Observations of the inner radian of the Galactic disk at very high energy (VHE) gamma-rays have r... more Observations of the inner radian of the Galactic disk at very high energy (VHE) gamma-rays have revealed at least 16 new sources. Besides shell type supernova remnants, pulsar wind nebulae (PWN) appear to be a dominant source population in the catalogue of VHE gamma-ray sources. Except for the Crab nebula, the newly discovered PWN are resolved at VHE gamma-rays to be spatially extended (5-20 pc). Currently, at least 3 middle aged (t > 10 kyrs) PWN (Vela X, G18.0-0.7, and G313.3+0.6 in the "Kookaburra" region) and 1 young PWN MSH 15-52 (t = 1.55 kyrs) have been identified to be VHE emitting PWN (sometimes called "TeV Plerions"). Two more candidate "TeV Plerions" have been identifed and have been reported at this conference [1]. In this contribution, the gamma-ray emission from Vela X is explained by a nucleonic component in the pulsar wind. The measured broad band spectral energy distribution is compared with the expected X-ray emission from primary and secondary electrons. The observed Xray emission and TeV emission from the three middle aged PWN are compared with each other.
Proceedings of 25th Texas Symposium on Relativistic Astrophysics — PoS(Texas 2010)
We will here discuss how the gamma-ray emission from molecular clouds can be used to probe the co... more We will here discuss how the gamma-ray emission from molecular clouds can be used to probe the cosmic ray flux in distant regions of the Galaxy and to constrain the highly unknown cosmic ray diffusion coefficient. In particular we will discuss the GeV to TeV emission from runaway cosmic rays penetrating molecular clouds close to young and old supernova remnants and in molecular clouds illuminated by the background cosmic ray flux.
During the period March-April 1997, a high flux level of TeV ??-rays was observed from Mkn 501, u... more During the period March-April 1997, a high flux level of TeV ??-rays was observed from Mkn 501, using the HEGRA stereoscopic system of four imaging atmospheric Cherenkov telescopes (IACTs). The almost background-free detection of ??-rays with a rate exceeding ...
We analyze the model of gamma-ray binaries, consisting of a massive star and a pulsar with ultrar... more We analyze the model of gamma-ray binaries, consisting of a massive star and a pulsar with ultrarelativistic wind. We consider radiation from energetic particles, accelerated at the pulsar wind termination shock, and feedback of this radiation on the wind through production of secondary electron-positron pairs. We show that the pair feedback limits the Lorentz factor of the pulsar wind and creates a population of very energetic pairs, whose radiation may be responsible for the observed gamma-ray signal.
ABSTRACT The recent surveys of the Milky Way with space and ground-based gamma-ray detectors reve... more ABSTRACT The recent surveys of the Milky Way with space and ground-based gamma-ray detectors revealed hundreds of high energy (HE) and tens of very high energy (VHE) gamma-ray emitters representing several galactic source populations - supernova remnants, giant molecular clouds, star forming regions, pulsars, pulsar wind nebulae, binary systems. The major fraction of these objects remains however unidentified. In this chapter I discuss the astrophysical implications of VHE gamma-ray observations of supernova remnants (SNRs) in the context of the origin of galactic cosmic rays. These observations confirm the earlier theoretical predictions of effective acceleration of multi-TeV particles in young SNRs by strong shock waves. The interpretation of VHE gamma-ray data from several prominent representatives of young SNRs within the so-called hadronic models requires hard energy spectra of protons extending to 100 TeV, with total energy released in relativistic protons and nuclei as large as 1050 erg. Formally, this can be considered as an observational proof of the so-called SNR paradigm of the origin of galactic cosmic rays. However, the hadronic models are not free of problems related to interpretation of multi-wavelength properties of these objects. Moreover, in most of the cases the gamma-ray data can be explained by the inverse Compton scattering of electrons which are responsible also for the synchrotron X-radiation of young SNRs. These circumstances prevent us from a firm statement about the contribution of SNRs to the overall flux of galactic cosmic rays. Further observations of young SNRs, especially in the highest energy band (well above 10 TeV), can be crucial in this regard. Quite important are also the complementary observations from massive molecular clouds located within the close proximity of mid-age SNRs.
