TeV Γ-ray Emission Research Articles

Temperature stratification in a molecular shock: Analysis of the emission of H2 pure rotational lines in IC443G

Context. Supernovae remnants (SNRs) represent a major source of feedback from stars on the interstellar medium of galaxies. During the latest stage of supernova explosions (which lasts 10–100 kyr), shock waves produced by the initial blast modify the chemistry of gas and dust, inject kinetic energy in the surroundings, and may alter star formation characteristics. Simultaneously, γ-ray emission is generated by the interaction between the ambient medium and cosmic rays, in particular those locally accelerated in the early stages of the explosion. Aims. We aim to estimate the total molecular mass, local density, and total column density of H2 and the temperature structure in a shocked clump interacting with the supernova remnant IC443 located in a region where cosmic rays interact with the interstellar medium. Measuring the mass of the dense and neutral component of the medium is a prerequisite to understanding the chemistry, energetics, and GeV to TeV γ-ray emission. Methods. Assuming that the emission of H2 pure rotational lines is produced by a collection of gas layers with variable temperature, we compared Spitzer/IRS emission maps for the ν = 0–0 S(0) to S(7) lines with a thermal admixture model. Our description is based on a power-law distribution of thermalized components with temperatures varying between Tmin = 25 K and Tmax = 1500 K. Results. Our thermal admixture model allows the level populations of H2 to be described by a power-law distribution dN = ΛT−ΓdT, with Γ ~ 2.2−4.7. We measured a total mass MH2 = 220−80+110 M⊙ across the Spitzer/IRS field of observations. Conclusions. Our analysis shows that an estimate of the cold molecular gas temperature is paramount to accurately constraining the H2 mass, although the mass remains affected by significant uncertainties due to the assumptions on the gas temperature distribution.

Revision of the GeV γ-Ray Emission in the Region of HESS J1813-178 with Fermi-LAT

HESS J1813-178 is one of the brightest and most compact TeV γ-ray sources, and whether its γ-ray emission is associated with supernova remnant (SNR), pulsar wind nebula (PWN), or young stellar cluster (YSC) is still under debate. By analyzing the GeV γ-ray data in the field of HESS J1813-178 using 14 yr of PASS 8 data recorded by the Fermi Large Area Telescope (Fermi-LAT), we report the discovery of three different sources with different spectra in this region. The hard source with a PL spectral index of 2.11 ± 0.08 has a small size extension, which is spatially and spectrally coincident with the TeV γ-ray emission from HESS J1813-178. CO observations display the dense molecular clouds surrounding HESS J1813-178 in the velocity range of 45–60 km s−1. The possible origins of the γ-ray emission from HESS J1813-178 are discussed, including SNR G12.820.02, the PWN driven by the energetic X-ray pulsar PSR J1813-1749 and YSC Cl 1813-178. However, none of them can be ruled out clearly. Note that the maximum energy of protons in the hadronic model should exceed a few hundred TeV, which makes HESS J1813-178 a promising PeVatron. A detailed LHAASO data analysis about the morphology and spectrum would be helpful to investigate the origin of the γ-ray emission in this region and test its PeVatron nature.

Could the TeV emission of starburst galaxies originate from pulsar wind nebulae?

ABSTRACT While the GeV γ-ray emission of starburst galaxies (SBGs) is commonly thought to arise from hadronic interactions between accelerated cosmic rays and interstellar gas, the origin of the TeV γ-ray emission is more uncertain. One possibility is that a population of pulsar wind nebulae (PWNe) in these galaxies could be responsible for the TeV γ-ray emission. In this work, we first synthesize a PWNe population in the Milky Way and assess their contribution to the γ-ray emission of the Galaxy, using a time-dependent model to calculate the evolution of the PWN population. Such a synthetic PWN population can reproduce the flux distribution of PWNe identified in the Milky Way given a distribution of the initial state of the pulsar population. We then apply it to starburst galaxies and quantitatively calculate the spectral energy distribution of all PWNe in the SBGs NGC 253 and M82. We propose that TeV γ-ray emission in starburst galaxies can be dominated by PWNe for a wide range of parameter space. The energetic argument requires that ηe × vSN > 0.01 yr−1, where ηe is the fraction of spin-down energy going to electrons and vSN is the supernova rate. By requiring the synchrotron emission flux of all PWNe in the galaxy not to exceed the hard X-ray measurement of NGC 253, we constrain the initial magnetic field strength of PWNe to be $\lesssim 400\,\mu{\rm G}$. Future observations at higher energies with the Large High Altitude Air Shower Observatory (LHAASO) or the next-generation neutrino observatory IceCube-Gen2 will help us to understand the origin of the TeV γ-ray emission in SBGs better.

