SSC Process Research Articles

Gamma-ray variability and multi-wavelength insights into the unprecedented outburst from 4C 31.03

The blazar 4C31.03 recently underwent a major γ-ray outburst at the beginning of 2023 after a prolonged quiescent phase. Fermi-LAT reported a daily average flux of 5×10−6 phs cm−2 s−1, which is about 60 times its average value. We investigated this extraordinary outbreak through temporal and multi-wavelength analysis. From the statistical analysis of the γ-ray lightcurves using Bayesian blocks, we identified 3 epochs of prominent flares. The fastest flux decay during this major outburst was observed within 5.5±0.7 hours. The highest energy of γ-ray photons found from the source during the active phase is ∼82GeV. Using the transparency of γ-rays against pair production and light crossing time argument, we could obtain the minimum jet Doppler factor as 17 corresponding to the flaring state. The broadband spectral energy distribution study performed using synchrotron, SSC and EC emission processes supports the external Compton scattering of IR photons as the likely mechanism for the γ-ray emission from the source. The results of this study support the scenario of the emission region in 4C31.03, being located beyond the Broad line region from the central blackhole. The long-term γ-ray flux distribution depicts a double log-normal variability, indicating that two distinct flux states are active in this energy band. The index distribution also reveals a two distinct variability pattern and hints that the γ-ray spectrum can be more precisely described by two photon indices.

The radio and γ $\gamma$ -ray variability analysis of S5 0716+714

We have studied the variability of \object{S5 0716+714} at radio 15 GHz and $\gamma$-ray band using three different methods. A possible periodicity of $P_{15 GHz}=266.0\pm11.5$ and $P_{\gamma}=344.0\pm16.4$ days are obtained for radio 15 GHz and $\gamma$-ray light curves, respectively. The variability may be related to the intrinsically emission mechanism. The difference between the variability timescales of radio 15 GHz and $\gamma$-ray may be due to that the emission of radio 15 GHz is produced via the synchrotron process, while the $\gamma$-ray is produced by both the SSC and EC processes.

Curvature of the spectral energy distribution, the inverse Compton component and the jet in Fermi 2LAC blazars

We fit the spectral energy distributions (SEDs) of members of a large sample of Fermi 2LAC blazars to synchrotron and inverse Compton (IC) models. Our main results are as follows. (i) As suggested by previous works, the correlation between peak frequency and curvature can be explained by statistical or stochastic particle acceleration mechanisms. For BL Lacs, we find a linear correlation between synchrotron peak frequency and its curvature. The slope of the correlation is consistent with the stochastic acceleration mechanisms and confirm previous studies. For FSRQs, we also find a linear correlation, but its slope cannot be explained by previous theoretical models. (ii) We find a significant correlation between IC luminosity and synchrotron luminosity. The slope of the correlation of FSRQs is consistent with the EC process. And the slope of the correlation of BL Lac is consistent with the SSC process. (iii) We find several significant correlations between IC curvature and several basic parameters of blazars (black hole mass, broad line luminosity, the Lorentz factor of jet). We also find significant correlations between bolometric luminosity and these basic parameters of blazars which suggest that the origin of jet is a mixture of the mechanisms proposed by Blandford $\&$ Znajek and by Blandford $\&$ Payne.

GAMMA-RAY POLARIZATION OF THE SYNCHROTRON SELF-COMPTON PROCESS FROM A HIGHLY RELATIVISTIC JET

The high polarization observed in the prompt phase of some gamma-ray bursts (GRBs) arouses extensive studies on the emission mechanism. In this paper, we investigate the polarization properties of the synchrotron-self-Compton (SSC) process from a highly relativistic jet. A magnetic-dominated, baryon-loaded jet ejected from the central engine travels with a large Lorentz factor. Shells with slightly different velocities collide with each other and produce shocks. The shocks accelerate electrons to power-law distribution, and at the same time, magnify the magnetic field. Electrons move in the magnetic field and produce synchrotron photons. The synchrotron photons suffer from the Compton scattering (CS) process and then are detected by an observer locating slightly off-axis. We derive analytically the formulae of photon polarization in the SSC process in two magnetic configurations: magnetic field in the shock plane and perpendicular to the shock plane. We show that photons induced by the SSC process can be highly polarized, with the maximum polarization $\Pi \sim 24\%$ in the energy band $[0.5,5]$ MeV. The polarization depends on the viewing angles, peaking in the plane perpendicular to the magnetic field. In the energy band $[0.05,0.5]$ MeV, in which most $\gamma$-ray polarimeters are active, the polarization is about twice of that in the Thomson limit, reaching to $\Pi\sim 20\%$. This implies that the Klein-Nishina effect, which is often neglected in literatures, should be carefully considered.

