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Abdalla, H., Adam, R., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arakawa, M., . . . Bordas, P. (2020). HESS and Fermi-LAT observations of PSR B1259-63/LS 2883 during its 2014 and 2017 periastron passages. Astronomy and Astrophysics, 633, 1-14, Article ID A102.
Open this publication in new window or tab >>HESS and Fermi-LAT observations of PSR B1259-63/LS 2883 during its 2014 and 2017 periastron passages
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2020 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 633, p. 1-14, article id A102Article in journal (Refereed) Published
Abstract [en]

Context. PSR B1259-63/LS 2883 is a gamma-ray binary system consisting of a pulsar in an eccentric orbit around a bright Oe stellar-type companion star that features a dense circumstellar disc. The bright broad-band emission observed at phases close to periastron offers a unique opportunity to study particle acceleration and radiation processes in binary systems. Observations at gamma-ray energies constrain these processes through variability and spectral characterisation studies. Aims. The high- and very-high-energy (HE, VHE) gamma-ray emission from PSR B1259-63/LS 2883 around the times of its periastron passage are characterised, in particular, at the time of the HE gamma-ray flares reported to have occurred in 2011, 2014, and 2017. Short-term and average emission characteristics of PSR B1259-63/LS 2883 are determined. Super-orbital variability is searched for in order to investigate possible cycle-to-cycle VHE flux changes due to different properties of the companion star's circumstellar disc and/or the conditions under which the HE gamma-ray flares develop. Methods. Spectra and light curves were derived from observations conducted with the H.E.S.S-II array in 2014 and 2017. Phase-folded light curves are compared with the results obtained in 2004, 2007, and 2011. Fermi-LAT observations from 2010/11, 2014, and 2017 are analysed. Results. A local double-peak profile with asymmetric peaks in the VHE light curve is measured, with a flux minimum at the time of periastron t(p) and two peaks coinciding with the times at which the neutron star crosses the companion's circumstellar disc (similar to t(p) 16 d). A high VHE gamma-ray flux is also observed at the times of the HE gamma-ray flares (similar to t(p) + 30 d) and at phases before the first disc crossing (similar to t(p) - 35 d). The spectral energy range now extends to below 200 GeV and up to similar to 45 TeV. Conclusions. PSR B1259-63/LS 2883 displays periodic flux variability at VHE gamma-rays without clear signatures of super-orbital modulation in the time span covered by the monitoring of the source with the H.E.S.S. telescopes. This flux variability is most probably caused by the changing environmental conditions, particularly at times close to periastron passage at which the neutron star is thought to cross the circumstellar disc of the companion star twice. In contrast, the photon index remains unchanged within uncertainties for about 200 d around periastron. At HE gamma-rays, PSR B1259-63/LS 2883 has now been detected also before and after periastron, close to the disc crossing times. Repetitive flares with distinct variability patterns are detected in this energy range. Such outbursts are not observed at VHEs, although a relatively high emission level is measured. The spectra obtained in both energy regimes displays a similar slope, although a common physical origin either in terms of a related particle population, emission mechanism, or emitter location is ruled out.

Place, publisher, year, edition, pages
EDP Sciences, 2020
Keywords
astroparticle physics, radiation mechanisms: non-thermal, shock waves, gamma-rays: general, binaries: general, pulsars: general
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-91799 (URN)10.1051/0004-6361/201936621 (DOI)000508412900001 ()
Available from: 2020-02-04 Created: 2020-02-04 Last updated: 2020-02-04Bibliographically approved
Abdalla, H., Adam, R., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arakawa, M., . . . Smith, P. S. (2020). HESS detection of very high-energy gamma-ray emission from the quasar PKS 0736+017. Astronomy and Astrophysics, 633, 1-11, Article ID A162.
Open this publication in new window or tab >>HESS detection of very high-energy gamma-ray emission from the quasar PKS 0736+017
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2020 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 633, p. 1-11, article id A162Article in journal (Refereed) Published
Abstract [en]

