Blazar Flares Research Articles

EXTERNAL COMPTON EMISSION IN BLAZARS OF NONLINEAR SYNCHROTRON SELF-COMPTON-COOLED ELECTRONS

The origin of the high-energy component in spectral energy distributions (SED) of blazars is still a bit of a mystery. While BL Lac objects can be rather successfully modelled within the one-zone synchrotron self-Compton (SSC) scenario, the SED of low peaked Flat Spectrum Radio Quasars (FSRQ) is more difficult to reproduce. Their high-energy component needs the abundance of strong external photon sources, giving rise to stronger cooling via the inverse Compton channel, and thusly to a powerful component in the SED. Recently, we were able to show that such a powerful inverse Compton component can also be achieved within the SSC framework. This, however, is only possible if the electrons cool by SSC, which results in a non-linear process, since the cooling depends on an energy integral over the electrons. In this paper we aim to compare the non-linear SSC framework with the external Compton (EC) output by calculating analytically the external Compton component with the underlying electron distribution being either linearly or non-linearly cooled. Due to the additional linear cooling of the electrons with the external photons, higher number densities of electrons are required to achieve non-linear cooling, resulting in more powerful inverse Compton components. If the electrons initially cool non-linearly, the resulting SED can exhibit a dominating SSC over the EC component. However, this dominance depends strongly on the input parameters. We conclude that with the correct time-dependent treatment the SSC component should be taken into account to model blazar flares.

Exploring the extreme universe with the Fermi Gamma-Ray Space Telescope

The Fermi orbiter, mapping the entire gamma-ray sky every three hours, monitors the cosmos for high-energy phenomena both fleeting and enduring.

DIFFERENCES OF LEPTONIC AND HADRONIC RADIATION PRODUCTION IN FLARING BLAZARS

In order to distinguish the leptonic from the hadronic origin of the non-thermal emission of a flaring blazar, we present a semi-analytical model that describes the temporal development of the emergent particles (i.e., photons and neutrinos) based on their leptonic and hadronic origins, respectively. The approach starts with the transport equation of the injected relativistic particles and takes spatial diffusion and continuous energy losses into account. On the one hand, a relativistic electron pick-up is considered, which leads to synchrotron, as well as external Compton emission; and on the other hand, a relativistic proton pick-up, which results in high-energy photons and neutrinos by inelastic proton-proton collisions. The temporal development of the emergent photon and neutrino intensities of blazar flares in hadronic and leptonic interaction scenarios are calculated, given useful predictions of flare durations and time lags between photons of different wavelength and high-energy neutrinos.

ON THE DURATION OF BLAZAR SYNCHROTRON FLARES

A semi-analytical model is presented that describes the temporal development of a blazar synchrotron flare for the case of a broadband synchrotron power spectrum. We examine three different injection scenarios and present its influence on the synchrotron flare. An accurate approximation of the half-life of a synchrotron flare is analytically computed and we give some illustrative examples of the time evolution of the emergent synchrotron intensity by using a numerical integration method. The synchrotron flare starts at all photon energies right after the injection of ultrarelativistic electrons into the spherical emission volume of radius R and its duration exceeds the light travel time 2R/c in the low energy regime. Furthermore, the flare duration extends by the period of injection of relativistic electrons into the emission knot. However, the energetic and spatial distribution of these injected electrons has no significant influence on the flare duration. We obtain a temporal behavior that agrees most favorably with the observations of PKS 2155–304 on 2006 July 29-30 and it differs considerably from the results that were recently achieved by using a monochromatic approximation of the synchrotron power.

A new ordering parameter of spectral energy distributions from synchrotron self-Compton emitting blazars

The broadband SEDs of blazars exhibit two broad spectral components, which in leptonic emission models are attributed to synchrotron radiation and synchrotron self-Compton (SSC) radiation of relativistic electrons. During high state phases, the high-frequency SSC component often dominates the low-frequency synchrotron component, implying that the inverse Compton SSC losses of electrons are at least equal to or greater than the synchrotron losses of electrons. We calculate from the analytical solution of the kinetic equation of relativistic electrons, subject to the combined linear synchrotron and nonlinear synchrotron self-Compton cooling, for monoenergetic injection the time-integrated total synchrotron and SSC radiation fluences and spectral energy distributions (SED). Depending on the ratio of the initial cooling terms, displayed by the injection parameter $\alpha$, we find for $\alpha\ll 1$, implying complete linear cooling, that the synchrotron peak dominates the inverse Compton peak and the usual results of the spectra are recovered. For $\alpha\gg 1$ the SSC peak dominates the synchrotron peak, proving our assumption that in such a case the cooling becomes initially non-linear. The spectra also show some unique features, which can be attributed directly to the non-linear cooling. To show the potential of the model, we apply it to outbursts of 3C 279 and 3C 454.3, successfully reproducing the SEDs. The results of our analysis are promising, and we argue that this non-equilibrium model should be considered in future modeling attempts for blazar flares.

