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Abdalla, H., Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Andersson, T., . . . Yoshiike, S. (2018). A search for new supernova remnant shells in the Galactic plane with HESS. Astronomy and Astrophysics, 612, Article ID A8.
Open this publication in new window or tab >>A search for new supernova remnant shells in the Galactic plane with HESS
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, article id A8Article in journal (Refereed) Published
Abstract [en]

A search for new supernova remnants (SNRs) has been conducted using TeV gamma-ray data from the H.E.S.S. Galactic plane survey. As an identification criterion, shell morphologies that are characteristic for known resolved TeV SNRs have been used. Three new SNR candidates were identified in the H.E.S.S. data set with this method. Extensive multiwavelength searches for counterparts were conducted. A radio SNR candidate has been identified to be a counterpart to HESS J1534-571. The TeV source is therefore classified as a SNR. For the other two sources, HESS J1614-518 and HESS J1912 + 101, no identifying counterparts have been found, thus they remain SNR candidates for the time being. TeV-emitting SNRs are key objects in the context of identifying the accelerators of Galactic cosmic rays. The TeV emission of the relativistic particles in the new sources is examined in view of possible leptonic and hadronic emission scenarios, taking the current multiwavelength knowledge into account.

Keywords
astroparticle physics, ISM: supernova remnants, cosmic rays
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-76475 (URN)10.1051/0004-6361/201730737 (DOI)000429404700008 ()
Available from: 2018-07-10 Created: 2018-07-10 Last updated: 2018-09-07Bibliographically approved
Abdalla, H., Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Angüner, E. O., . . . Zywucka, N. (2018). A search for very high-energy flares from the microquasars GRS 1915+105, Circinus X-1, and V4641 Sgr using contemporaneous HESS and RXTE observations. Astronomy and Astrophysics, 612, Article ID A10.
Open this publication in new window or tab >>A search for very high-energy flares from the microquasars GRS 1915+105, Circinus X-1, and V4641 Sgr using contemporaneous HESS and RXTE observations
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, article id A10Article in journal (Refereed) Published
Abstract [en]

Context. Microquasars are potential gamma-ray emitters. Indications of transient episodes of gamma-ray emission were recently reported in at least two systems: Cyg X-1 and Cyg X-3. The identification of additional gamma-ray-emitting microquasars is required to better understand how gamma-ray emission can be produced in these systems. Aims. Theoretical models have predicted very high-energy (VHE) gamma-ray emission from microquasars during periods of transient outburst. Observations reported herein were undertaken with the objective of observing a broadband flaring event in the gamma-ray and X-ray bands. Methods. Contemporaneous observations of three microquasars, GRS 1915+105, Circinus X-1, and V4641 Sgr, were obtained using the High Energy Spectroscopic System (H.E.S.S.) telescope array and the Rossi X-ray Timing Explorer (RXTE) satellite. X-ray analyses for each microquasar were performed and VHE gamma-ray upper limits from contemporaneous H.E.S.S. observations were derived. Results. No significant gamma-ray signal has been detected in any of the three systems. The integral gamma-ray photon flux at the observational epochs is constrained to be I(>560 GeV) < 7.3 x 10(-13) cm(-2) S-1, I(>560 GeV) < 1.2 x 10-(12) cm s(-1), and I(>240 GeV) < 4.5 x 10(-12) cm(-2) s(-1) for GRS 1915+105, Circinus X-1, and V4641 Sgr, respectively. Conclusions. The gamma-ray upper limits obtained using H.E.S.S. are examined in the context of previous Cherenkov telescope observations of microquasars. The effect of intrinsic absorption is modelled for each target and found to have negligible impact on the flux of escaping gamma-rays. When combined with the X-ray behaviour observed using RXTE, the derived results indicate that if detectable VHE gamma-ray emission from microquasars is commonplace, then it is likely to be highly transient.

