transient well suited for intergalactic γ-ray propagation studies. Using publicly available Fermi-LAT data, we obtain upper limits on the spectrum of delayed emission from GRB 221009A during the time windows of 10, 30, and 90 d after the burst, and compare these with model spectra calculated for various EGMF strengths B, obtaining constraints on B. We show that the values of B between 10−20 and 10−18 G are excluded.

Modeling the propagation of very-high-energy γ-rays with the CRbeam code: Comparison with CRPropa and ELMAG codes

Context. Very-high-energy γ-rays produce electron positron pairs in interactions with low-energy photons of extragalactic background light during propagation through the intergalactic medium. The electron-positron pairs generate secondary γ-rays detectable by γ-ray telescopes. This secondary emission can be used to detect intergalactic magnetic fields (IGMF) in the voids of large-scale structure. Aims. A new γ-ray observatory, namely, Cherenkov Telescope Array (CTA), will provide an increase in sensitivity for detections of these secondary γ-ray emission and enable the measurement of its properties for sources at cosmological distances. The interpretation of the CTA data, including the detection of IGMF and study of its properties and origins, will require precision modeling of the primary and secondary γ-ray fluxes. Methods. We assess the precision of the modeling of the secondary γ-ray emission using model calculations with publicly available Monte Carlo codes CRPropa and ELMAG and compare their predictions with theoretical expectations and with model calculations of a newly developed CRbeam code. Results. We find that model predictions of different codes differ by up to 50% for low-redshift sources, with discrepancies increasing up to order-of-magnitude level with the increasing source redshifts. We identify the origin of these discrepancies and demonstrate that after eliminating the inaccuracies found, the discrepancies between the three codes are reduced to 10% when modeling nearby sources with z ~ 0.1. We argue that the new CRbeam code provides reliable predictions for the spectral, timing, and imaging properties of the secondary γ-ray signal for both nearby and distant sources with z ~ 1. Thus, it can be used to study gamma-ray sources and IGMF with a level of precision that is appropriate for the prospective CTA study of the effects of γ-ray propagation through the intergalactic medium.

Prospects of additional contribution at optical-NIR band of EBL in the light of VHE spectra

ABSTRACT The extragalactic background light (EBL) that spans the ultraviolet-infrared (UV-IR) band originates from direct and dust-reprocessed starlight integrated over the history of the Universe. EBL measurements are very challenging due to foreground emission like the zodiacal light and interplanetary dust emission. Indeed, some optical/NIR (near infrared) direct measurements overpredict EBL models based on galaxy counts. On the other hand, there is some debate on possible additional components of the optical-NIR photon density, e.g. population-III stars, axion-photon decay, direct collapse of black holes, intrahalo light, etc. Owing to the absorption of very high energy (VHE) gamma-rays by interaction with EBL photons, we study the prospects of accommodating an additional population of EBL sources in the optical-NIR band on top of the standard galaxy-count–based component. To this aim we use 105 VHE spectra of 37 blazars with known redshifts, 0.03 < z < 0.94. We correct the observed spectra for absorption by our model EBL. By requiring the intrinsic spectra to be non-concave and with a VHE spectral index >1.5, we estimate, at different wavelengths, upper limits to the additional low-energy photon fields that would contribute to the absorption of gamma-rays. Considering these limits, we suggest that there is room for photons from Pop III stars and axion-like particle annihilation. However, these additional hypothetical photon fields are bound to fall significantly below direct published EBL measurements by several instruments, and therefore, our limits are either in tension or even inconsistent with such measurements.