Context. Flat-spectrum radio-quasars (FSRQs) are rarely detected at very high energies (E& x2004;>=& x2004;100 GeV) due to their low-frequency-peaked spectral energy distributions. At present, only six FSRQs are known to emit very high-energy (VHE) photons, representing only 7% of the VHE extragalactic catalog, which is largely dominated by high-frequency-peaked BL Lacertae objects. Aims. Following the detection of MeV-GeV gamma-ray flaring activity from the FSRQ PKS 0736+017 (z& x2004;=& x2004;0.189) with Fermi-LAT, the H.E.S.S. array of Cherenkov telescopes triggered target-of-opportunity (ToO) observations on February 18, 2015, with the goal of studying the gamma-ray emission in the VHE band. Methods. H.E.S.S. ToO observations were carried out during the nights of February 18, 19, 21, and 24, 2015. Together with Fermi-LAT, the multi-wavelength coverage of the flare includes Swift observations in soft X-ray and optical-UV bands, and optical monitoring (photometry and spectro-polarimetry) by the Steward Observatory, and the ATOM, the KAIT, and the ASAS-SN telescopes. Results. VHE emission from PKS 0736+017 was detected with H.E.S.S. only during the night of February 19, 2015. Fermi-LAT data indicate the presence of a gamma-ray flare, peaking at the time of the H.E.S.S. detection, with a flux doubling timescale of around six hours. The gamma-ray flare was accompanied by at least a 1 mag brightening of the non-thermal optical continuum. No simultaneous observations at longer wavelengths are available for the night of the H.E.S.S. detection. The gamma-ray observations with H.E.S.S. and Fermi-LAT are used to put constraints on the location of the gamma-ray emitting region during the flare: it is constrained to be just outside the radius of the broad-line region r(BLR) with a bulk Lorentz factor Gamma& x2004;similar or equal to& x2004;20, or at the level of the radius of the dusty torus r(torus) with Gamma& x2004;similar or equal to& x2004;60. Conclusions. PKS 0736+017 is the seventh FSRQ known to emit VHE photons, and at z& x2004;=& x2004;0.189 is the nearest so far. The location of the gamma-ray emitting region during the flare can be tightly constrained thanks to opacity, variability, and collimation arguments.

Place, publisher, year, edition, pages
EDP Sciences, 2020
Keywords
gamma rays, general, astroparticle physics, relativistic processes, quasars, individual, PKS 0736+017
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-92321 (URN)10.1051/0004-6361/201935906 (DOI)000509938700005 ()
Available from: 2020-02-21 Created: 2020-02-21 Last updated: 2020-02-21Bibliographically approved
Abdalla, H., Aharonian, F., Ait Benkhali, F., Anguner, E. O., Arakawa, M., Arcaro, C., . . . Zywucka, N. (2020). Resolving the Crab pulsar wind nebula at teraelectronvolt energies. Nature Astronomy, 4(2), 167-173
Open this publication in new window or tab >>Resolving the Crab pulsar wind nebula at teraelectronvolt energies
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2020 (English)In: Nature Astronomy, E-ISSN 2397-3366, Vol. 4, no 2, p. 167-173Article in journal (Refereed) Published
Abstract [en]

An angular extension at gamma-ray energies of 52 arcseconds is detected for the Crab nebula, revealing the emission region of the highest-energy gamma rays; simulations of the electromagnetic emission provide a non-trivial test of our understanding of particle acceleration in the Crab nebula. The Crab nebula is one of the most-studied cosmic particle accelerators, shining brightly across the entire electromagnetic spectrum up to very-high-energy gamma rays(1,2). It is known from observations in the radio to gamma-ray part of the spectrum that the nebula is powered by a pulsar, which converts most of its rotational energy losses into a highly relativistic outflow. This outflow powers a pulsar wind nebula, a region of up to ten light-years across, filled with relativistic electrons and positrons. These particles emit synchrotron photons in the ambient magnetic field and produce very-high-energy gamma rays by Compton up-scattering of ambient low-energy photons. Although the synchrotron morphology of the nebula is well established, it has not been known from which region the very-high-energy gamma rays are emitted(3-8). Here we report that the Crab nebula has an angular extension at gamma-ray energies of 52 arcseconds (assuming a Gaussian source width), much larger than at X-ray energies. This result closes a gap in the multi-wavelength coverage of the nebula, revealing the emission region of the highest-energy gamma rays. These gamma rays enable us to probe a previously inaccessible electron and positron energy range. We find that simulations of the electromagnetic emission reproduce our measurement, providing a non-trivial test of our understanding of particle acceleration in the Crab nebula.