Keywords
gamma rays: general, X-rays: binaries, X-rays: individuals: GRS 1915+105, X-rays: individuals: Circinus X-1, X-rays: individuals: V4641 Sgr
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-76466 (URN)10.1051/0004-6361/201527773 (DOI)000429404700010 ()
Available from: 2018-07-10 Created: 2018-07-10 Last updated: 2018-08-28Bibliographically approved
Abdalla, H., Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Andersson, T., . . . Zywucka, N. (2018). Characterising the VHE diffuse emission in the central 200 parsecs of our Galaxy with HESS. Astronomy and Astrophysics, 612, Article ID A9.
Open this publication in new window or tab >>Characterising the VHE diffuse emission in the central 200 parsecs of our Galaxy with HESS
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, article id A9Article in journal (Refereed) Published
Abstract [en]

The diffuse very high-energy (VHE; > 100 GeV) gamma-ray emission observed in the central 200 pc of the Milky Way by H.E.S.S. was found to follow dense matter distribution in the central molecular zone (CMZ) up to a longitudinal distance of about 130 pc to the Galactic centre (GC), where the flux rapidly decreases. This was initially interpreted as the result of a burst-like injection of energetic particles 104 yr ago, but a recent more sensitive H.E.S.S. analysis revealed that the cosmic-ray (CR) density profile drops with the distance to the centre, making data compatible with a steady cosmic PeVatron at the GC. In this paper, we extend this analysis to obtain, for the first time, a detailed characterisation of the correlation with matter and to search for additional features and individual gamma-ray sources in the inner 200 pc. Taking advantage of 250 h of H.E.S.S. data and improved analysis techniques, we perform a detailed morphology study of the diffuse VHE emission observed from the GC ridge and reconstruct its total spectrum. To test the various contributions to the total gamma-ray emission, we used an iterative 2D maximum-likelihood approach that allows us to build a phenomenological model of the emission by summing a number of different spatial components. We show that the emission correlated with dense matter covers the full CMZ and that its flux is about half the total diffuse emission flux. We also detect some emission at higher latitude that is likely produced by hadronic collisions of CRs in less dense regions of the GC interstellar medium. We detect an additional emission component centred on the GC and extending over about 15 pc that is consistent with the existence of a strong CR density gradient and confirms the presence of a CR accelerator at the very centre of our Galaxy. We show that the spectrum of full ridge diffuse emission is compatible with that previously derived from the central regions, suggesting that a single population of particles fills the entire CMZ. Finally, we report the discovery of a VHE gamma-ray source near the GC radio arc and argue that it is produced by the pulsar wind nebula candidate G0.13-0.11.

Keywords
gamma rays: general, gamma rays: ISM, Galaxy: center, cosmic rays
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-76467 (URN)10.1051/0004-6361/201730824 (DOI)000429404700009 ()
Available from: 2018-07-10 Created: 2018-07-10 Last updated: 2018-08-28Bibliographically approved
Ahnen, M. L., Ansoldi, S., Antonelli, L. A., Arcaro, C., Babie, A., Banerjee, B., . . . Prokoph, H. (2018). Constraints on particle acceleration in SS433/W50 from MAGIC and HESS observations. Astronomy and Astrophysics, 612, Article ID A14.
Open this publication in new window or tab >>Constraints on particle acceleration in SS433/W50 from MAGIC and HESS observations
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, article id A14Article in journal (Refereed) Published
Abstract [en]