Why are some BL Lacertaes detected byFermi, but others not?

By cross-correlating an archival sample of 170 BL Lacs with 2 year \fermilat AGN sample, we have compiled a sample of 100 BL Lacs with \fermi detection (FBLs), and a sample of 70 non-\fermi BL Lacs (NFBLs). We compared various parameters of FBLs with those of NFBLs, including the redshift, the low frequency radio luminosity at 408 MHz ($L_{\rm 408MHz}$), the absolute magnitude of host galaxies ($M_{\rm host}$), the polarization fraction from NVSS survey ($P_{\rm NVSS}$), the observed arcsecond scale radio core flux at 5 GHz ($F_{\rm core}$) and jet Doppler factor; all the parameters are directly \textbf{measured} or derived from available data from literatures. We found that the Doppler factor is on average larger in FBLs than in NFBLs, and the $Fermi~ \gamma$-ray detection rate is higher in sources with higher Doppler factor. In contrast, there are no significant differences in terms of the intrinsic parameters of redshift, $ L_{\rm 408MHz}$, $ M_{\rm host}$ and $ P_{\rm NVSS}$. FBLs seem to have a higher probability of exhibiting measurable proper motion. These results strongly indicate a higher beaming effect in FBLs compared to NFBLs. The radio core flux is found to be strongly correlated with $\gamma$-ray flux, which remains after excluding the common dependence of the Doppler factor. At the fixed Doppler factor, FBLs have systematically larger radio core flux than NFBLs, implying lower $\gamma$-ray emission in NFBLs since the radio and $\gamma$-ray flux are significantly correlated. Our results indicate that the Doppler factor is an important parameter of $\gamma$-ray detection, the non-detection of $\gamma$-ray emission in NFBLs is likely due to low beaming effect, and/or low intrinsic $\gamma$-ray flux, and the gamma-rays are likely produced co-spatially with the arcsecond-scale radio core radiation and mainly through the SSC process.

Modelling the very high energy flare of 3C 279 using one-zone leptonic model

We model the simultaneous observations of the flat spectrum radio quasar 3C 279 at radio, optical, X-ray and very high energy (VHE) gamma ray energies during 2006 flare using a simple one zone leptonic model. We consider synchrotron emission due to cooling of a non-thermal electron distribution in an equipartition magnetic field and inverse Compton emission due to the scattering off synchrotron photons (SSC) and external soft photons (EC) by the same distribution of electrons. We show that the VHE gamma ray flux cannot be explained by SSC process thereby suggesting the EC mechanism as a plausible emission process at this energy. The EC scattering of BLR photons to VHE energies will be in Klein-Nishina regime predicting a steep spectrum which is contrary to the observations. However the infrared photons from the dusty torus can be boosted to VHE energies with the scattering process remaining in the Thomson regime. Though EC process can successfully explain the observed VHE flux, it require a magnetic field much lower than the equipartition value to reproduce the observed X-ray flux. Hence we attribute the X-ray emission due to SSC process. We derive the physical parameters of 3C 279 considering the above mentioned emission processes. In addition we assume the size of emission region constrained by a variability timescale of one day. This model can successfully reproduce the broadband spectrum of 3C 279 but predicts substantially large flux at MeV-GeV energies.

The long-lasting activity of 3C 454.3

We present multiwavelength observations of 3C 454.3 from April 2008 to March 2010. The radio to optical data are mostly from the GASP-WEBT, UV and X-ray data from Swift, and gamma-ray data from the AGILE and Fermi satellites. We improved the calibration of optical-UV data from the UVOT and OM instruments and estimated the Lyalpha flux to disentangle the contributions from different components in this spectral region. The observations reveal prominent variability above 8 GHz. In the optical-UV band, the variability amplitude decreases with increasing frequency due to a steadier radiation from both a broad line region and an accretion disc. The optical flux reaches nearly the same levels in the 2008-2009 and 2009-2010 observing seasons; the mm one shows similar behaviour, whereas the gamma and X-ray flux levels rise in the second period. Two prominent gamma-ray flares in mid 2008 and late 2009 show a double-peaked structure, with a variable gamma/optical flux ratio. The X-ray flux variations seem to follow the gamma-ray and optical ones by about 0.5 and 1 d, respectively. We interpret the multifrequency behaviour in terms of an inhomogeneous curved jet, where synchrotron radiation of increasing wavelength is produced in progressively outer and wider jet regions, which can change their orientation in time. In particular, we assume that the long-term variability is due to this geometrical effect. By combining the optical and mm light curves to fit the gamma and X-ray ones, we find that the gamma (X-ray) emission may be explained by inverse-Comptonisation of synchrotron optical (IR) photons by their parent relativistic electrons (SSC process). A slight, variable misalignment between the synchrotron and Comptonisation zones would explain the increased gamma and X-ray flux levels in 2009-2010, as well as the change in the gamma/optical flux ratio during the outbursts peaks.

