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  • 1.
    Axelsson, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Jackson, Miranda
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Lundman, Christoffer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Moretti, Elena
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Nymark, Tanja
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Pe'Er, A.
    et al.,
    GRB110721A: An extreme peak energy and signatures of the photosphere2012In: The Astrophysical Journal. Letters, ISSN 2041-8205, Vol. 757, no 2, p. L31-Article in journal (Refereed)
    Abstract [en]

    GRB110721A was observed by the Fermi Gamma-ray Space Telescope using its two instruments the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM). The burst consisted of one major emission episode which lasted for ~24.5 seconds (in the GBM) and had a peak flux of 5.7\pm0.2 x 10^{-5} erg/s/cm^2. The time-resolved emission spectrum is best modeled with a combination of a Band function and a blackbody spectrum. The peak energy of the Band component was initially 15\pm2 MeV, which is the highest value ever detected in a GRB. This measurement was made possible by combining GBM/BGO data with LAT Low Energy Events to achieve continuous 10--100 MeV coverage. The peak energy later decreased as a power law in time with an index of -1.89\pm0.10. The temperature of the blackbody component also decreased, starting from ~80 keV, and the decay showed a significant break after ~2 seconds. The spectrum provides strong constraints on the standard synchrotron model, indicating that alternative mechanisms may give rise to the emission at these energies.

  • 2. Iyyani, S
    et al.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Axelsson, Magnus
    KTH, School of Engineering Sciences (SCI), Physics.
    Burgess, J. M.
    Guiriec, S.
    Larsson, Josefin
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Lundman, Christoffer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Moretti, Elena
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    McGlynn, S.
    Nymark, Tanja
    KTH, School of Engineering Sciences (SCI), Physics.
    Rosquist, K.
    Variable jet properties in GRB 110721A: time resolved observations of the jet photosphere2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 433, no 4, p. 2739-2748Article in journal (Refereed)
    Abstract [en]

    Fermi Gamma-ray Space Telescope observations of GRB 110721A have revealed two emission components from the relativistic jet: emission from the photosphere, peaking at similar to 100 keV, and a non-thermal component, which peaks at similar to 1000 keV. We use the photospheric component to calculate the properties of the relativistic outflow. We find a strong evolution in the flow properties: the Lorentz factor decreases with time during the bursts from G similar to 1000 to similar to 150 (assuming a redshift z = 2; the values are only weakly dependent on unknown efficiency parameters). Such a decrease is contrary to the expectations from the internal shocks and the isolated magnetar birth models. Moreover, the position of the flow nozzle measured from the central engine, r(0), increases by more than two orders of magnitude. Assuming a moderately magnetized outflow we estimate that r(0) varies from 10(6) to similar to 10(9) cm during the burst. We suggest that the maximal value reflects the size of the progenitor core. Finally, we show that these jet properties naturally explain the observed broken power-law decay of the temperature which has been reported as a characteristic for gamma-ray burst pulses.

  • 3.
    Iyyani, Shabnam
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Ahlgren, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Burgess, J. Michael
    Larsson, Josefin
    Pe'er, A.
    Lundman, Christopher
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Axelsson, M.
    McGlynn, S.
    Extremely narrow spectrum of GRB110920A: further evidence for localized, subphotospheric dissipation2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 450, no 2, p. 1651-1663Article in journal (Refereed)
    Abstract [en]

    Much evidence points towards that the photosphere in the relativistic outflow in GRBs plays an important role in shaping the observed MeV spectrum. However, it is unclear whether the spectrum is fully produced by the photosphere or whether a substantial part of the spectrum is added by processes far above the photosphere. Here we make a detailed study of the. ray emission from single pulse GRB110920A which has a spectrum that becomes extremely narrow towards the end of the burst. We show that the emission can be interpreted as Comptonization of thermal photons by cold electrons in an unmagnetized outflow at an optical depth of tau similar to 20. The electrons receive their energy by a local dissipation occurring close to the saturation radius. The main spectral component of GRB110920A and its evolution is thus, in this interpretation, fully explained by the emission from the photosphere including localized dissipation at high optical depths.