We present exactly simultaneous X-ray and TeV monitoring with RXT E and HEGRA of the TeV blazar M... more We present exactly simultaneous X-ray and TeV monitoring with RXT E and HEGRA of the TeV blazar Mrk 501 during 15 days in 1998 June. After an initial period of very low ux at both wavelengths, the source underwent a remarkable are in the TeV and X-ray energy bands, lasting for about 6 days and with a larger amplitude at TeV energies than in the X-ray band. At the peak of the TeV are, rapid TeV ux variability on subhour timescales is found. Large spectral variations are observed at X-rays, with the 320 keV photon index of a pure power-law continuum attening from ! 2.3 to ! 1.8 on a timescale of 23 days. This implies that during the maximum of the TeV activity the synchrotron peak shifted to energies keV, a behavior similar to that observed during the longer lasting, Z50 more intense are in 1997 April. The TeV spectrum during the are is described by a power law with photon index ! 1.9 and an exponential cuto+ at D4 TeV ; an indication for spectral softening during the are decay is obs...
The results of follow-up observations of the TeV gamma-ray source HESSJ 1640-465 from 2004 to 201... more The results of follow-up observations of the TeV gamma-ray source HESSJ 1640-465 from 2004 to 2011 with the High Energy Stereoscopic System (H.E.S.S.) are reported in this work. The spectrum is well described by an exponential cut-off power law with photon index Gamma=2.11 +/- 0.09_stat +/- 0.10_sys, and a cut-off energy of E_c = (6.0 +2.0 -1.2) TeV. The TeV emission is significantly extended and overlaps with the north-western part of the shell of the SNR G338.3-0.0. The new H.E.S.S. results, a re-analysis of archival XMM-Newton data, and multi-wavelength observations suggest that a significant part of the gamma-ray emission from HESS J1640-465 originates in the SNR shell. In a hadronic scenario, as suggested by the smooth connection of the GeV and TeV spectra, the product of total proton energy and mean target density could be as high as W_p n_H ~ 4 x 10^52 (d/10kpc)^2 erg cm^-3.
Thanks to extensive observations with X-ray missions and facilities working in other wavelengths,... more Thanks to extensive observations with X-ray missions and facilities working in other wavelengths, as well as rapidly-advancing numerical simulations of accretion flows, our knowledge of astrophysical black holes has been remarkably enriched. Rapid progress has opened new areas of enquiry, including measurements of black hole spin, the properties and driving mechanisms of jets and disk winds, the impact of feedback into local environments, the origin of periodic and aperiodic X-ray variations, and the nature of super-Eddington accretion flows, among others. The goal of this White Paper is to illustrate how ASTRO-H can make dramatic progress in the study of astrophysical black holes, particularly the study of black hole X-ray binaries.
ABSTRACT Since the detection of gamma-ray binaries in the GeV range with Fermi, the GeV and TeV s... more ABSTRACT Since the detection of gamma-ray binaries in the GeV range with Fermi, the GeV and TeV spectra of these systems have challenged our picture of their high-energy emission. The exponential cutoff at a few GeV, combined with the hard spectrum at TeV and the anticorrelation between the two bands, points towards two different origins for the GeV and TeV emission from LS 5039 and LS I +61 303. If these systems are powered by a young pulsar, one of the locations could be the apex of the contact discontinuity between the pulsar and stellar winds, located between the pulsar and the star. This location is a good candidate for emission of the GeV component, but the strong photon field density from the star makes it an unsuitable TeV emitter owing to pair production opacity. However, the rotation of the pulsar around the star gives rise to a bending of the wind interaction region by Coriolis forces at distances larger than the size of the binary system. We show that the particles accelerated in the pulsar wind shock at this location could be responsible for the TeV component in gamma-ray binaries. The spectral and lightcurve resulting from this simple two-emitter model match satisfactorily the Fermi and HESS observations of LS 5039, and provide a starting point for a future broadband analysis, from radio to TeV, of the properties of the system.