Effects of estrogens on extracellular matrix synthesis in cultures of human normal and scleroderma skin fibroblasts

To investigate the effects of 17beta-estradiol (E2) on extracellular matrix (ECM) protein synthesis (collagen type I, fibronectin, and laminin) using cultures of normal and scleroderma (SSc) skin fibroblasts. Primary fibroblasts cultures, obtained from skin biopsies of six female voluntary subjects and three female SSc patients, were treated for 24 h with E2 (10(-10)M) alone or in combination with tamoxifene (TAM, 10(-7)M) as an estrogen receptor (ER) antagonist. ECM protein synthesis was analyzed by immunocytochemistry and Western blotting. E2 induced a significant increase of fibronectin, collagen type I, and laminin synthesis both in normal (P < 0.01, P < 0.05, P < 0.01, respectively) and SSc fibroblasts (P < 0.001, P < 0.05, P < 0.001, respectively) when compared to untreated fibroblasts. TAM induced a significant decrease of ECM protein synthesis when compared to E2-treated TAM-untreated fibroblasts. This study seems to support important modulatory effects of E2 in the fibrotic progression of the SSc process via ER interactions.

Statistical characteristics of gamma ray and radio emission of BL Lac object

A sample including 22 BL Lac objects was collected. The possible correlations between the γ-ray emission at 1 GeV and the radio emission at 8.4GHz or 5.0GHz were investigated. Main results are as follows: 1） for minimum γ-ray data, there is no correlation between γ-ray and radio flux density. For maximum and average data, there are good correlations between γ-ray and radio emissions, with the maximum correlation coefficient reaching 0.85, and the chance probability p-4; 2） there are weak correlations between γ-ray spectral index and the radio spectral index. Therefore, the γ-ray radiation was the SSC process.

Intergalactic absorption and blazar γ-ray spectra

The distribution of TeV spectral slopes versus redshift for currently known TeV blazars (16 sources with z ≤ 0.21, and one with z > 0.25) is essentially a scatter plot with hardly any hint of a global trend. We suggest that this is the outcome of two combined effects of intergalactic γγ absorption, plus an inherent feature of the SSC (synchro-self-Compton) process of blazar emission. First, flux dimming introduces a bias that favors detection of progressively more flaring sources at higher redshifts. According to mainstream SSC models, more flaring source states imply sources with flatter TeV slopes. This results in a structured relation between intrinsic TeV slope and redshift. The second effect, spectral steepening by intergalactic absorption, affects sources progressively with distance and effectively wipes out the intrinsic slope-redshift correlation.

A general scheme for modelling γ-ray burst prompt emission

We describe a general method for modelling γ-ray burst (GRB) prompt emission, and determine the range of magnetic field strength, electron energy, Lorentz factor of the source and the distance of the source from the central explosion that is needed to account for the prompt γ-ray emission of a typical long-duration burst. We find that for the burst to be produced via the synchrotron process unphysical conditions are required – the distance of the source from the centre of the explosion (Rγ) must be larger than ∼1017cm and the source Lorentz factor ≳103; for such a high Lorentz factor the deceleration radius (Rd) is less than Rγ even if the number density of particles in the surrounding medium is as small as ∼0.1 cm−3. The result, Rγ > Rd, is in contradiction with the early X-ray and optical afterglow data that show that γ-rays precede the afterglow flux that is produced by a decelerating forward shock. This problem for the synchrotron process applies to all long GRBs other than those that have the low-energy spectrum precisely ν−1/2. In order for the synchrotron process to be a viable mechanism for long bursts, the energy of electrons radiating in the γ-ray band needs to be continuously replenished by some acceleration mechanism during much of the observed spike in GRB light curve – this is not possible if GRB-prompt radiation is produced in shocks (at least the kind that has been usually considered for GRBs) where particles are accelerated at the shock front and not as they travel downstream and emit γ-rays, but might work in some different scenarios such as magnetic outflows. The synchrotron-self-Compton (SSC) process fares much better. There is a large solution space for a typical GRB-prompt emission to be produced via the SSC process. The prompt optical emission accompanying the burst is found to be very bright (≲14 mag; for z∼ 2) in the SSC model, which exceeds the observed flux (or upper limit) for most GRBs. The prompt optical is predicted to be even brighter for the subclass of bursts that have the spectrum fν∝να with α∼ 1 below the peak of νfν. Surprisingly, there are no SSC solutions for bursts that have α∼ 1/3; these bursts might require continuous or repeated acceleration of electrons or some physics beyond the simplified, although generic, SSC model considered in this work. Continuous acceleration of electrons can also significantly reduce the optical flux that would otherwise accompany γ-rays in the SSC model.