Context. The large jet kinetic power and non-thermal processes occurring in the microquasar SS 433 make this source a good candidate for a very high-energy (VHE) gamma-ray emitter. Gamma-ray fluxes above the sensitivity limits of current Cherenkov telescopes have been predicted for both the central X-ray binary system and the interaction regions of SS 433 jets with the surrounding W50 nebula. Non-thermal emission at lower energies has been previously reported, indicating that efficient particle acceleration is taking place in the system. Aims. We explore the capability of SS 433 to emit VHE gamma rays during periods in which the expected flux attenuation due to periodic eclipses (P-orb similar to 13.1 days) and precession of the circumstellar disk (P-pre similar to 162 days) periodically covering the central binary system is expected to be at its minimum. The eastern and western SS 433/W50 interaction regions are also examined using the whole data set available. We aim to constrain some theoretical models previously developed for this system with our observations. Methods. We made use of dedicated observations from the Major Atmospheric Gamma Imaging Cherenkov telescopes (MAGIC) and High Energy Spectroscopic System (H.E.S.S.) of SS 433 taken from 2006 to 2011. These observation were combined for the first time and accounted for a total effective observation time of 16.5 h, which were scheduled considering the expected phases of minimum absorption of the putative VHE emission. Gamma-ray attenuation does not affect the jet/medium interaction regions. In this case, the analysis of a larger data set amounting to similar to 40-80 h, depending on the region, was employed. Results. No evidence of VHE gamma-ray emission either from the central binary system or from the eastern/western interaction regions was found. Upper limits were computed for the combined data set. Differential fluxes from the central system are found to be less than or similar to 10(-12)-10(-13) TeV-1 cm(-2) s(-1) in an energy interval ranging from similar to few x 100 GeV to similar to few TeV. Integral flux limits down to similar to 10(-12)-10(-13) ph cm(-2) s(-1) and similar to 10(-13)-10(-14) ph cm(-2) s(-1) are obtained at 300 and 800 GeV, respectively. Our results are used to place constraints on the particle acceleration fraction at the inner jet regions and on the physics of the jet/medium interactions. Conclusions. Our findings suggest that the fraction of the jet kinetic power that is transferred to relativistic protons must be relatively small in SS 433, q(p) <= 2.5 x 10(-5), to explain the lack of TeV and neutrino emission from the central system. At the SS 433/W50 interface, the presence of magnetic fields greater than or similar to 10 mu G is derived assuming a synchrotron origin for the observed X-ray emission. This also implies the presence of high-energy electrons with E-e up to 50 TeV, preventing an efficient production of gamma-ray fluxes in these interaction regions.

Keywords
gamma rays: general, stars: black holes, X-rays: binaries, ISM: jets and outflows
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-76478 (URN)10.1051/0004-6361/201731169 (DOI)000429404700014 ()
Available from: 2018-07-10 Created: 2018-07-10 Last updated: 2018-12-11Bibliographically approved
Abdalla, H., Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Angüner, E. O., . . . Zywucka, N. (2018). Deeper HESS observations of Vela Junior (RX J0852.0-4622): Morphology studies and resolved spectroscopy. Astronomy and Astrophysics, 612, Article ID A7.
Open this publication in new window or tab >>Deeper HESS observations of Vela Junior (RX J0852.0-4622): Morphology studies and resolved spectroscopy
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, article id A7Article in journal (Refereed) Published
Abstract [en]

Aims. We study gamma-ray emission from the shell-type supernova remnant (SNR) RXJ0852.0-4622 to better characterize its spectral properties and its distribution over the SNR. Methods. The analysis of an extended High Energy Spectroscopic System (H.E.S.S.) data set at very high energies (E > 100 GeV) permits detailed studies, as well as spatially resolved spectroscopy, of the morphology and spectrum of the whole RXJ0852.0-4622 region. The H.E.S.S. data are combined with archival data from other wavebands and interpreted in the framework of leptonic and hadronic models. The joint Fermi-LAT-H.E.S.S. spectrum allows the direct determination of the spectral characteristics of the parent particle population in leptonic and hadronic scenarios using only GeV-TeV data. Results. An updated analysis of the H.E.S.S. data shows that the spectrum of the entire SNR connects smoothly to the high-energy spectrum measured by Fermi-LAT. The increased data set makes it possible to demonstrate that the H.E.S.S. spectrum deviates significantly from a power law and is well described by both a curved power law and a power law with an exponential cutoff at an energy of E-cut = (6.7 +/- 1.2(stat) +/- 1.2(syst)) TeV. The joint Fermi-LAT-H.E.S.S. spectrum allows the unambiguous identification of the spectral shape as a power law with an exponential cutoff. No significant evidence is found for a variation of the spectral parameters across the SNR, suggesting similar conditions of particle acceleration across the remnant. A simple modeling using one particle population to model the SNR emission demonstrates that both leptonic and hadronic emission scenarios remain plausible. It is also shown that at least a part of the shell emission is likely due to the presence of a pulsar wind nebula around PSR J0855-4644.