Evidence for Multiple Shocks from the γ-Ray Emission of RS Ophiuchi

In 2021 August, the Fermi Large Area Telescope, H.E.S.S., and MAGIC detected GeV and TeV γ-ray emission from an outburst of recurrent nova RS Ophiuchi. This detection represents the first very high-energy γ-rays observed from a nova, and it opens a new window to study particle acceleration. Both H.E.S.S. and MAGIC described the observed γ-rays as arising from a single, external shock. In this paper, we perform detailed, multi-zone modeling of RS Ophiuchi’s 2021 outburst, including a self-consistent prescription for particle acceleration and magnetic field amplification. We demonstrate that, contrary to previous work, a single shock cannot simultaneously explain RS Ophiuchi’s GeV and TeV emission, in particular the spectral shape and distinct light-curve peaks. Instead, we put forward a model involving multiple shocks that reproduces the observed γ-ray spectrum and temporal evolution. The simultaneous appearance of multiple distinct velocity components in the nova optical spectrum over the first several days of the outburst supports the presence of distinct shocks, which may arise either from the strong latitudinal dependence of the density of the external circumbinary medium (e.g., in the binary equatorial plane versus the poles) or due to internal collisions within the white dwarf ejecta (which power the γ-ray emission in classical novae).

Revision of the γ-Ray Emission from SNR CTB 109 with the Fermi Large Area Telescope

CTB 109 is a middle-aged shell-type supernova remnant (SNR) with bright thermal X-ray emission. We reanalyze the GeV γ-ray emission from CTB 109 using 13 yr of Pass 8 data recorded by the Fermi Large Area Telescope. The γ-ray emission of CTB 109 shows a center bright morphology, which is well consistent with its thermal X-ray emission rather than the shell-type structure in the radio band. The spectral analysis shows an evident spectral curvature at ∼several GeV for the GeV γ-ray spectrum, which can naturally explain the lack of TeV γ-ray emission from CTB 109. Although either a leptonic or a hadronic model could fit the multiwavelength observations of CTB 109, the hadronic model is favored considering its γ-ray morphology and the spectral curvature of the GeV spectrum. The unusual γ-ray spectrum of CTB 109 with other SNRs and the luminosity-diameter squared relation make CTB 109 distinguished both from the young-aged SNRs with hard GeV γ-ray spectra and several old-aged SNRs interacting with molecular clouds.

γ-Ray Emission from Classical Nova V392 Per: Measurements from Fermi and HAWC

This paper reports on the γ-ray properties of the 2018 Galactic nova V392 Per, spanning photon energies ∼0.1 GeV–100 TeV by combining observations from the Fermi Gamma-ray Space Telescope and the HAWC Observatory. As one of the most rapidly evolving γ-ray signals yet observed for a nova, GeV γ-rays with a power-law spectrum with an index Γ = 2.0 ± 0.1 were detected over 8 days following V392 Per’s optical maximum. HAWC observations constrain the TeV γ-ray signal during this time and also before and after. We observe no statistically significant evidence of TeV γ-ray emission from V392 Per, but present flux limits. Tests disfavor the extension of the Fermi Large Area Telescope spectrum to energies above 5 TeV by 2 standard deviations (95%) or more. We fit V392 Per’s GeV γ-rays with hadronic acceleration models, incorporating optical observations, and compare the calculations with HAWC limits.