The Correlation between Gamma-ray and Radio Emissions in γ-ray Loud Blazars

We collect 119 γ-ray-loud blazars (97 flat spectrum radio quasars (FSRQs) and 22 BL Lacertae objects (BL Lacs)), and investigate possible correlations between their γ-ray emission (maximum, minimum and average values) at 1 GeV and the radio emission at 8.4 GHz. Our main results are as follows. For the lower state γ-ray data, there is no correlation between the γ-ray and radio flux density; For the high state γ-ray data, there are good correlations for the whole 119 blazars and 97 FSRQs, and a weak correlation for the 22 BL Lac objects; For the average γ-ray data, there are good correlations. According to our analysis, we propose that the γ-rays are associated with the radio emission from the jet, and that the γ-ray emission is likely from the SSC process in this case.

Chandradiscovery of extended non-thermal emission in 3C 207 and the spectrum of the relativistic electrons

We report on the Chandra discovery of large scale non–thermal emission features in the double lobed SSRL quasar 3C 207 (). These are: a diffuse emission well correlated with the western radio lobe, a bright one sided jet whose structure coincides with that of the eastern radio jet and an X-ray source at the tip of the jet coincident with the hot spot of the eastern lobe. The diffuse X-ray structure is best interpreted as inverse Compton (IC) scattering of the IR photons from the nuclear source and provides direct observational support to an earlier conjecture (Brunetti et al. [CITE]) that the spectrum of the relativistic electrons in the lobes of radio galaxies extends to much lower energies than those involved in the synchrotron radio emission. The X-ray luminous and spatially resolved knot along the jet is of particular interest: by combining VLA and Chandra data we show that a SSC model is ruled out, while the X-ray spectrum and flux can be accounted for by the IC scattering of the CMB photons (EIC) under the assumptions of a relatively strong boosting and of an energy distribution of the relativistic electrons as that expected from shock acceleration mechanisms. The X-ray properties of the hot spot are consistent with a SSC model. In all cases we find that the inferred magnetic field strength are lower, but close to the equipartition values. The constraints on the energy distribution of the relativistic electrons, imposed by the X-ray spectra of the observed features, are discussed. To this aim we derive in the Appendices precise semi–analytic formulae for the emissivities due to the SSC and EIC processes.

A global synthesis model of dipolarization at substorm expansion onset

In this paper, we describe a global synthesis dipolarization model combining coupled processes in the midtail, inner tail and auroral ionosphere: In the late growth phase, magnetic reconnection releases the magnetic energy stored in the magnetotail. Magnetic flux and energy are transported earthward and tailward. As earthward flow slows down in the near-earth plasma sheet (NEPS), it compresses the magnetic field and plasmas near and earthward of the inner edge of the NEPS and pushes them further inward (earthward and equatorward). This sets up a favorable condition for generating the drift ballooning mode (DBM) instability in the inner tail. The unstable DBMs generate coupled Alfvén-slow magnetosonic waves and field-aligned currents (FACs), resulting in a turbulent state in the equatorial region and enhancing the ionospheric conductance Σ. As soon as Σ and FACs increase to a threshold level, the substorm current wedge is formed, leading to an explosive intensification of the auroral electrojet and magnetic field dipolarization at substorm onset. Moreover, we regard the “substorm trigger phase” (Ohtani et al., Planet. Space Sci. 37 (1989) 579–588) as the interval during which the inner tail is being further compressed inward and the DBMs explosively develop to trigger magnetic field dipolarization. We suggest that the dawn-dusk electric field E y which causes further compression of the inner tail may either be associated with flow braking, or produced by storm SSC and other magnetospheric processes. The present model is thus applicable to the cases either with or without neutral-line formation. Furthermore, in the former case, the enhanced E y in the inner tail may either appear somewhat later than or simultaneously with magnetic reconnection in the midtail. It seems that a variety of expansion onset features can be explained in terms of this synthesis dipolarization model.