Place, publisher, year, edition, pages
Nature Publishing Group, 2020
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-92466 (URN)10.1038/s41550-019-0910-0 (DOI)000512983200019 ()
Available from: 2020-03-02 Created: 2020-03-02 Last updated: 2020-03-02Bibliographically approved
Abdalla, H., Adam, R., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arakawa, M., . . . Roberts, O. J. (2019). A very-high-energy component deep in the gamma-ray burst afterglow. Nature, 575(7783), 464-467
Open this publication in new window or tab >>A very-high-energy component deep in the gamma-ray burst afterglow
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2019 (English)In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 575, no 7783, p. 464-467Article in journal (Refereed) Published
Abstract [en]

Gamma-ray bursts (GRBs) are brief flashes of gamma-rays and are considered to be the most energetic explosive phenomena in the Universe(1). The emission from GRBs comprises a short (typically tens of seconds) and bright prompt emission, followed by a much longer afterglow phase. During the afterglow phase, the shocked outflow-produced by the interaction between the ejected matter and the circumburst medium-slows down, and a gradual decrease in brightness is observed(2). GRBs typically emit most of their energy via.-rays with energies in the kiloelectronvolt-to-megaelectronvolt range, but a few photons with energies of tens of gigaelectronvolts have been detected by space-based instruments(3). However, the origins of such high-energy (above one gigaelectronvolt) photons and the presence of very-high-energy (more than 100 gigaelectronvolts) emission have remained elusive(4). Here we report observations of very-high-energy emission in the bright GRB 180720B deep in the GRB afterglow-ten hours after the end of the prompt emission phase, when the X-ray flux had already decayed by four orders of magnitude. Two possible explanations exist for the observed radiation: inverse Compton emission and synchrotron emission of ultrarelativistic electrons. Our observations show that the energy fluxes in the X-ray and gamma-ray range and their photon indices remain comparable to each other throughout the afterglow. This discovery places distinct constraints on the GRB environment for both emission mechanisms, with the inverse Compton explanation alleviating the particle energy requirements for the emission observed at late times. The late timing of this detection has consequences for the future observations of GRBs at the highest energies.

Place, publisher, year, edition, pages
Nature Publishing Group, 2019
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-90546 (URN)10.1038/s41586-019-1743-9 (DOI)000498812200044 ()31748724 (PubMedID)
Available from: 2019-12-13 Created: 2019-12-13 Last updated: 2019-12-13Bibliographically approved
Abdalla, H., Adam, R., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arakawa, M., . . . Meyer, M. (2019). Constraints on the emission region of 3C 279 during strong flares in 2014 and 2015 through VHE gamma-ray observations with HESS. Astronomy and Astrophysics, 627, 1-19, Article ID A159.
Open this publication in new window or tab >>Constraints on the emission region of 3C 279 during strong flares in 2014 and 2015 through VHE gamma-ray observations with HESS
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 627, p. 1-19, article id A159Article in journal (Refereed) Published
Abstract [en]

The flat spectrum radio quasar 3C 279 is known to exhibit pronounced variability in the high-energy (100MeV < E < 100 GeV) gamma-ray band, which is continuously monitored with Fermi-LAT. During two periods of high activity in April 2014 and June 2015 target-of-opportunity observations were undertaken with the High Energy Stereoscopic System (H.E.S.S.) in the very-high-energy (VHE, E > 100 GeV) gamma-ray domain. While the observation in 2014 provides an upper limit, the observation in 2015 results in a signal with 8 : 7 sigma significance above an energy threshold of 66 GeV. No VHE variability was detected during the 2015 observations. The VHE photon spectrum is soft and described by a power-law index of 4.2 +/- 0.3. The H.E.S.S. data along with a detailed and contemporaneous multiwavelength data set provide constraints on the physical parameters of the emission region. The minimum distance of the emission region from the central black hole was estimated using two plausible geometries of the broad-line region and three potential intrinsic spectra. The emission region is confidently placed at r greater than or similar to 1 : 7 X 1017 cm from the black hole, that is beyond the assumed distance of the broad-line region. Time-dependent leptonic and lepto-hadronic one-zone models were used to describe the evolution of the 2015 flare. Neither model can fully reproduce the observations, despite testing various parameter sets. Furthermore, the H.E.S.S. data were used to derive constraints on Lorentz invariance violation given the large redshift of 3C 279.

Place, publisher, year, edition, pages
EDP Sciences, 2019
Keywords
radiation mechanisms: non-thermal, quasars: individual: 3C 279, galaxies: active, relativistic processes
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-87058 (URN)10.1051/0004-6361/201935704 (DOI)000475554500004 ()2-s2.0-85069435438 (Scopus ID)
Available from: 2019-08-01 Created: 2019-08-01 Last updated: 2019-08-29Bibliographically approved
Abdalla, H., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arakawa, M., Arcaro, C., . . . Zywucka, N. (2019). HESS and Suzaku observations of the Vela X pulsar wind nebula. Astronomy and Astrophysics, 627, 1-16, Article ID A100.
Open this publication in new window or tab >>HESS and Suzaku observations of the Vela X pulsar wind nebula
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 627, p. 1-16, article id A100Article in journal (Refereed) Published
Abstract [en]