Aims. Previous observations with the High Energy Stereoscopic System (H.E.S.S.) have revealed the... more Aims. Previous observations with the High Energy Stereoscopic System (H.E.S.S.) have revealed the existence of an extended very-high-energy (VHE; E>100 GeV) γ-ray source, HESS J1834−087, coincident with the supernova remnant (SNR) W41. The origin of the γ-ray emission has been further investigated with the H.E.S.S. array and the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi). Methods. The γ-ray data provided by 61 hours of observations with H.E.S.S. and 4 years with the Fermi LAT have been analyzed, covering over 5 decades in energy from 1.8 GeV up to 30 TeV. The morphology and spectrum of the TeV and GeV sources have been studied and multi-wavelength data have been used to investigate the origin of the γ-ray emission towards W41. Results. The TeV source can be modeled with a sum of two components: one point-like and one significantly extended (σTeV = 0.17 • ± 0.01 •), both centered on SNR W41 and exhibiting spectra described by a power law with index ΓTeV ≃ 2.6. The GeV source detected with Fermi LAT is extended (σGeV = 0.15 • ± 0.03 •) and morphologically matches the VHE emission. Its spectrum can be described by a power-law model with an index ΓGeV = 2.15 ± 0.12 and joins smoothly the one of the whole TeV source. A break appears in the γ-ray spectra around 100 GeV. No pulsations have been found in the GeV range. Conclusions. Two main scenarios are proposed to explain the observed emission: a pulsar wind nebula (PWN) or the interaction of SNR W41 with an associated molecular cloud. X-ray observations suggest the presence of a point-like source (a pulsar candidate) near the center of the remnant and non-thermal X-ray diffuse emission which could arise from the possibly associated PWN. The PWN scenario is supported by the compatible positions of the TeV and GeV sources with the putative pulsar. However, the spectral energy distribution from radio to γ-rays is reproduced by a one-zone leptonic model only if an excess of low-energy electrons is injected following a Maxwellian distribution by a pulsar with a high spin-down power (> 10 37 erg s −1). This additional low-energy component is not needed if we consider that the point-like TeV source is unrelated to the extended GeV and TeV sources. The interacting SNR scenario is supported by the spatial coincidence between the γ-ray sources, the detection of OH (1720 MHz) maser lines and the hadronic modeling.
ABSTRACT We explore the correlation of diffuse TeV gamma rays in the Galactic centre with optical... more ABSTRACT We explore the correlation of diffuse TeV gamma rays in the Galactic centre with optical-depth-corrected CO(1-0) emission. We present our analysis of the data, in which the 12CO(1-0) molecular line emission is corrected for optical-depth by using the the 13C and 18O isotopes of the CO(1-0) line. Using simple scatter plots, we find no evidence of a correlation with the lower-density tracer.
Long-term monitoring of PKS 2155−304 with ATOM and H.E.S.S.: investigation of optical/γ-ray corre... more Long-term monitoring of PKS 2155−304 with ATOM and H.E.S.S.: investigation of optical/γ-ray correlations in different spectral states (Affiliations can be found after the references)
International Journal of Modern Physics: Conference Series, 2014
We summarize recent results based on an analysis of Fermi-LAT data for the lobes and the core of ... more We summarize recent results based on an analysis of Fermi-LAT data for the lobes and the core of the nearby radio galaxy Centaurus A (Cen A). In the case of the core, high-energy (HE; > 100 MeV) γ-rays up to 50 GeV have been detected with a detection significance of about 44σ. The average gamma-ray spectrum of the core reveals interesting evidence for a possible deviation from a simple power-law. A likelihood analysis with a broken power-law model shows that the photon index becomes substantially harder above Eb ≃ 4 GeV, changing from Γ1 = 2.74 ± 0.03 below to Γ2 = 2.09 ± 0.20 above. It seems possible that this hardening marks the contribution of an additional high-energy component beyond the common synchrotron-self Compton jet emission. In the case of the lobes, the high-energy gamma-ray emission extends up to 6 GeV, with a significance of more than 10 and 20 σ for the north and the south lobe, respectively. Based on a detailed spatial analysis and comparison with the associated...