Constraints on the extragalactic magnetic field strength from blazar spectra based on 145 months of Fermi-LAT observations

ABSTRACT Properties of the extragalactic magnetic field (EGMF) outside of clusters and filaments of the large-scale structure are essentially unknown. The EGMF could be probed with γ-ray observations of distant (redshift z > 0.1) blazars. TeV γ-rays from these sources are strongly absorbed on extragalactic background light photons; secondary electrons and positrons produce cascade γ-rays with the observable flux dependent on EGMF parameters. We put constraints on the EGMF strength using 145 months of Fermi-LAT observations of the blazars 1ES 1218+304, 1ES 1101−232, and 1ES 0347−121, and imaging atmospheric Cherenkov telescope observations of the same sources. We perform a series of full direct Monte Carlo simulations of intergalactic electromagnetic cascades with the elmag 3.01 code and construct a model of the observable spectra inside the point spread functions of the observing instruments for a range of EGMF strengths. We compare the observed spectra with the models for various values of the EGMF strength B and calculate the exclusion statistical significance for every value of B. We find that the values of the EGMF strength B ≤ 10−17 G are excluded at a high level of the statistical significance Z > 4σ for all the four options of the intrinsic spectral shape considered (power law, power law with exponential cutoff, log-parabola, and log-parabola with exponential cutoff). The value of B = 10−16 G is not excluded; it is still a viable option of the EGMF strength. These results were obtained for the case of steady sources.

Nonthermal Radiation of the Extreme TeV Blazar 1ES 0229+200 from Electromagnetic Cascades on Infrared Photon Field

Extreme TeV blazars (ETBs) are active galactic nuclei with jets presumably pointing towards the observer having their intrinsic (compensated for the effect of γ-ray absorption on extragalactic background light photons) spectral energy distributions (SEDs) peaked at an energy in excess of 1 TeV. These sources typically reveal relatively weak and slow variability as well as higher frequency of the low-energy SED peak compared to other classes of blazars. It proved to be exceedingly hard to incorporate all these peculiar properties of ETBs into the framework of conventional γ-ray emission models. ETB physics have recently attracted great attention in the astrophysical community, underlying the importance of the development of self-consistent ETB emission model(s). We propose a new scenario for the formation of X-ray and γ-ray spectra of ETBs assuming that electromagnetic cascades develop in the infrared photon field surrounding the central blazar engine. This scenario does not invoke compact fast-moving sources of radiation (so-called “blobs”), in agreement with the apparent absence of fast and strong variability of ETBs. For the case of the extreme TeV blazar 1ES 0229+200 we propose a specific emission model in the framework of the considered scenario. We demonstrate that this model allows to obtain a good fit to the measured SED of 1ES 0229+200.

Artificial neural networks for cosmic gamma-ray propagation in the universe

We explore the potential of an artificial neural network (ANN) based method to probe the propagation of cosmic γ-ray photons in the extragalactic Universe. The journey of γ-rays emitted from a distant source like blazar to the observer at the Earth is impeded by the absorption through the interaction with the extragalactic background light (EBL), leading to an electron-positron pair production. This process dominates for gamma ray photons with energy above 10 GeV propagating over the cosmological distances. The effect of γ-ray attenuation is characterized by a physical quantity called optical depth, which strongly depends on the γ-ray photon energy, redshift of the source, and density of the EBL photons. We estimate the optical depth values for γ-ray energies above 10 GeV emitted from the sources at redshifts in the range 0.01 to 1 using three different and most promising EBL models. These optical depth estimates are randomly divided into two data sets for training and testing of the ANN using energy, redshift as inputs and optical depth as output. The optimization of ANN-performance for each EBL model employs standard back-propagation (BP) and radial-basis function (RBF) algorithms. The performance of the ANN model using the RBF is found to be superior to the BP method. In particular, the RBF-ANN with 40 neurons in the hidden layer corresponding to the EBL model proposed by Finke et al. (2010) shows the best performance for the propagation of γ-rays in the Universe.