  • 4. Larsson, J.
    et al.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Lundman, Christoffer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    McGlynn, Sinead
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Larsson, Stefan
    Ohno, M.
    Yamaoka, K.
    Spectral components in the bright, long GRB 061007: properties of the photosphere and the nature of the outflow2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 414, no 3, p. 2642-2649Article in journal (Refereed)
    Abstract [en]

    We present a time-resolved spectral analysis of the bright, long GRB 061007 (z = 1.261) using Swift and Suzaku data. We find that the prompt emission of the burst can be equally well explained by a photospheric component together with a power law as by a Band function, and we explore the implications of the former model. The photospheric component, which we model with a multicolour blackbody, dominates the spectra and has a very stable shape throughout the burst. This component provides a natural explanation for the hardness-intensity correlation seen within the burst and also allows us to estimate the bulk Lorentz factor and the radius of the photosphere. The power-law component dominates the fit at high energies and has a nearly constant slope of -1.5. We discuss the possibility that this component is of the same origin as the high-energy power laws recently observed in some Fermi bursts.

  • 5.
    Lundman, Christoffer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Photospheric emission from structured, relativistic jets: applications to gamma-ray burst spectra and polarization2013Doctoral thesis, monograph (Other academic)
    Abstract [en]

    The radiative mechanism responsible for the prompt gamma-ray burst (GRB) emission remains elusive. For the last decade, optically thin synchrotron emission from shocks internal to the GRB jet appeared to be the most plausible explanation. However, the synchrotron interpretation is incompatible with a significant fraction of GRB observations, highlighting the need for new ideas. In this thesis, it is shown that the narrow, dominating component of the prompt emission from the bright GRB090902B is initially consistent only with emission released at the optically thick jet photosphere. However, this emission component then broadens in time into a more typical GRB spectrum, which calls for an explanation. In this thesis, a previously unconsidered way of broadening the spectrum of photospheric emission, based on considerations of the lateral jet structure, is presented and explored. Expressions for the spectral features, as well as polarization properties, of the photospheric emission observed from structured, relativistic jets are derived analytically under simplifying assumptions on the radiative transfer close to the photosphere. The full, polarized radiative transfer is solved through Monte Carlo simulations, using a code which has been constructed for this unique purpose. It is shown that the typical observed GRB spectrum can be obtained from the photosphere, without the need for additional, commonly assumed, physical processes (e.g. energy dissipation, particle acceleration, or additional radiative processes). Furthermore, contrary to common expectations, it is found that the observed photospheric emission can be highly linearly polarized (up to $\sim 40 \, \%$). In particular, it is shown that a shift of $\pi/2$ of the angle of polarization is the only shift allowed by the proposed model, consistent with the only measurement preformed to date. A number of ways to test the theory is proposed, mainly involving simultaneous spectral and polarization measurements. The simplest measurement, which tests not only the proposed theory but also common assumptions on the jet structure, involves only two consecutive measurements of the angle of polarization during the prompt emission.

  • 6.
    Lundman, Christoffer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Photospheric emission in gamma-ray bursts2012Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis considers emission from gamma-ray bursts (GRBs), the most powerful explosions known in the Universe. Most GRBs are likely associated with the final stages of stellar evolution, where the core of a massive star collapses, and gives birth to a highly compact object such as a neutron star or black hole. The wide energy range of the Fermi Gamma-ray Space Telescope allows for unprecedented studies of GRBs. Fermi data is used to study the emission released at the photosphere of the relativistic outow ejected from the central compact object. The thesis present studies of two of the strongest GRBs ever detected; GRB 090902B (Papers I, II) and GRB 110721A (Paper III). Photospheric emission is identied and its properties are studied for both GRBs. For the first time, observational evidence is found for spectral broadening of photospheric emission. Motivated by these results, possible mechanisms to make the emission from the photosphere appear broader than the Planck spectrum are examined. Two separate theoretical explanations are presented. Apart from the possibility of energy dissipation below the photosphere (Paper II), geometrical effects in outflows with angle dependent properties is shown to significantly broaden the photospheric spectrum (Paper IV). Most importantly, the observed spectrum below the peak energy may become significantly softer inthe latter case. This thesis thus concludes that photospheric emission in GRBs may be more common than previously thought. This is because the emission spectrum from the jet photosphere does not necessarily need to be a Planck function. On the contrary it is shown that broader and/or multicomponent spectra naturally arise, consistent with what is generally observed. In particular, the thesis presents a new mechanism for spectral broadening due to geometrical effects, which must be taken into consideration in the study of GRB emission.