Very-high-energy (VHE; E ≥ 100 GeV) and high-energy (HE; 100 MeV ≤ E ≤ 100 GeV) data from γray ob... more Very-high-energy (VHE; E ≥ 100 GeV) and high-energy (HE; 100 MeV ≤ E ≤ 100 GeV) data from γray observations performed with the H.E.S.S. telescope array and the Fermi-LAT instrument, respectively, are analysed in order to investigate the non-thermal processes in the starburst galaxy NGC 253. The VHE γ-ray data can be described by a power law in energy with differential photon index Γ = 2.14 ± 0.18 stat ± 0.30 sys and differential flux normalisation at 1 TeV of F 0 = (9.6 ± 1.5 stat (+5.7, −2.9) sys) × 10 −14 TeV −1 cm −2 s −1. A powerlaw fit to the differential HE γ-ray spectrum reveals a photon index of Γ = 2.24±0.14 stat ±0.03 sys and an integral flux between 200 MeV and 200 GeV of F(0.2 − 200 GeV) = (4.9 ± 1.0 stat ± 0.3 sys) × 10 −9 cm −2 s −1. No evidence for a spectral break or turnover is found over the dynamic range of both the LAT instrument and the H.E.S.S. experiment: a combined fit of a power law to the HE and VHE γ-ray data results in a differential photon index Γ = 2.34 ± 0.03 with a p-value of 30%. The γ-ray observations indicate that at least about 20% of the energy of the cosmic rays capable of producing hadronic interactions is channeled into pion production. The smooth alignment between the spectra in the HE and VHE γ-ray domain suggests that the same transport processes dominate in the entire energy range. Advection is most likely responsible for charged particle removal from the starburst nucleus from GeV to multiple TeV energies. In a hadronic scenario for the γ-ray production, the single overall power-law spectrum observed would therefore correspond to the mean energy spectrum produced by the ensemble of cosmic-ray sources in the starburst region.
HESS J0632+057 is the only gamma-ray binary known so far whose position in the sky allows observa... more HESS J0632+057 is the only gamma-ray binary known so far whose position in the sky allows observations with ground-based observatories both in the northern and southern hemispheres. Here we report on long-term observations of HESS J0632+057 conducted with the VERITAS and H.E.S.S. Cherenkov Telescopes and the X-ray Satellite Swift, spanning a time range from 2004 to
We report on the first completely simultaneous observation of a gamma-ray burst (GRB) using an ar... more We report on the first completely simultaneous observation of a gamma-ray burst (GRB) using an array of Imaging Atmospheric Cherenkov Telescopes, which is sensitive to photons in the very high energy (VHE) γ-ray range (100 GeV). On 2006 June 2, the Swift Burst Alert Telescope (BAT) registered an unusually soft γ-ray burst (GRB 060602B). The burst position was under observation using the High Energy Stereoscopic System (HESS) at the time the burst occurred. Data were taken before, during, and after the burst. A total of 5 hr of observations were obtained during the night of 2006 June 2-3, and five additional hours were obtained over the next three nights. No VHE γ-ray signal was found during the period covered by the HESS observations. The 99% confidence level flux upper limit (> 1 TeV) for the prompt phase (9 s) of GRB 060602B is 2.9 × 10 −9 erg cm −2 s −1. Due to the very soft BAT spectrum of the burst compared with other Swift GRBs and its proximity to the Galactic center, the burst is likely associated with a Galactic X-ray burster, although the possibility of it being a cosmological GRB cannot be ruled out. We discuss the implications of our flux limits in the context of these two bursting scenarios.