Characterization of the Gamma-ray Emission from the Kepler Supernova Remnant with Fermi-LAT

The Kepler supernova remnant (SNR) had been the only historic SNR that lacked a detection at GeV and TeV energies, which probe particle acceleration. A recent analysis of Fermi-LAT data reported a likely GeV γ-ray candidate in the direction of the SNR. Using approximately the same data set but with an optimized analysis configuration, we confirm the γ-ray candidate to a solid >6σ detection and report a spectral index of 2.14 ± 0.12stat ± 0.15syst for an energy flux above 100 MeV of (3.1 ± 0.6stat ± 0.3syst) × 10−12 erg cm−2 s−1. The γ-ray excess is not significantly extended and is fully compatible with the radio, infrared, and X-ray spatial distribution of the SNR. We successfully characterized this multiwavelength emission with a model in which accelerated particles interact with the dense circumstellar material in the northwest portion of the SNR and radiate GeV γ rays through π° decay. The X-ray synchrotron and inverse-Compton emission mostly stem from the fast shocks in the southern regions with a magnetic field B ~ 100 μG or higher. Depending on the exact magnetic field amplitude, the TeV γ-ray emission could arise from either the south region (inverse-Compton dominated) or the interaction region (π° decay dominated).

Prospect of detecting X-ray haloes around middle-aged pulsars with eROSITA

ABSTRACT The detection of extended TeV γ-ray emission (dubbed ‘TeV haloes’) around Geminga and Monogem pulsars by High Altitude Water Cherenkov collaboration implies that the halo-like morphologies around middle-aged pulsars may be common. The γ-ray emission above 10 TeV is thought to arise from inverse Compton scattering of relativistic electrons/positrons in the pulsar haloes off cosmic microwave background photons. In the meanwhile, these electrons and positrons can produce X-ray synchrotron emission in the interstellar magnetic field, resulting in a diffuse emission in the X-ray band (namely X-ray haloes). Here, we study the prospect of detecting X-ray haloes with extended Roentgen Survey with an Imaging Telescope Array (eROSITA) from 10 middle-aged pulsars with characteristic age τc larger than tens of thousands of years in the Australia Telescope National Facility pulsar catalogue. Assuming a benchmark value (i.e. B = 3 $\mu$G) for the magnetic field, most of the X-ray haloes are found to be bright enough to be detected by eROSITA in the energy range of 0.5–2 keV with a 20 ks targeted survey. Among these pulsar haloes, four are detectable in the ongoing 4-yr eROSITA all-sky survey. Thanks to the large grasp in the soft X-ray band, eROSITA is expected to be able to measure the surface brightness profiles of the X-ray haloes from sub-pc up to tens of pc scales, which can be used to constrain the magnetic field and the diffusion coefficient in the pulsar haloes.

Multiwavelength emission from leptonic processes in ageing galaxy bubbles

ABSTRACT The evolutionary behaviour and multiwavelength emission properties of bubbles around galaxies, such as the Fermi bubbles of the Milky Way, is unsettled. We perform 3D magneto-hydrodynamical simulations to investigate the evolution of leptonic galaxy bubbles driven by a 0.3-Myr intense explosive outburst from the nucleus of Milky-Way-like galaxies. Adopting an ageing model for their leptonic cosmic rays, we post-process our simulations to compute the multiwavelength emission properties of these bubbles. We calculate the resulting spectra emitted from the bubbles from radio frequencies to γ-rays, and construct emission maps in four energy bands to show the the development of the spatial emission structure of the bubbles. The simulated bubbles show a progression in their spectral properties as they age. In particular, the TeV γ-ray emission is initially strong and dominated by inverse Compton scattering, but falls rapidly after ∼1 Myr. In contrast, the radio synchrotron emission remains relatively stable and fades slowly over the lifetime of the bubble. Based on the emission properties of our post-processed simulations, we demonstrate that γ-ray observations will be limited in their ability to detect galaxy bubbles, with only young bubbles around nearby galaxies being within reach. However, radio observations with, e.g. the upcoming Square Kilometer Array, would be able to detect substantially older bubbles at much greater distances, and would be better placed to capture the evolutionary progression and diversity of galaxy bubble populations.

Cosmic Ray production in SNRs of Pulsar Wind Nebulae type at different ages

The overall observations of plerions from radio to the very high energy gamma-rays could provide information about the evolution of PWN from the young Crab-like to the older stages. The extended MeV-TeV emission from Geminga middle-aged pulsar of ~ 3.4 × 105year age in SHALON, Milagro, HAWC observations, and Fermi-LAT detection could arise from the PWN associated with the Geminga SNR. The TeVγ-ray emission from 3C 58 PWN whose estimated age varies from ~ 800 to (5 − 7) × 103years was first detected in year 2011 by SHALON. The overall spectral energy distribution and information about the extension of PWN from radio to GeV-TeV energies from Fermi-LAT and SHALON observations can contribute to particle transport models and also to the understanding of the mechanisms of PWN expanding, which is, in turn, can shed light on the age of 3C 58 and the history of progenitor SN explosion.