Context. Pulsar wind nebulae (PWNe) represent the most prominent population of Galactic very-high-energy gamma-ray sources and are thought to be an efficient source of leptonic cosmic rays. Vela X is a nearby middle-aged PWN, which shows bright X-ray and TeV gamma-ray emission towards an elongated structure called the cocoon. Aims. Since TeV emission is likely inverse-Compton emission of electrons, predominantly from interactions with the cosmic microwave background, while X-ray emission is synchrotron radiation of the same electrons, we aim to derive the properties of the relativistic particles and of magnetic fields with minimal modelling. Methods. We used data from the Suzaku XIS to derive the spectra from three compact regions in Vela X covering distances from 0.3 to 4 pc from the pulsar along the cocoon. We obtained gamma-ray spectra of the same regions from H.E.S.S. observations and fitted a radiative model to the multi-wavelength spectra. Results. The TeV electron spectra and magnetic field strengths are consistent within the uncertainties for the three regions, with energy densities of the order 10(-12) erg cm(-3). The data indicate the presence of a cutoff in the electron spectrum at energies of similar to 100 TeV and a magnetic field strength of similar to 6 mu G. Constraints on the presence of turbulent magnetic fields are weak. Conclusions. The pressure of TeV electrons and magnetic fields in the cocoon is dynamically negligible, requiring the presence of another dominant pressure component to balance the pulsar wind at the termination shock. Sub-TeV electrons cannot completely account for the missing pressure, which may be provided either by relativistic ions or from mixing of the ejecta with the pulsar wind. The electron spectra are consistent with expectations from transport scenarios dominated either by advection via the reverse shock or by diffusion, but for the latter the role of radiative losses near the termination shock needs to be further investigated in the light of the measured cutoff energies. Constraints on turbulent magnetic fields and the shape of the electron cutoff can be improved by spectral measurements in the energy range greater than or similar to 10 keV.

Place, publisher, year, edition, pages
EDP Sciences, 2019
Keywords
stars: winds, outflows, gamma rays: stars, radiation mechanisms: non-thermal, acceleration of particles, pulsars: individual: PSR B0833-45
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-86983 (URN)10.1051/0004-6361/201935458 (DOI)000474334300001 ()2-s2.0-85069766171 (Scopus ID)
Available from: 2019-07-25 Created: 2019-07-25 Last updated: 2019-08-29Bibliographically approved
Abdalla, H., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arakawa, M., Arcaro, C., . . . Zywucka, N. (2019). HESS observations of the flaring gravitationally lensed galaxy PKS 1830-211. Monthly notices of the Royal Astronomical Society, 486(3), 3886-3891
Open this publication in new window or tab >>HESS observations of the flaring gravitationally lensed galaxy PKS 1830-211
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2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 486, no 3, p. 3886-3891Article in journal (Refereed) Published
Abstract [en]

PKS 1830-211 is a known macrolensed quasar located at a redshift of z = 2.5. Its highenergy gamma-ray emission has been detected with the Fermi-Large Area Telescope (LAT) instrument and evidence for lensing was obtained by several authors from its high-energy data. Observations of PKS 1830-211 were taken with the High Energy Stereoscopic System (H.E.S.S.) array of Imaging Atmospheric Cherenkov Telescopes in 2014 August, following a flare alert by the Fermi-LAT Collaboration. The H.E.S.S observations were aimed at detecting a gamma-ray flare delayed by 20-27 d from the alert flare, as expected from observations at other wavelengths. More than 12 h of good-quality data were taken with an analysis threshold of similar to 67 GeV. The significance of a potential signal is computed as a function of the date and the average significance over the whole period. Data are compared to simultaneous observations by Fermi-LAT. No photon excess or significant signal is detected. An upper limit on PKS 1830-211 flux above 67 GeV is computed and compared to the extrapolation of the Fermi-LAT flare spectrum.

Place, publisher, year, edition, pages
Oxford University Press, 2019
Keywords
gravitational lensing: strong, diffuse radiation, gamma-rays: galaxies
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-86986 (URN)10.1093/mnras/stz1031 (DOI)000474903500069 ()
Available from: 2019-07-25 Created: 2019-07-25 Last updated: 2019-07-25Bibliographically approved
Acharyya, A., Agudo, I., Anguner, E. O., Alfaro, R., Alfaro, J., Alispach, C., . . . Punch, M. (2019). Monte Carlo studies for the optimisation of the Cherenkov Telescope Array layout. Astroparticle physics, 111, 35-53
Open this publication in new window or tab >>Monte Carlo studies for the optimisation of the Cherenkov Telescope Array layout