Observations of the inner radian of the Galactic disk at very high energy (VHE) gamma-rays have r... more Observations of the inner radian of the Galactic disk at very high energy (VHE) gamma-rays have revealed at least 16 new sources. Besides shell type supernova remnants, pulsar wind nebulae (PWN) appear to be a dominant source population in the catalogue of VHE gamma-ray sources. Except for the Crab nebula, the newly discovered PWN are resolved at VHE gamma-rays to be spatially extended (5-20 pc). Currently, at least 3 middle aged (t > 10 kyrs) PWN (Vela X, G18.0-0.7, and G313.3+0.6 in the "Kookaburra" region) and 1 young PWN MSH 15-52 (t = 1.55 kyrs) have been identified to be VHE emitting PWN (sometimes called "TeV Plerions"). Two more candidate "TeV Plerions" have been identifed and have been reported at this conference [1]. In this contribution, the gamma-ray emission from Vela X is explained by a nucleonic component in the pulsar wind. The measured broad band spectral energy distribution is compared with the expected X-ray emission from primary and secondary electrons. The observed Xray emission and TeV emission from the three middle aged PWN are compared with each other.
Proceedings of 25th Texas Symposium on Relativistic Astrophysics — PoS(Texas 2010)
We will here discuss how the gamma-ray emission from molecular clouds can be used to probe the co... more We will here discuss how the gamma-ray emission from molecular clouds can be used to probe the cosmic ray flux in distant regions of the Galaxy and to constrain the highly unknown cosmic ray diffusion coefficient. In particular we will discuss the GeV to TeV emission from runaway cosmic rays penetrating molecular clouds close to young and old supernova remnants and in molecular clouds illuminated by the background cosmic ray flux.
During the period March-April 1997, a high flux level of TeV ??-rays was observed from Mkn 501, u... more During the period March-April 1997, a high flux level of TeV ??-rays was observed from Mkn 501, using the HEGRA stereoscopic system of four imaging atmospheric Cherenkov telescopes (IACTs). The almost background-free detection of ??-rays with a rate exceeding ...
We analyze the model of gamma-ray binaries, consisting of a massive star and a pulsar with ultrar... more We analyze the model of gamma-ray binaries, consisting of a massive star and a pulsar with ultrarelativistic wind. We consider radiation from energetic particles, accelerated at the pulsar wind termination shock, and feedback of this radiation on the wind through production of secondary electron-positron pairs. We show that the pair feedback limits the Lorentz factor of the pulsar wind and creates a population of very energetic pairs, whose radiation may be responsible for the observed gamma-ray signal.
ABSTRACT The recent surveys of the Milky Way with space and ground-based gamma-ray detectors reve... more ABSTRACT The recent surveys of the Milky Way with space and ground-based gamma-ray detectors revealed hundreds of high energy (HE) and tens of very high energy (VHE) gamma-ray emitters representing several galactic source populations - supernova remnants, giant molecular clouds, star forming regions, pulsars, pulsar wind nebulae, binary systems. The major fraction of these objects remains however unidentified. In this chapter I discuss the astrophysical implications of VHE gamma-ray observations of supernova remnants (SNRs) in the context of the origin of galactic cosmic rays. These observations confirm the earlier theoretical predictions of effective acceleration of multi-TeV particles in young SNRs by strong shock waves. The interpretation of VHE gamma-ray data from several prominent representatives of young SNRs within the so-called hadronic models requires hard energy spectra of protons extending to 100 TeV, with total energy released in relativistic protons and nuclei as large as 1050 erg. Formally, this can be considered as an observational proof of the so-called SNR paradigm of the origin of galactic cosmic rays. However, the hadronic models are not free of problems related to interpretation of multi-wavelength properties of these objects. Moreover, in most of the cases the gamma-ray data can be explained by the inverse Compton scattering of electrons which are responsible also for the synchrotron X-radiation of young SNRs. These circumstances prevent us from a firm statement about the contribution of SNRs to the overall flux of galactic cosmic rays. Further observations of young SNRs, especially in the highest energy band (well above 10 TeV), can be crucial in this regard. Quite important are also the complementary observations from massive molecular clouds located within the close proximity of mid-age SNRs.