TeV Cosmic-Ray Nucleus Acceleration in Shell-type Supernova Remnants with Hard γ-Ray Spectra

Abstract The emission mechanism for hard γ-ray spectra from supernova remnants (SNRs) is still a matter of debate. Recent multiwavelength observations of the TeV source HESS J1912+101 show that it is associated with an SNR with an age of ∼100 kyr, making it unlikely produce the TeV γ-ray emission via leptonic processes. We analyzed Fermi observations of it and found an extended source with a hard spectrum. HESS J1912+101 may represent a peculiar stage of SNR evolution that dominates the acceleration of TeV cosmic rays. By fitting the multiwavelength spectra of 13 SNRs with hard GeV γ-ray spectra with simple emission models with a density ratio of GeV electrons to protons of ∼10−2, we obtain reasonable mean densities and magnetic fields with a total energy of ∼1050 erg for relativistic ions in each SNR. Among these sources, only two of them, namely SN 1006 and RCW 86, favor a leptonic origin for the γ-ray emission. The magnetic field energy is found to be comparable to that of accelerated relativistic ions and their ratio has a tendency to increase with the age of SNRs. These results suggest that TeV cosmic rays mainly originate from SNRs with hard γ-ray spectra.

Near-infrared [Fe II] and H2 Line Mapping of the Supernova Remnant IC 443 with the IRSF/SIRIUS

Abstract We investigate properties of the interstellar medium (ISM) interacting with shocks around the Galactic supernova remnant IC 443, using the results of near-infrared [Fe ii] and H2 line mapping with the InfraRed Survey Facility/Simultaneous Infrared Imager for Unbiased Survey. In the present study, we newly performed H2 1–0 S(1) and 2–1 S(1) line mapping with the narrowband filters tuned for these lines, covering the entire remnant ( 30 ′ × 35 ′ ). Combined with an [Fe ii] line map, our result shows that the H2 line emission is significantly detected in the southern region, while the [Fe ii] line emission is detected all over the remnant, suggesting that slow and fast shocks propagate through the southern region and the entire remnant, respectively. In particular, the H2 line emission is relatively strong compared to the [Fe ii] line emission in the southwestern region, where TeV γ-ray emission is detected. As the strong H2 line emission indicates the dominance of the dense ISM, this result supports the scenario that the γ-ray emission is likely to be produced through a heavy interaction between cosmic-ray protons and the dense ISM in the southwestern shell. We also find that the H2 and [Fe ii] line emissions show an anti-correlated spatial distribution in the same region, suggesting the presence of the clumpy ISM. Such a clumpy morphology of the ISM around IC 443 may assist cosmic-ray protons to efficiently interact with large amounts of the ISM protons.

High-velocity interstellar absorption associated with the supernova remnant W28

ABSTRACT We present an analysis of moderately high-resolution optical spectra obtained for the sightline to CD−23 13777, an O9 supergiant that probes high-velocity interstellar gas associated with the supernova remnant W28. Absorption components at both high positive and high negative velocity are seen in the interstellar Na i D and Ca ii H and K lines towards CD−23 13777. The high-velocity components exhibit low Na i/Ca ii ratios, suggesting efficient grain destruction by shock sputtering. High column densities of CH+, and high CH+/CH ratios, for the components seen at lower velocity may be indicative of enhanced turbulence in the clouds interacting with W28. The highest positive and negative velocities of the components seen in Na i and Ca ii absorption towards CD−23 13777 imply that the velocity of the blast wave associated with W28 is at least 150 km s−1, a value that is significantly higher than most previous estimates. The line of sight to CD−23 13777 passes very close to a well-known site of interaction between the supernova remnant and a molecular cloud to the north-east. The north-east molecular cloud exhibits broad molecular line emission, OH maser emission from numerous locations, and bright extended GeV and TeV γ-ray emission. The sightline to CD−23 13777 is thus a unique and valuable probe of the interaction between W28 and dense molecular gas in its environs. Future observations at ultraviolet and visible wavelengths will help to better constrain the abundances, kinematics, and physical conditions in the shocked and quiescent gas along this line of sight.