Keywords
astroparticle physics, gamma rays: general, acceleration of particles, cosmic rays, ISM: supernova remnants
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-76465 (URN)10.1051/0004-6361/201630002 (DOI)000429404700007 ()
Available from: 2018-07-10 Created: 2018-07-10 Last updated: 2018-08-28Bibliographically approved
Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Angüner, E. O., Backes, M., . . . Zywucka, N. (2018). Detailed spectral and morphological analysis of the shell type supernova remnant RCW 86. Astronomy and Astrophysics, 612, Article ID A4.
Open this publication in new window or tab >>Detailed spectral and morphological analysis of the shell type supernova remnant RCW 86
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, article id A4Article in journal (Refereed) Published
Abstract [en]

Aims. We aim for an understanding of the morphological and spectral properties of the supernova remnant RCW 86 and for insights into the production mechanism leading to the RCW 86 very high-energy gamma-ray emission. Methods. We analyzed High Energy Spectroscopic System (H.E.S.S.) data that had increased sensitivity compared to the observations presented in the RCW 86 H.E.S.S. discovery publication. Studies of the morphological correlation between the 0.5-1 keV X-ray band, the 2-5 keV X-ray band, radio, and gamma-ray emissions have been performed as well as broadband modeling of the spectral energy distribution with two different emission models. Results. We present the first conclusive evidence that the TeV gamma-ray emission region is shell-like based on our morphological studies. The comparison with 2-5 keV X-ray data reveals a correlation with the 0.4-50 TeV gamma-ray emission. The spectrum of RCW 86 is best described by a power law with an exponential cutoff at E-cut = (3.5 +/- 1.2(stat)) TeV and a spectral index of Gamma approximate to 1.6 +/- 0.2. A static leptonic one-zone model adequately describes the measured spectral energy distribution of RCW 86, with the resultant total kinetic energy of the electrons above 1 GeV being equivalent to similar to 0.1% of the initial kinetic energy of a Type Ia supernova explosion (10(51) erg). When using a hadronic model, a magnetic field of B approximate to 100 mu G is needed to represent the measured data. Although this is comparable to formerly published estimates, a standard E-2 spectrum for the proton distribution cannot describe the gamma-ray data. Instead, a spectral index of Gamma(p) approximate to 1.7 would be required, which implies that similar to 7 x 10(49)/n(cm-3) erg has been transferred into high-energy protons with the effective density n(cm-3) = n/1 cm(-3). This is about 10% of the kinetic energy of a typical Type Ia supernova under the assumption of a density of 1 cm(-3).

Keywords
astroparticle physics, gamma rays: general, ISM: supernova remnants, cosmic rays
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-76477 (URN)10.1051/0004-6361/201526545 (DOI)000429404700004 ()
Available from: 2018-07-10 Created: 2018-07-10 Last updated: 2018-08-28Bibliographically approved
Abdalla, H., Abramowski, A., Aharonian, F., Benkhali, F. A., Angüner, E. O., Arakawa, M., . . . Zywucka, N. (2018). Detection of variable VHE gamma-ray emission from the extra-galactic gamma-ray binary LMC P3 [Letter to the editor]. Astronomy and Astrophysics, 610, Article ID L17.
Open this publication in new window or tab >>Detection of variable VHE gamma-ray emission from the extra-galactic gamma-ray binary LMC P3