Probing the evolution of the EBL photon density out to $z\sim 1$ via $\gamma $-ray propagation measurements with Fermi

The redshift ($z$) evolution of the Extragalactic Background Light (EBL) photon density is very important to understand the history of cosmological structure formation of galaxies and stars since the epoch of recombination. The EBL photons with the characteristic spectral energy distribution ranging from ultraviolet/optical to far-infrared provide a major source of opacity of the Universe to the GeV-TeV $\gamma$-rays travelling over cosmological distances. The effect of the EBL is very significant through $\gamma \gamma \rightarrow e^- e^+$ absorption process on the propagation of the $\gamma$-ray photons with energy $E >$ 50 GeV emitted from the sources at $z \sim 1$. This effect is characterized by the optical depth ($\tau$) which strongly depends on $E$, $z$ and density of the EBL photons. The proper density of the EBL photons increases with $z$ due to expansion of the Universe whereas evolution of radiation sources contributing to the EBL leads to a decrease in the density with increasing $z$. Therefore, the resultant volumetric evolution of the EBL photon density is approximated by a modified redshift dependence. In this work, we probe evolution of the EBL photon density predicted by two prominent models using cosmic gamma-ray horizon ($\tau (E,z)=$ 1) determined by the measurements from the \emph{Fermi}-Large Area Telescope (LAT) observations. The modified redshift dependence of the EBL photon density is optimized for a given EBL model by estimating the same gamma-ray horizon as predicted by the \emph{Fermi}-LAT observations. We further compare the optical depth estimates in the energy range $E =$ 4 GeV-1 TeV and redshift range $z =0.01-1$ from the \emph{Fermi}-LAT observations with the values derived from the two EBL models to further constrain the evolution of the EBL photon density in the $z~\sim 1$ Universe.

Studies of extragalactic background light with TeV BL Lacertae objects

ABSTRACT Very high energy (VHE; E ≥ 100 GeV) gamma-rays from cosmological distances are attenuated by the extragalactic background light (EBL) in the infrared to ultraviolet bands. By contrasting measured versus intrinsic emission,we can derive the EBL photon density. However, we do not know the intrinsic spectra and the EBL separately, only their combined effect. Here we first present a flexible model-dependent optical depth method to study the EBL by fitting the emission spectra of TeV BL Lacertae objects (BL Lacs) via a one-zone leptonic synchrotron self-Compton model (SSC). We have little information about electron energy distributions (EEDs) in the jet, which is critically important to build spectral energy distributions (SEDs) in the SSC scenario. Based on current particle acceleration models, we use two types of EEDs to fit the observed spectra: a power-law log-parabola (PLLP) EED and a broken power-law (BPL) EED. We find that the upper limit of the EBL density is about 30 n W m−2 sr−1, which is similar to the published measurement. Furthermore, we propose an unprecedented method to test the radiation mechanisms involved in TeV objects, by simply comparing the reduced EBL density with the limit obtained by galaxy counts. We demonstrate that for some BL Lacs, at least, the one-zone SSC model should be reconsidered.

Extragalactic background light: Constraints from the TeV gamma-ray observations

The cosmological processes including the history of stars and galaxies formation will be observed in the energy spectrum of Extragalactic Background Light (EBL) radiation. Observations of the very high energy γ-ray sources at redshifts from z = 0.018 up to z = 2.979 with SHALON telescopes gives an opportunity to constrain the EBL density, as the TeV γ-rays can be absorbed due to interaction of low-energy photons of EBL. The detection of TeV γ-ray sources at high redshifts is the evidence of less average spectral density of EBL and thus the less star formation rate at early evolution stage as well as the less expansion rate of the universe, than it is previously believed.

ELMAG 3.01: A three-dimensional Monte Carlo simulation of electromagnetic cascades on the extragalactic background light and in magnetic fields