We present exactly simultaneous X-ray and TeV monitoring with RXT E and HEGRA of the TeV blazar M... more We present exactly simultaneous X-ray and TeV monitoring with RXT E and HEGRA of the TeV blazar Mrk 501 during 15 days in 1998 June. After an initial period of very low ux at both wavelengths, the source underwent a remarkable are in the TeV and X-ray energy bands, lasting for about 6 days and with a larger amplitude at TeV energies than in the X-ray band. At the peak of the TeV are, rapid TeV ux variability on subhour timescales is found. Large spectral variations are observed at X-rays, with the 320 keV photon index of a pure power-law continuum attening from ! 2.3 to ! 1.8 on a timescale of 23 days. This implies that during the maximum of the TeV activity the synchrotron peak shifted to energies keV, a behavior similar to that observed during the longer lasting, Z50 more intense are in 1997 April. The TeV spectrum during the are is described by a power law with photon index ! 1.9 and an exponential cuto+ at D4 TeV ; an indication for spectral softening during the are decay is obs...
The results of follow-up observations of the TeV gamma-ray source HESSJ 1640-465 from 2004 to 201... more The results of follow-up observations of the TeV gamma-ray source HESSJ 1640-465 from 2004 to 2011 with the High Energy Stereoscopic System (H.E.S.S.) are reported in this work. The spectrum is well described by an exponential cut-off power law with photon index Gamma=2.11 +/- 0.09_stat +/- 0.10_sys, and a cut-off energy of E_c = (6.0 +2.0 -1.2) TeV. The TeV emission is significantly extended and overlaps with the north-western part of the shell of the SNR G338.3-0.0. The new H.E.S.S. results, a re-analysis of archival XMM-Newton data, and multi-wavelength observations suggest that a significant part of the gamma-ray emission from HESS J1640-465 originates in the SNR shell. In a hadronic scenario, as suggested by the smooth connection of the GeV and TeV spectra, the product of total proton energy and mean target density could be as high as W_p n_H ~ 4 x 10^52 (d/10kpc)^2 erg cm^-3.
Thanks to extensive observations with X-ray missions and facilities working in other wavelengths,... more Thanks to extensive observations with X-ray missions and facilities working in other wavelengths, as well as rapidly-advancing numerical simulations of accretion flows, our knowledge of astrophysical black holes has been remarkably enriched. Rapid progress has opened new areas of enquiry, including measurements of black hole spin, the properties and driving mechanisms of jets and disk winds, the impact of feedback into local environments, the origin of periodic and aperiodic X-ray variations, and the nature of super-Eddington accretion flows, among others. The goal of this White Paper is to illustrate how ASTRO-H can make dramatic progress in the study of astrophysical black holes, particularly the study of black hole X-ray binaries.
ABSTRACT Since the detection of gamma-ray binaries in the GeV range with Fermi, the GeV and TeV s... more ABSTRACT Since the detection of gamma-ray binaries in the GeV range with Fermi, the GeV and TeV spectra of these systems have challenged our picture of their high-energy emission. The exponential cutoff at a few GeV, combined with the hard spectrum at TeV and the anticorrelation between the two bands, points towards two different origins for the GeV and TeV emission from LS 5039 and LS I +61 303. If these systems are powered by a young pulsar, one of the locations could be the apex of the contact discontinuity between the pulsar and stellar winds, located between the pulsar and the star. This location is a good candidate for emission of the GeV component, but the strong photon field density from the star makes it an unsuitable TeV emitter owing to pair production opacity. However, the rotation of the pulsar around the star gives rise to a bending of the wind interaction region by Coriolis forces at distances larger than the size of the binary system. We show that the particles accelerated in the pulsar wind shock at this location could be responsible for the TeV component in gamma-ray binaries. The spectral and lightcurve resulting from this simple two-emitter model match satisfactorily the Fermi and HESS observations of LS 5039, and provide a starting point for a future broadband analysis, from radio to TeV, of the properties of the system.