Fermi-LAT View of Bright Flaring Gamma-Ray Blazars

The Fermi LAT provides a continuous and uniform monitoring of the Universe in the gamma-ray band. During the first year many gamma-ray blazar flares, some unidentified transients and emission by the Sun while in a quiet state were promptly detected. This is mainly due to the design of the mission, featuring a detector, the LAT with a wide field of view, and to the operation of the spacecraft itself, that can cover every region of the sky every 3 hours. Nevertheless, the scientific exploitation of this monitoring is more fruitful when early information about transients reaches a broader community. In this respect, the indefatigable activity of flare advocates, who worked on weekly shifts to validate the results and quickly broadcast information about flares and new detections, was the key to most scientific results.

Fermi gamma-ray space telescope: Science highlights for the first 8 months

The Fermi Gamma-ray Space Telescope was launched on June 11, 2008 and since August 2008 has successfully been conducting routine science observations of high energy phenomena in the gamma-ray sky. A number of exciting discoveries have been made during its first year of operation, including blazar flares, high-energy gamma-ray bursts, and numerous new gamma-ray sources of different types, among them pulsars and Active Galactic Nuclei (AGN). Fermi-LAT also performed accurate measurement of the diffuse gamma-radiation which clarifies the “GeV excess” reported by EGRET almost 10 years ago, high precision measurement of the high energy electron spectrum, and other observations. An overview of the observatory status and recent results as of April 30, 2009, are presented.

FLARING BEHAVIOR OF THE QUASAR 3C 454.3 ACROSS THE ELECTROMAGNETIC SPECTRUM

We analyze the behavior of the parsec-scale jet of the quasar 3C~454.3 during pronounced flaring activity in 2005-2008. Three major disturbances propagated down the jet along different trajectories with Lorentz factors $\Gamma>$10. The disturbances show a clear connection with millimeter-wave outbursts, in 2005 May/June, 2007 July, and 2007 December. High-amplitude optical events in the $R$-band light curve precede peaks of the millimeter-wave outbursts by 15-50 days. Each optical outburst is accompanied by an increase in X-ray activity. We associate the optical outbursts with propagation of the superluminal knots and derive the location of sites of energy dissipation in the form of radiation. The most prominent and long-lasting of these, in 2005 May, occurred closer to the black hole, while the outbursts with a shorter duration in 2005 Autumn and in 2007 might be connected with the passage of a disturbance through the millimeter-wave core of the jet. The optical outbursts, which coincide with the passage of superluminal radio knots through the core, are accompanied by systematic rotation of the position angle of optical linear polarization. Such rotation appears to be a common feature during the early stages of flares in blazars. We find correlations between optical variations and those at X-ray and $\gamma$-ray energies. We conclude that the emergence of a superluminal knot from the core yields a series of optical and high-energy outbursts, and that the mm-wave core lies at the end of the jet's acceleration and collimation zone.

Proposed method for searches of gravitational waves from PKS 2155-304 and other blazar flares

We propose to search for gravitational waves from PKS 2155-304 as well as other blazars. PKS 2155-304 emitted a long-duration energetic flare in July 2006 with total isotropic equivalent energy released in TeV gamma rays of approximately 1045 ergs. Possible gravitational wave signals associated with this outburst are expected to be seen at the same time as the electromagnetic signal. During this flare, the two LIGO interferometers at Hanford and the GEO detector were in operation and collecting data. For this search we will use the data from multiple gravitational wave detectors. The method we use for this purpose is a coherent network analysis algorithm which is called RIDGE. To estimate the sensitivity of the search, we perform numerical simulations. For a detection probability of 20%, we estimate that this method is sensitive to total isotropic equivalent energy at the source of about 2.5 × 1055 ergs. For this analysis, an end-to-end pipeline has been developed, which takes into account the motion of the source across the sky.