X-ray observations of TeV blazars and multi-frequency analysis

The nonthermal spectra of blazars, observed from radio to GeV/TeV γ-rays, reveal two pronounced components, both produced by radiation by energetic particles. One peaks in the IR-to-soft X-ray band, radiating via the synchrotron process; the other, peaking in the high-energy γ-rays, is produced by the Compton process. These spectra - and, in particular, the ASCA data - suggest that the origin of the seed photons for Comptonization is diverse. In the High-energy peaked BL Lac objects (HBLs), the dominant seed photons for Comptonization appear to be the synchrotron photons internal to the jet (SSC process). In the quasar-hosted blazars (QHBs), on the other hand, the X-ray band emission is still dominated by the SSC process, while the MeV to GeV range is produced by Comptonization of external photons such as the emission line light. In the context of this three-component model, we derive the magnetic field of 0.1–1 Gauss for all classes of blazars. Lorentz factors γ peak of electrons radiating at each peak of the vF( v) spectra are estimated to be ∼ 10 5 for HBLs; this is much higher than ∼ 10 3 for QHBs. This difference is consistent with the fact that the four sources that are known to emit TeV γ-rays (TeV blazars) are all classified as HBLs. Among the TeV blazars, Mkn 421 is one of the brightest and most variable emitters from ultraviolet (eV) to hard γ-ray (TeV) energies, and its correlated inter-band variability suggests that both keV and TeV spectral regimes are produced by the same, most energetic end of the electron population radiating via the synchrotron process in the keV, and the SSC process in TeV band. The multi-frequency observations including TeV energy band provide the best opportunity to understand high-energy emission from blazar jets. In this paper, we discuss results of multi-frequency analysis and review the results of intensive campaigns for Mkn 421 from 1994 to 1998.

High-Energy Gamma and Radio Variability of Blazars in the Model of Non-Stationary Jets

A model has been developed for impulsive VLBI jet formation and gamma ray outbursts of Blazars. Propagation of newly expelled matter in the old channel of a jet is calculated supposing that the main driving force is the electromagnetic field. The new outflowing matter overtakes the old matter and forms double, fast or slow magnetosonic shock fronts. In the region of the fronts, the number of particles and their energy increase continuously with propagation time from the central object (Romanova and Lovelace, 1995). Accelerated electrons and positrons in the front interact with a diffuse field of UV photons (inverse Compton scattering), with the magnetic field (synchrotron radiation), and with synchrotron photons (SSC processes), thus creating radiation in a very wide range of bands. The self-consistent relativistic equations for the number of particles, the momentum, energy, and magnetic flux in the front are derived and solved numerically (Lovelace and Romanova, 1995). The time-dependent apparent luminosities in the radio to gamma ray bands are calculated taking into account the Doppler boost of the photons. The model predicts a short outburst of radiation in gamma rays (weeks or so) connected with Compton processes, a sharp (less than a day) outburst in the X-rays with a smooth decrease of the luminosity connected with SSC processes, and synchrotron radiation changing from infrared to radio bands (Fig. 1A). The lepton distribution function was taken as fl = K1/γ2 in the main energy containing range, γ1 ≤ γ ≤ γ2, steeper distribution fl = K2/γ3 for γ2 ≤ γ ≤ γ3, and even steeper for γ ≥ γ3. For γ &lt; γ1, fl is assumed negligible as a result of synchrotron self-absorption. The lowest frequency f(syn1), determined by self-absorption, corresponds initially to the infrared band, and later - to the radio band. From Fig.1B, one can see that radio at 3 mm may start to appear after 2 weeks after outburst. But its maximum may correspond to much later times (months), because f(syn1) decreases slowly with time. The appearance of the new VLBI component in QSO 0528+134, which approximately coincides with the strong gamma-ray flash and with the beginning of the strong mm radio outburst (Krichbaum, et al. 1995; Pohl, et al. 1995), supports the proposed model.Both authors were supported in part by NSF grant AST-9320068. MMR is grateful to RFBR and Organizers of the Symposium for the partial support.