Aims. Previous observations with the High Energy Stereoscopic System (H.E.S.S.) have revealed the... more Aims. Previous observations with the High Energy Stereoscopic System (H.E.S.S.) have revealed the existence of an extended very-high-energy (VHE; E>100 GeV) γ-ray source, HESS J1834−087, coincident with the supernova remnant (SNR) W41. The origin of the γ-ray emission has been further investigated with the H.E.S.S. array and the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi). Methods. The γ-ray data provided by 61 hours of observations with H.E.S.S. and 4 years with the Fermi LAT have been analyzed, covering over 5 decades in energy from 1.8 GeV up to 30 TeV. The morphology and spectrum of the TeV and GeV sources have been studied and multi-wavelength data have been used to investigate the origin of the γ-ray emission towards W41. Results. The TeV source can be modeled with a sum of two components: one point-like and one significantly extended (σTeV = 0.17 • ± 0.01 •), both centered on SNR W41 and exhibiting spectra described by a power law with index ΓTeV ≃ 2.6. The GeV source detected with Fermi LAT is extended (σGeV = 0.15 • ± 0.03 •) and morphologically matches the VHE emission. Its spectrum can be described by a power-law model with an index ΓGeV = 2.15 ± 0.12 and joins smoothly the one of the whole TeV source. A break appears in the γ-ray spectra around 100 GeV. No pulsations have been found in the GeV range. Conclusions. Two main scenarios are proposed to explain the observed emission: a pulsar wind nebula (PWN) or the interaction of SNR W41 with an associated molecular cloud. X-ray observations suggest the presence of a point-like source (a pulsar candidate) near the center of the remnant and non-thermal X-ray diffuse emission which could arise from the possibly associated PWN. The PWN scenario is supported by the compatible positions of the TeV and GeV sources with the putative pulsar. However, the spectral energy distribution from radio to γ-rays is reproduced by a one-zone leptonic model only if an excess of low-energy electrons is injected following a Maxwellian distribution by a pulsar with a high spin-down power (> 10 37 erg s −1). This additional low-energy component is not needed if we consider that the point-like TeV source is unrelated to the extended GeV and TeV sources. The interacting SNR scenario is supported by the spatial coincidence between the γ-ray sources, the detection of OH (1720 MHz) maser lines and the hadronic modeling.
ABSTRACT We explore the correlation of diffuse TeV gamma rays in the Galactic centre with optical... more ABSTRACT We explore the correlation of diffuse TeV gamma rays in the Galactic centre with optical-depth-corrected CO(1-0) emission. We present our analysis of the data, in which the 12CO(1-0) molecular line emission is corrected for optical-depth by using the the 13C and 18O isotopes of the CO(1-0) line. Using simple scatter plots, we find no evidence of a correlation with the lower-density tracer.
Long-term monitoring of PKS 2155−304 with ATOM and H.E.S.S.: investigation of optical/γ-ray corre... more Long-term monitoring of PKS 2155−304 with ATOM and H.E.S.S.: investigation of optical/γ-ray correlations in different spectral states (Affiliations can be found after the references)
International Journal of Modern Physics: Conference Series, 2014
We summarize recent results based on an analysis of Fermi-LAT data for the lobes and the core of ... more We summarize recent results based on an analysis of Fermi-LAT data for the lobes and the core of the nearby radio galaxy Centaurus A (Cen A). In the case of the core, high-energy (HE; > 100 MeV) γ-rays up to 50 GeV have been detected with a detection significance of about 44σ. The average gamma-ray spectrum of the core reveals interesting evidence for a possible deviation from a simple power-law. A likelihood analysis with a broken power-law model shows that the photon index becomes substantially harder above Eb ≃ 4 GeV, changing from Γ1 = 2.74 ± 0.03 below to Γ2 = 2.09 ± 0.20 above. It seems possible that this hardening marks the contribution of an additional high-energy component beyond the common synchrotron-self Compton jet emission. In the case of the lobes, the high-energy gamma-ray emission extends up to 6 GeV, with a significance of more than 10 and 20 σ for the north and the south lobe, respectively. Based on a detailed spatial analysis and comparison with the associated...