The spectrum of 1ES0229 + 200 and the cosmic infrared background

Aims. We reexamine the implications of the recent HESS observations of the blazar 1ES0229+200 for constraining the extragalactic mid-infrared background radiation. Methods. We examine the effect of gamma-ray absorption by the extragalactic infrared radiation on predicted intrinsic spectra for this blazar and compare our results with the observational data. Results. We find agreement with our previous results on the shape of the infrared spectral energy distribution, contrary to the recent assertion of the HESS group. Our analysis indicates that 1ES0229+200 has a very hard intrinsic spectrum with a spectral index between 1.1 +/- 0.3 and 1.5 0.3 in the energy range between 0.5 TeV and 15 TeV. Conclusions. Under the assumptions that (1) the models of Stecker et al. as derived from numerous detailed infrared observations are reasonable, and (2) spectral indexes in the range 1 < Gamma < 1.5 are obtainable from relativistic shock acceleration under the astrophysical conditions extant in blazar flares, the fits to the observations of 1ES0229+200 using our previous infrared spectral energy distributions are consistent with both the infrared and -ray observations. Our analysis presents evidence indicating that the energy spectrum of relativistic particles in 1ES0229+200 is produced by relativistic shock acceleration, producing an intrinsic gamma-ray spectrum with index 1 < Gamma < 1.5 and with no evidence of a peak in the spectral energy distribution up to energies approximately 15 TeV.

Statistics of Cosmological Black Hole Jet Sources: Blazar Predictions for theGamma-Ray Large Area Space Telescope

A study of the statistics of cosmological black hole jet sources is applied to EGRET blazar data and predictions are made for GLAST. Black hole jet sources are modeled as collimated relativistic plasma outflows with radiation beamed along the jet axis due to strong Doppler boosting. The comoving rate density of blazar flares is assumed to follow a blazar formation rate (BFR), modeled by analytic functions based on astronomical observations and fits to EGRET data. The redshift and size distributions of γ-ray blazars observed with EGRET, separated into BL Lac objects (BLs) and flat spectrum radio quasar (FSRQ) distributions, are fit with monoparametric functions for the distributions of the jet Lorentz factor Γ, comoving directional power l, and spectral slope. A BFR factor ≈10× greater at z ≳ 1 than at present is found to fit the FSRQ data. A smaller comoving rate density and greater luminosity of BL flares at early times compared to the present epoch fits the BL data. Based on the EGRET observations, ≈1000 blazars consisting of ≈800 FSRQs and FR 2 radio galaxies and ≈200 BL Lac objects and FR 1 radio galaxies will be detected with GLAST during the first year of the mission. Additional AGN classes, such as hard-spectrum BL Lac objects that were mostly missed with EGRET, could add more GLAST sources. The FSRQ and BL contributions to the EGRET γ-ray background at 1 GeV are estimated at the level of ≈10%-15% and ≈2%-4%, respectively. EGRET and GLAST sensitivities to blazar flares are considered in the optimal case, and a GLAST analysis method for blazar detection is outlined.

A Structured Leptonic Jet Model of the “Orphan” TeV Gamma‐Ray Flares in TeV Blazars

The emission spectra of TeV blazars extend up to tens of TeV, and the emission mechanism of the TeV γ-rays is explained by synchrotron self-Compton scattering in leptonic models. In these models the time variabilities of X-rays and TeV γ-rays are correlated. However, recent observations of 1ES 1959+65.0 and Mrk 421 have found the TeV γ-ray flares, i.e., TeV γ-ray flares without simultaneous X-ray flares. In this paper we propose a model for the orphan TeV γ-ray flares, employing an inhomogeneous leptonic jet model. After a primary flare that accompanies flare-up both in X-rays and TeV γ-rays, radiation propagates in various directions in the comoving frame of the jet. When a dense region in the jet receives the radiation, X-rays are scattered by relativistic electrons/positrons to become TeV γ-rays. These γ-ray photons are observed as an orphan TeV γ-ray flare. The observed delay time between the primary and orphan flares is about 2 weeks, and this is accounted for in our model for parameters such as Γ = 20, d = 4 × 1017 cm, α = 3, and η = 1, where Γ is the bulk Lorentz factor of the jet, d is the distance between the central black hole and the primary flare site, α/Γ is the angle between the jet axis and the direction of the motion of the dense region that scatters incoming X-rays produced by the primary flare, and η/Γ is the angle between the jet axis and the line of sight.