Morphology of TeV Gamma-Ray Emission from the Kiloparsec-scale Jets in Radio Galaxies

Abstract TeV γ rays are observed from a few nearby radio galaxies whose jets are viewed at relatively large angles toward the observer. This emission can be produced in kiloparsec-scale jets whose Lorentz factors are decelerated from values of the order of several at parsec-scale distances. We consider in detail the model in which TeV γ-ray emission is produced by the relativistic electrons in kiloparsec-scale jets that comptonize strongly beamed radiation from inner (parsec-scale) jets. As an example, we study the morphology of the TeV γ-ray emission from the decelerated kiloparsec-scale jet in the nearby radio galaxy Cen A. We show that TeV γ-ray emission can extend throughout kiloparsec-scale distances, as it is relatively smoothly distributed along jets for some parameters of the model we consider. Investigation of the morphological structure of such specific γ-ray emission by the future Cerenkov Telescope Array should provide important constraints on the content and dynamics of the kiloparsec-scale jet in Cen A.

TeV gamma-rays from the region of Perseus Cluster

The results of 20-year observations of the Perseus cluster centering on the NGC 1275 including IC 310 radio galaxy and extragalactic supernova SN 2006gy at energies 800 GeV - 45 TeV by the SHALON telescope are presented. It was found, that the TeV γ-ray emission at energies > 800 GeV has an extended structure with a distinct core centered at the NGC 1275 nucleus and well correlates with the photon emission regions viewed in X-rays by Chandra and anti-correlates with radio-structures. Also, the variations of TeV γ-ray flux both at year- and day- scales were found. The obtained data indicate that the part of TeV γ-ray emission is generated by relativistic jets in the nucleus of NGC 1275. Whereas, the presence of an extended structure around NGC 1275 and the slow rise of the γ-ray flux is the evidence of the interaction of cosmic rays and magnetic fields generated in the jets at the galactic center with the gas of the Perseus cluster.

Cygnus X-3 at very high energies

Cyg X-3 is a well-known binary system which also belongs to a microquasar-type objects. It is actively studied through the wide range of electromagnetic spectrum including ultrahigh energies. Here, we present the results of more than 20-year-long studies of Cyg X-3 in the range of 800 GeV-100 TeV with the SHALON telescope. The detected TeV γ-ray source was identified with Cyg X-s based on the detection of the γ-ray flux modulation at the orbital period of this binary system of 4.8 hours. Detected modulation of TeV γ-ray emission with orbit together with the high luminosity of the companion star of Cyg X-3 and the close orbit leads to an efficient generation of the part of γ-ray emission in the inverse Compton scattering. The correlation of TeV flux increases with the flaring activity of Cyg X-3 at X-ray and radio ranges is found which could be associated with powerful mass ejections from the central regions around the black hole.

On the origin of the TeV gamma-ray emission from Cygnus X-3

Cygnus X-3 binary system is a famous object studied over the wide range of electromagnetic spectrum. Early detections of ultra-high energy gamma-rays from Cygnus X-3 by Kiel, Havera Park and then by Akeno triggered the construction of several large air shower detectors. Also, Cygnus X-3 has been proposed to be one of the most powerful sources of charged cosmic ray particles in the Galaxy. The results of twenty-year observations of the Cyg X-3 binary at energies 800 GeV - 85 TeV are presented with images, spectra during periods of flaring activity and at low flux periods. The correlation of TeV flux increases with flaring activity at the lower energy range of X-ray and radio emission from the relativistic jets of Cygnus X-3 is found as well as 4.8-hour orbital modulation of TeV γ-ray intensity. Detected modulation of TeV γ-ray emission with orbit and important characteristics of Cyg X-3 such as the high luminosity of the companion star and the close orbit leads to an efficient generation of γ-ray emission through inverse Compton scattering in this object. The different type variability of very high-energy γ-emission and correlation of radiation activity in the wide energy range can provide essential information on the mechanism of particle production up to very high energies.