Very-high-energy (VHE; E ≥ 100 GeV) and high-energy (HE; 100 MeV ≤ E ≤ 100 GeV) data from γray ob... more Very-high-energy (VHE; E ≥ 100 GeV) and high-energy (HE; 100 MeV ≤ E ≤ 100 GeV) data from γray observations performed with the H.E.S.S. telescope array and the Fermi-LAT instrument, respectively, are analysed in order to investigate the non-thermal processes in the starburst galaxy NGC 253. The VHE γ-ray data can be described by a power law in energy with differential photon index Γ = 2.14 ± 0.18 stat ± 0.30 sys and differential flux normalisation at 1 TeV of F 0 = (9.6 ± 1.5 stat (+5.7, −2.9) sys) × 10 −14 TeV −1 cm −2 s −1. A powerlaw fit to the differential HE γ-ray spectrum reveals a photon index of Γ = 2.24±0.14 stat ±0.03 sys and an integral flux between 200 MeV and 200 GeV of F(0.2 − 200 GeV) = (4.9 ± 1.0 stat ± 0.3 sys) × 10 −9 cm −2 s −1. No evidence for a spectral break or turnover is found over the dynamic range of both the LAT instrument and the H.E.S.S. experiment: a combined fit of a power law to the HE and VHE γ-ray data results in a differential photon index Γ = 2.34 ± 0.03 with a p-value of 30%. The γ-ray observations indicate that at least about 20% of the energy of the cosmic rays capable of producing hadronic interactions is channeled into pion production. The smooth alignment between the spectra in the HE and VHE γ-ray domain suggests that the same transport processes dominate in the entire energy range. Advection is most likely responsible for charged particle removal from the starburst nucleus from GeV to multiple TeV energies. In a hadronic scenario for the γ-ray production, the single overall power-law spectrum observed would therefore correspond to the mean energy spectrum produced by the ensemble of cosmic-ray sources in the starburst region.
HESS J0632+057 is the only gamma-ray binary known so far whose position in the sky allows observa... more HESS J0632+057 is the only gamma-ray binary known so far whose position in the sky allows observations with ground-based observatories both in the northern and southern hemispheres. Here we report on long-term observations of HESS J0632+057 conducted with the VERITAS and H.E.S.S. Cherenkov Telescopes and the X-ray Satellite Swift, spanning a time range from 2004 to
We report on the first completely simultaneous observation of a gamma-ray burst (GRB) using an ar... more We report on the first completely simultaneous observation of a gamma-ray burst (GRB) using an array of Imaging Atmospheric Cherenkov Telescopes, which is sensitive to photons in the very high energy (VHE) γ-ray range (100 GeV). On 2006 June 2, the Swift Burst Alert Telescope (BAT) registered an unusually soft γ-ray burst (GRB 060602B). The burst position was under observation using the High Energy Stereoscopic System (HESS) at the time the burst occurred. Data were taken before, during, and after the burst. A total of 5 hr of observations were obtained during the night of 2006 June 2-3, and five additional hours were obtained over the next three nights. No VHE γ-ray signal was found during the period covered by the HESS observations. The 99% confidence level flux upper limit (> 1 TeV) for the prompt phase (9 s) of GRB 060602B is 2.9 × 10 −9 erg cm −2 s −1. Due to the very soft BAT spectrum of the burst compared with other Swift GRBs and its proximity to the Galactic center, the burst is likely associated with a Galactic X-ray burster, although the possibility of it being a cosmological GRB cannot be ruled out. We discuss the implications of our flux limits in the context of these two bursting scenarios.
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Papers by F. Aharonian