External Compton Radiation from Rapid Nonthermal Flares in Blazars

In this paper we extend our approach to modeling multifrequency emission variability from blazars to include external-radiation Compton (ERC) emission and electron energy losses from inverse Compton scattering of seed photons originating outside the jet. We consider seed-photon emission from a dusty molecular torus and the broad line region (BLR) surrounding the central engine. We establish constraints on the properties of the molecular torus and BLR under which synchrotron self-Compton (SSC) emission dominates such that the results obtained in our previous paper are applicable. The focus of this study is on relative time delays between the light curves observed at different frequencies. For definiteness, we consider emission resulting from a collision between relativistic shocks, but the results apply more generally to conditions involving acceleration of relativistic electrons and/or magnetic field amplification at any type of front. Unlike SSC emission, ERC flares involving a constant field of seed photons are not delayed by light travel time of the seed photons. The main cause of delays is from radiative energy losses, which result in frequency stratification behind the front and quench the flare first at the highest frequencies, progressing to lower frequencies as time advances. However, if the spectrum of electrons injected at the shock front is characterized by a relatively high value of the minimum energy (a Lorentz factor $\gamma_{min}\sim100$ is sufficient), the ERC flare in the X-ray band can be delayed and may even peak after the injection has ceased. This effect is strongly frequency dependent, with a longer lag at lower frequencies and an X-ray spectral index that changes rapidly from positive (inverted spectrum) to steep values.

Soft X-ray precursors of the non-thermal flares in blazars

Soft X-ray precursors of the non-thermal flares in blazars

Blazar flaring rates measured with GLAST

We derive the minimum observing time scales to detect a blazar at a given flux level with the LAT on the Gamma-ray Large Area Space Telescope (GLAST) in the scanning and pointing modes. Based upon Phase 1 observations with EGRET, we predict the GLAST detection rate of blazar flares at different flux levels. With some uncertainty given the poor statistics of bright blazars, we predict that a blazar flare with integral flux ≳200×10 −8 photons (>100 MeV) cm −2 s −1, which are the best candidates for Target of Opportunity pointings and extensive temporal and spectral studies, should occur every few days.

Numerical simulations of radiation from blazar jets

We present a description of our numerical code BLAZAR . This code calculates spectra and light curves of blazars during outbursts. The code is based on a model in which the non-thermal flares in blazars are produced in thin shells propagating down a conical jet with relativistic velocities. Such shells may represent layers of a shocked plasma, enclosed between the forward and reverse fronts of an internal shock. In the model adopted by us, the production of non-thermal radiation is assumed to be dominated by electrons and positrons which are accelerated directly, rather then injected by pair cascades. The code includes synchrotron emission and inverse-Compton process as the radiation mechanisms. Both synchrotron photons and external photons are included as the seed photons for Comptonization. At the present stage, the code is limited to treat the inverse Compton process only within the Thomson limit and is specialized to model radiation production in the flat spectrum radio quasars. As an example, we present the results of modeling an outburst in 3C 279 – the most extensively monitored γ -ray – bright quasar.

Radiative Hydromagnetic Shocks in Relativistic Outflow Sources

We calculate the structure of a relativistic shock wave in which the internal energy of the shocked fluid is radiated away on a time scale much shorter than the characteristic shock propagation time. The shock is assumed to move through a uniformly magnetized, neutral plasma consisting of protons, electrons, and positrons, and allowance is made for the possible production of electron/positron pairs in the shock itself. The radiation mechanism is taken to be synchrotron and inverse-Compton emission (the latter involving both synchrotron-produced and externally supplied seed photons) by the electrons and positrons. We simplify the discussion by considering a shock in which the magnetic field is transverse to the direction of propagation and focus attention on the properties of the radiative zone that forms behind the shock transition. In particular, we investigate the possibility that the compression induced by the cooling of the gas amplifies the magnetic pressure until it reaches (and ultimately exceeds) equipartition with the thermal pressure (which, in turn, limits the overall compression). We show that, if a significant fraction of the postshock thermal energy is deposited in the electron/positron component, then a considerable portion of the emitted radiation will come from regions of strong magnetic field even if the field immediately behind the shock transition is well below equipartition. This field amplification mechanism may be relevant to the production of synchrotron flares in blazars, miniquasars, and gamma-ray burst sources. We consider the latter application in some detail and show that this process may play a role in the prompt $\gamma$-ray and possibly also the optical ``flash'' and radio ``flare'' emission, but probably not in the afterglow.