The version 3.01 of ELMAG, a Monte Carlo program for the simulation of electromagnetic cascades initiated by high-energy photons and electrons interacting with extragalactic background light (EBL), is presented. Pair production and inverse Compton scattering on EBL photons as well as synchrotron losses are implemented using weighted sampling of the cascade development. New features include, among others, the implementation of turbulent extragalactic magnetic fields and the calculation of three-dimensional electron and positron trajectories, solving the Lorentz force equation. As final result of the three-dimensional simulations, the program provides two-dimensional source images as function of the energy and the time delay of secondary cascade particles. Program summaryProgram Title:ELMAG 3.01Program Files doi:http://dx.doi.org/10.17632/gsr7dfnyx6.1Licensing provisions: CC by NC 3.0.Programming language: Fortran 90Supplementary material: See http://elmag.sourceforge.net/Nature of problem: Calculation of secondaries produced by electromagnetic cascades on the extragalactic background light (EBL), including deflections in magnetic fields.Solution method: Monte Carlo simulation of pair production and inverse Compton scattering on EBL photons; weighted sampling of the cascading secondaries; recording of energy, observation angle and time delay of secondary particles at the present epoch in a 1.5-dimensional approximation or of sky-maps in a three-dimensional approach.Restrictions: Using the three-dimensional approach, the energies of cascade photons should be above 1 GeV for Brms≲10−10 G.

Very high-energy constraints on the infrared extragalactic background light

Context. Measurements of the extragalactic background light (EBL) are a fundamental source of information on the collective emission of cosmic sources. Aims. At infrared wavelengths, however, these measurements are precluded by the overwhelming dominance from interplanetary dust emission and the Galactic infrared foreground. Only at λ > 300 μm, where the foregrounds are minimal, has the infrared EBL (IR EBL) been inferred from analysis of the COBE maps. The present paper aims to assess the possibility of evaluating the IR EBL from a few micrometers up to the peak of the emission at > 100 μm using an indirect method that avoids the foreground problem. Methods. To this purpose we exploit the effect of pair-production from gamma-gamma interaction by considering the highest-energy photons emitted by extragalactic sources and their interaction with the IR EBL photons. We simulate observations of a variety of low-redshift emitters with the forthcoming Imaging Atmospheric Cherenkov Telescope (IACT) arrays (CTA in particular) and water Cherenkov observatories (LHAASO, HAWC, SWGO) to assess their suitability to constrain the EBL at such long wavelengths. Results. We find that even under the most extremely favorable conditions of huge emission flares, extremely high-energy emitting blazars are not very useful for our purpose because they are much too distant (> 100 Mpc the nearest ones, MKN 501 and MKN 421). Observations of more local AGNs displaying very high-energy emission, like low-redshift radio galaxies (M 87, IC 310, Centaurus A), are better suited and will potentially allow us to constrain the EBL up to λ ≃ 100 μm.

Extragalactic photon–axion-like particle oscillations up to 1000 TeV

Axion-like particles (ALPs) are attracting increasing interest since, among other things, they are a prediction of many extensions of the standard model of elementary particles physics and in particular of superstrings and superbranes. ALPs are very light, neutral, pseudo-scalar bosons which are supposed to interact with two photons. For their mass ma≪1eV and two-photon coupling gγγa in a suitable range they can give rise to very interesting astrophysical effects taking place in the X- and γ-ray bands. Specifically, throughout the present paper we are concerned with photon–ALP oscillations in the very-high-energy band (VHE,100GeV≲E≲100TeV) and beyond, which ought to occur in the photon beam emitted by far-away blazars and are triggered by the domain-like random extragalactic magnetic field Bext. Because of the presence of the extragalactic background light (EBL) – which is the infrared/optical/ultraviolet radiation emitted by all galaxies during the cosmic evolution – when a VHE photon scatters off an EBL photon an e+e− pair can be created, which causes a rather strong dimming of the source. In the presence of photon–ALP oscillation things are different, since a photon travels sometimes as a true photon and sometimes as an ALP. Since ALPs do not interact with the EBL, the effective optical depth is somewhat reduced. But – as a consequence – the photon survival probability gets strongly enhanced with respect to the prediction of conventional physics, thereby greatly increasing the photon transparency in the VHE band so that the corresponding horizon gets enlarged to a considerable extent. While all this is well known and already studied in detail (De Angelis et al., 2011, 2013a), the new effect of photon dispersion on the cosmic microwave background (CMB) becomes very important at high enough energies. The aim of the present paper is to take it systematically into account. Actually, two widely different energy scales are associated with it. One is EH=O(5TeV), above which the effect in question starts to become dominant and makes the single random realizations of the beam propagation process – the only ones that are observable – to exhibit small energy oscillations: this is a crucial prediction of our model. The other energy scale is Eeq above which the oscillation length becomes smaller than the coherence length of Bext: typically Eeq=O(40TeV) with a large uncertainty. Thus, previously used domain-like models of Bext would generally give wrong results above Eeq and a more realistic model for Bext becomes compelling, like the one very recently developed by the authors. Remarkably, we have been able to derive the corresponding photon survival probability Pγ→γALP(E0,z)analytically and exactly up to observed energies E0=1000TeV and redshift up to z=2, a fact that drastically shortens the computation time in the derivation of the results presented in this paper. Specifically, for 7 simulated blazars we exhibit the plots of the Pγ→γALP(E0,z) along 1000 random realizations versus E0, for different values of z and four values of the model parameters. Our predictions can be tested by the new generation of γ-ray observatories like CTA, HAWC, GAMMA-400, LHAASO, TAIGA-HiSCORE and HERD. Finally, for our guessed values of ma and gγγa our ALP can be detected in the upgrade of ALPS II at DESY, the planned experiments IAXO, STAX and ABRACADABRA as well as with other techniques developed by Avignone and collaborators.