Magnetic field estimates from the X-ray synchrotron emitting rims of the 30 Dor C superbubble and the implications for the nature of 30 Dor C’s TeV emission

Context. The 30 Dor C superbubble is unique for its synchrotron X-ray shell, as well as being the first superbubble to be detected in TeV γ-rays, though which is the dominant TeV emission mechanism, leptonic or hadronic, is still unclear. Aims. We aim to use new Chandra observations of 30 Dor C to resolve the synchrotron shell in unprecedented detail and to estimate the magnetic (B) field in the postshock region, a key discriminator between TeV γ-ray emission mechanisms. Methods. We extracted radial profiles in the 1.5–8 keV range from various sectors around the synchrotron shell and fitted these with a projected and point spread function convolved postshock volumetric emissivity model to determine the filament widths. We then calculated the postshock magnetic field strength from these widths. Results. We find that most of the sectors were well fitted with our postshock model and the determined B-field values were low, all with best fits ≲20 μG. Upper limits on the confidence intervals of three sectors reached ≳30 μG though these were poorly constrained. The generally low B-field values suggests a leptonic-dominated origin for the TeV γ-rays. Our postshock model did not provide adequate fits to two sectors. We find that one sector simply did not provide a clean enough radial profile, while the other could be fitted with a modified postshock model where the projected profile falls off abruptly below ~0.8 times the shell radius, yielding a postshock B-field of 4.8 (3.7–11.8) μG which is again consistent with the leptonic TeV γ-ray mechanism. Alternatively, the observed profiles in these sectors could result from synchrotron enhancements around a shock–cloud interaction as suggested in previous works. Conclusions. The average postshock B-field determined around the X-ray synchrotron shell of 30 Dor C suggests the leptonic scenario as the dominant emission mechanism for the TeV γ-rays.

Discovery of TeV γ-ray emission from the neighbourhood of the supernova remnant G24.7+0.6 by MAGIC

SNR G24.7+0.6 is a 9.5 kyrs radio and $\gamma$-ray supernova remnant evolving in a dense medium. In the GeV regime, SNR G24.7+0.6 (3FHL\,J1834.1--0706e/FGES\,J1834.1--0706) shows a hard spectral index ($\Gamma$$\sim$2) up to $200$\,GeV, which makes it a good candidate to be observed with Cherenkov telescopes such as MAGIC. We observed the field of view of \snr\ with the MAGIC telescopes for a total of 31 hours. We detect very high energy $\gamma$-ray emission from an extended source located 0.34\degr\ away from the center of the radio SNR. The new source, named \mgc\ is detected up to 5\,TeV, and its spectrum is well-represented by a power-law function with spectral index of $2.74 \pm 0.08$. The complexity of the region makes the identification of the origin of the very-high energy emission difficult, however the spectral agreement with the LAT source and overlapping position at less than 1.5$\sigma$ point to a common origin. We analysed 8 years of \fermi-LAT data to extend the spectrum of the source down to 60\,MeV. \fermi-LAT and MAGIC spectra overlap within errors and the global broad band spectrum is described by a power-law with exponential cutoff at $1.9\pm0.5$\,TeV. The detected $\gamma$-ray emission can be interpreted as the results of proton-proton interaction between the supernova and the CO-rich surrounding.

TeV γ-ray emission from Mrk 421 observed with TACTIC during December 2014–February 2015

We observed the TeV blazar Mrk421 in the very high energy (VHE) region with TACTIC from 21 December, 2014 to 21 February, 2015. The VHE γ-ray signal is detected from the source at a statistical significance of ∼ 9σ with 648 ± 74 γ-ray like events using a total observation time of 65.6 h of clean data. To denote the active period of the source, the entire observation has been divided into two spells: Spell I (21–30 December, 2014; MJD: 57012–57021) and Spell II (15 January-21 February, 2015; MJD: 57037–57074). The Spell I corresponds to relatively higher activity state wherein 337 ± 47 γ-rays are detected at a statistical significance of 7.39σ in 21.7 h. During Spell II, 311 ± 57 γ-ray events have been detected in ∼ 44c at a statistical significance of 5.6σ. The analysis of high energy (HE) γ-ray data from Fermi-LAT has been carried out for the period of TACTIC observations. In addition, near simultaneous multi-wavelength data from Swift-BAT and MAXI in the X-ray region, SPOL in optical V-band and OVRO at 15 GHz radio band have also been considered. The HE γ-ray and optical observations indicate variability during both the spells whereas hard X-ray observations are consistent with the average emission through out the TACTIC observations. The broad-band spectral energy distributions of the source are reproduced by single zone homogeneous synchrotron self Compton process for the two spells. The model parameters estimated for the two spells are similar except for the particle energy density which is obtained to be slightly higher during Spell I.