On the hadronic beam model of TeV γ-ray flares from blazars

We propose two models, based on the hadronic beam-cloud collision, for production of TeV γ -ray flares in blazars. In the first model, a change in bulk Lorentz factor of the jet of the Active Galactic Nucleus (AGN) produces the flare. In the second model we consider a two-stage mechanism for high-energy particle acceleration in the jet. This system produces a power law spectrum with two different power indices at low and high energies, which is more efficient than one-stage shock acceleration at high energies. Using this spectrum in a hadronic beam-cloud model for γ -ray emission of blazars, a variable TeV as well as a stable GeV γ -ray flux is predicted. The break energy in the spectrum, related to distance between the jet base and terminal shock or boundary layer thickness, is the parameter used to obtain TeV flares.

X-Ray Spectral Variability of PKS 2005−489 During the Spectacular 1998 November Flare

We report on monitoring of the BL Lac object PKS 2005-489 by the Rossi X-Ray Timing Explorer (RXTE) in 1998 October-December. During these months, the source underwent a spectacular flare; at its peak on November 10, its 2-10 keV flux was 3.33 × 10-10 ergs cm-2 s-1, over 30 times brighter than in quiescence. During the rising phase, the X-ray spectrum of PKS 2005-489 hardened considerably, reaching α = 1.32 (Fν ∝ ν-α) near maximum. During the declining phase, the X-ray spectrum steepened rapidly, reaching α = 1.82, then became somewhat harder toward the end of December (α ~ 1.6). While such behavior has been seen before, the simplicity, magnitude, and duration of this flare allowed us to study it in great detail. We argue that this flare was caused by either the injection of particles into the jet or in situ particle acceleration and that the spectral steepening which followed the flare maximum was the result of synchrotron cooling. Contrary to other recently observed blazar flares (e.g., Mrk 501, 3C 279, PKS 2155-304), our results do not imply a major shift in the location of the synchrotron peak during this flare.

Photohadronic neutrinos from transients in astrophysical sources

We investigate the spectrum of photohadronically produced neutrinos at very high energies (VHE, $\ensuremath{\gtrsim}{10}^{14}\mathrm{eV}$) in astrophysical sources whose physical properties are constrained by their variability, in particular jets in active galactic nuclei (blazars) and gamma-ray bursts (GRBs). We discuss in detail the various competing cooling processes for energetic protons, as well as the cooling of pions and muons in the hadronic cascade, which impose limits on both the efficiency of neutrino production and the maximum neutrino energy. If the proton acceleration process is of the Fermi type, we can derive a model independent upper limit on the neutrino energy from the observed properties of any cosmic transient, which depends only on the assumed total energy of the transient. For standard energetic constraints, we can rule out major contributions above ${10}^{19}\mathrm{eV}$ from current models of both blazars and GRBs; and in most models much stronger limits apply in order to produce measurable neutrino fluxes. For GRBs, we show that the cooling of pions and muons in the hadronic cascade imposes the strongest limit on the neutrino energy, leading to cutoff energies of the electron and muon neutrino spectrum at the source differing by about one order of magnitude. We also discuss the relation of maximum cosmic ray energies to maximum neutrino energies and fluxes in GRBs, and find that the production of both the highest energy cosmic rays and observable neutrino fluxes at the same site can only be realized under extreme conditions; a test implication of this joint scenario would be the existence of strong fluxes of GRB correlated muon neutrinos up to ultrahigh energies, $g{10}^{17}\mathrm{eV}.$ Secondary particle cooling also leads to slightly revised estimates for the neutrino fluxes from (nontransient) active galactic nuclei cores, which are commonly used in estimates for VHE detector event rates. Since our approach is quite general we conclude that the detection or nondetection of neutrinos above $\ensuremath{\sim}{10}^{19}\mathrm{eV}$ correlated with blazar flares or GRBs (e.g., with the Pierre Auger Observatory) would provide strong evidence against or in favor of current models for cosmic ray acceleration and neutrino production in these sources.