The extragalactic background light revisited and the cosmic photon-photon opacity

In addition to its relevant astrophysical and cosmological significance, the Extragalactic Background Light (EBL) is a fundamental source of opacity for cosmic high energy photons, as well as a limitation for the propagation of high-energy particles in the Universe. We review our previously published determinations of the EBL photon density in the Universe and its evolution with cosmic time, in the light of recent surveys of IR sources at long wavelengths. We exploit deep survey observations by the Herschel Space Observatory and the Spitzer telescope, matched to optical and near-IR photometric and spectroscopic data, to re-estimate number counts and luminosity functions longwards of a few microns, and the contribution of resolved sources to the EBL. These new data indicate slightly lower photon densities in the mid- and far-infrared and sub-millimeter compared to previous determinations. This implies slightly lower cosmic opacity for photon-photon interactions. The new data do not modify previously published EBL modeling in the UV-optical and near-IR up to several microns, while reducing the photon density at longer wavelengths. This improved model of the EBL alleviates some tension that had emerged in the interpretation of the highest-energy TeV observations of local \textit{blazar}s, reducing the case for new physics beyond the standard model (like violations of the Lorenz Invariance, LIV, at the highest particle energies), or for exotic astrophysics, that had sometimes been called for to explain it. Applications of this improved EBL model on current data are considered, as well as perspectives for future instrumentation, the Cherenkov Telescope Array (CTA) in particular.

Effects of spatial fluctuations in the extragalactic background light on hard gamma-ray spectra

This study investigates the impact of the fluctuations in the extra galactic background light (EBL) on the attenuation of the hard $\gamma$-ray spectra of distant blazars. EBL fluctuations occur on the scales up to 100 Mpc and are caused by clustering of galaxies. The EBL photons interact with high energy $\gamma$-rays via the electron-positron pair production mechanism: $\gamma + \gamma' \rightarrow e^+ + e^-$. The attenuation of $\gamma$-rays depends on their energy and the density of the intervening EBL photon field. Using a simple model for the evolution of the mean EBL photon density, we implement an analytical description of the EBL fluctuations. We find that the amplitudes of the EBL energy density can vary by $\pm 1\%$ as a function of environment. The EBL fluctuations lead to mild alterations of the optical depth or equivalently the transmissivity for $\gamma$-rays from distant blazars. Our model predicts maximum changes of $\pm 10\%$ in the $\gamma$-ray transmissivity. However, this translates into marginal differences in the power law slopes of currently observed $\gamma$-ray spectra. The slopes of deabsorbed $\gamma$-ray spectra differ by not more than $\pm 1\%$ if EBL fluctuations are included.

All the light there ever was.

The article discusses light particles and photons of extragalactic background light, astronomical photometry, and sky brightness. The authors comment on German astronomer Wilhelm Olbers and Olbers' Paradox, extragalactic distances, and gamma ray research. Other topics include research in astronomy and wavelengths.

STAR FORMATION HISTORY, DUST ATTENUATION, AND EXTRAGALACTIC BACKGROUND LIGHT

At any given epoch, the Extragalactic Background Light (EBL) carries imprints of integrated star formation activities in the universe till that epoch. On the other hand, in order to estimate the EBL, when direct observations are not possible, one requires an accurate estimation of the star formation rate density (SFRD) and the dust attenuation ($A_\nu$) in galaxies. Here, we present a 'progressive fitting method' that determines global average SFRD($z$) and $A_\nu(z)$ for any given extinction curve by using the available multi-wavelength multi-epoch galaxy luminosity function measurements. Using the available observations, we determine the best fitted combinations of SFRD($z$) and $A_\nu(z)$, in a simple fitting form, up to $z\sim8$ for five well known extinction curves. We find, irrespective of the extinction curve used, the $z$ at which the SFRD($z$) peaks is higher than the $z$ above which $A_\nu(z)$ begins to decline. For each case, we compute the EBL from ultra-violet to the far-infrared and optical depth ($\tau_\gamma$) encountered by the high energy $\gamma$-rays due to pair production upon collisions with these EBL photons. We compare these with measurements of the local EBL, $\gamma$-ray horizon and $\tau_\gamma$ measurements using Fermi-LAT. All these and the comparison of independent SFRD($z$) and $A_\nu(z)$ measurements from the literature with our predictions favor the extinction curve similar to that of Large Magellanic Cloud Supershell.

Very high-energyγ-ray emission from high-redshift blazars

We study the possible detection of and properties of very high-energy (VHE) gamma-ray emission (in the energy band above 100 GeV) from high redshift sources. We report on the detection of VHE gamma-ray flux from blazars with redshifts z>0.5. We use the data of Fermi telescope in the energy band above 100 GeV and identify significant sources via cross-correlation of arrival directions of individual VHE gamma-rays with the positions of known Fermi sources. There are thirteen high-redshift sources detected in the VHE band by Fermi/LAT telescope. The present statistics of the Fermi signal from these sources is too low for a sensible study of the effects of suppression of the VHE flux by pair production through interactions with Extragalactic Background Light photons. We find that the detection of these sources with ground-based gamma-ray telescopes would be challenging. However, several sources including BL Lacs PKS 0426-380 at z=1.11, KUV 00311-1938 at z=0.61, B3 1307+433 at z=0.69, PG 1246+586 at z=0.84, Ton 116 at z=1.065 as well as a flat-spectrum radio quasar 4C +55.17 at z=0.89 should be detectable by HESS-II, MAGIC-II and CTA. A high-statistics study of a much larger number of VHE gamma-ray sources at cosmological distances would be possible with the proposed high-altitude Cherenkov telescope 5@5.

A correlation between hard gamma-ray sources and cosmic voids along the line of sight

We estimate the galaxy density along lines of sight to hard extragalactic gamma-ray sources by correlating source positions on the sky with a void catalog based on the Sloan Digital Sky Survey (SDSS). Extragalactic gamma-ray sources that are detected at very high energy (VHE; E>100 GeV) or have been highlighted as VHE-emitting candidates in the Fermi Large Area Telescope hard source catalog (together referred to as "VHE-like" sources) are distributed along underdense lines of sight at the 2.4 sigma level. There is also a less suggestive correlation for the Fermi hard source population (1.7 sigma). A correlation between 10-500 GeV flux and underdense fraction along the line of sight for VHE-like and Fermi hard sources is found at 2.4 sigma and 2.6 sigma, respectively. The preference for underdense sight lines is not displayed by gamma-ray emitting galaxies within the second Fermi catalog, containing sources detected above 100 MeV, or the SDSS DR7 quasar catalog. We investigate whether this marginal correlation might be a result of lower extragalactic background light (EBL) photon density within the underdense regions and find that, even in the most extreme case of a entirely underdense sight line, the EBL photon density is only 2% less than the nominal EBL density. Translating this into gamma-ray attenuation along the line of sight for a highly attenuated source with opacity tau(E,z) ~5, we estimate that the attentuation of gamma-rays decreases no more than 10%. This decrease, although non-neglible, is unable to account for the apparent hard source correlation with underdense lines of sight.

Multi-wavelength study of Mrk 421 TeV flare observed with TACTIC telescope in February 2010

We present results from multi-wavelength study of intense flaring activity from a high frequency peaked BL Lac object Mrk 421. The source was observed in its flaring state on February 16, 2010 with the TACTIC at energies above 1.5TeV. Near simultaneous multi-wavelength data were obtained from high energy (MeV–GeV) γ-ray observations with Fermi–LAT, X-ray observations by the Swift and MAXI satellites, optical V-band observation by SPOL at Steward Observatory and radio 15GHz observation at OVRO 40 meter-telescope. We have performed a detailed spectral and temporal analysis of TACTIC, Fermi–LAT and Swift–XRT observations of Mrk 421 during February 10–23, 2010 (MJD 55237–55250). The flaring activity of the source is studied by investigating the properties of daily light curves from radio to TeV energy range and we present the correlation and variability analysis in each energy band. The TeV flare detected by TACTIC on February 16, 2010 is well correlated with the activity in lower energy bands. The differential energy spectrum of the source, in the energy range 1.5–11TeV, as measured by TACTIC on this night is described by a power law (dN/dE∝E-Γ) with spectral index Γ=2.6±0.3. After accounting for absorption of TeV photons by low energy extragalactic background light photons via pair production, the intrinsic TeV spectrum reveals a power law index of 2.3±0.3. Finally the broad band spectral energy distribution of the source in flaring state is reproduced using a simple emission model involving synchrotron and synchrotron self Compton processes. The obtained parameters are then used to understand the energetics of the source during the flaring episode.

THE COSMOLOGICAL IMPACT OF LUMINOUS TeV BLAZARS. I. IMPLICATIONS OF PLASMA INSTABILITIES FOR THE INTERGALACTIC MAGNETIC FIELD AND EXTRAGALACTIC GAMMA-RAY BACKGROUND

Inverse-Compton cascades initiated by energetic gamma rays (E>100 GeV) enhance the GeV emission from bright, extragalactic TeV sources. The absence of this emission from bright TeV blazars has been used to constrain the intergalactic magnetic field (IGMF), and the stringent limits placed upon the unresolved extragalactic gamma-ray background (EGRB) by Fermi has been used to argue against a large number of such objects at high redshifts. However, these are predicated upon the assumption that inverse-Compton scattering is the primary energy-loss mechanism for the ultra-relativistic pairs produced by the annihilation of the energetic gamma rays on extragalactic background light photons. Here we show that for sufficiently bright TeV sources (isotropic-equivalent luminosities >10^{42} erg/s) plasma beam instabilities, specifically the "oblique" instability, present a plausible mechanism by which the energy of these pairs can be dissipated locally, heating the intergalactic medium. Since these instabilities typically grow on timescales short in comparison to the inverse-Compton cooling rate, they necessarily suppress the inverse-Compton cascades. As a consequence, this places a severe constraint upon efforts to limit the IGMF from the lack of a discernible GeV bump in TeV sources. Similarly, it considerably weakens the Fermi limits upon the evolution of blazar populations. Specifically, we construct a TeV-blazar luminosity function from those objects presently observed and find that it is very well described by the quasar luminosity function at z~0.1, shifted to lower luminosities and number densities, suggesting that both classes of sources are regulated by similar processes. Extending this relationship to higher redshifts, we show that the magnitude and shape of the EGRB above ~10 GeV is naturally reproduced with this particular example of a rapidly evolving TeV-blazar luminosity function.