  • 7.
    Lundman, Christoffer
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Pe'er, A.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    A theory of photospheric emission from relativistic, collimated outflows2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 428, no 3, p. 2430-2442Article in journal (Refereed)
    Abstract [en]

    Relativistic outflows in the form of jets are common in many astrophysical objects. By their very nature, jets have angle-dependent velocity profiles, Γ=Γ(r, Θ, φ), where Γ is the outflow Lorentz factor. In this work we consider photospheric emission from non-dissipative jets with various Lorentz factor profiles, of the approximate form Γ ≈ Γ0/[(Θ/Θj)p + 1], where Θj is the characteristic jet opening angle. In collimated jets, the observed spectrum depends on the viewing angle, Θv. We show that for narrow jets (ΘjΓ0 < few), the obtained low-energy photon index is α ≈-1 (dN/dE / Eα), independent of viewing angle, and weakly dependent on the Lorentz factor gradient (p). A similar result is obtained for wider jets observed at Θv ≈ Θj. This result is surprisingly similar to the average low-energy photon index seen in gamma-ray bursts. For wide jets (ΘjΓ0 > few) observed at Θv > Θj, a multicolour blackbody spectrum is obtained. We discuss the consequences of this theory on our understanding of the prompt emission in gamma-ray bursts.

  • 8.
    Lundman, Christoffer
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Pe'er, A.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Polarization properties of photospheric emission from relativistic, collimated outflows2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 440, no 4, p. 3292-3308Article in journal (Refereed)
    Abstract [en]

    We consider the polarization properties of photospheric emission originating in jets consisting of a highly relativistic core of opening angle θj and Lorentz factor Γ0, and a surrounding shear layer where the Lorentz factor is decreasing as a power law of index p with angle from the jet axis. We find significant degrees of linear polarization for observers located at viewing angles θv ≳ θj. In particular, the polarization degree of emission from narrow jets (θj ≈ 1/Γ0) with steep Lorentz factor gradients (p ≳ 4) reaches ∼40 per cent. The angle of polarization may shift by Π/2 for time-variable jets. The spectrum below the thermal peak of the polarized emission appears non-thermal due to aberration of light, without the need for additional radiative processes or energy dissipation. Furthermore, above the thermal peak a power law of photons forms due to Comptonization of photons that repeatedly scatter between regions of different Lorentz factor before escaping. We show that polarization degrees of a few tens of per cent and broken power-law spectra are natural in the context of photospheric emission from structured jets. Applying the model to gamma-ray bursts, we discuss expected correlations between the spectral shape and the polarization degree of the prompt emission.

  • 9.
    Lundman, Christoffer
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Pe'er, Asaf
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    A theory of photospheric emission from collimated outflowsManuscript (preprint) (Other academic)
    Abstract [en]

    Relativistic outflows in the form of jets are common in many astrophysical objects. By their very nature, jets have angle dependent velocity profiles, Gamma = Gamma(r, theta, phi), where Gamma is the outflow Lorentz factor. In this work we consider photospheric emission from non-dissipative jets with various Lorentz factor profiles, of the approximate form Gamma \approx Gamma_0/[(theta/theta_j)^p + 1], were theta_j is the characteristic jet opening angle. In collimated jets, the observed spectrum depends on the viewing angle, theta_v. We show that for narrow jets (theta_j Gamma_0 \lesssim few), the obtained low energy photon index is alpha \approx -1 (dN/dE \propto E^alpha), independent of viewing angle, and weakly dependent on the Lorentz factor gradient (p). A similar result is obtained for wider jets observed at theta_v \approx theta_j. This result is surprisingly similar to the average low energy photon index seen in gamma-ray bursts. For wide jets (theta_j Gamma_0 \gtrsim few) observed at theta_v \ll theta_j, a multicolor blackbody spectrum is obtained. We discuss the consequences of this theory on our understanding of the prompt emission in gamma-ray bursts.

  • 10.
    Lundman, Christoffer
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Columbia Univ.
    Vurm, Indrek
    Columbia Univ, Phys Dept, 538 West 120th St, New York, NY 10027 USA.;Columbia Univ, Columbia Astrophys Lab, 538 West 120th St, New York, NY 10027 USA.;Tartu Observ, EE-61602 Toravere, Tartumaa, Estonia..
    Beloborodov, Andrei M.
    Columbia Univ, Phys Dept, 538 West 120th St, New York, NY 10027 USA.;Columbia Univ, Columbia Astrophys Lab, 538 West 120th St, New York, NY 10027 USA..
    Polarization of Gamma-Ray Bursts in the Dissipative Photosphere Model2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 856, no 2, article id 145Article in journal (Refereed)
    Abstract [en]

    The MeV spectral peak of gamma-ray bursts (GRBs) is best explained as photospheric emission from a dissipative relativistic jet. The observed non-blackbody spectrum shows that sub-photospheric dissipation involves both thermal plasma heating and injection of nonthermal particles, which quickly cool through inverse Compton scattering and emission of synchrotron radiation. Synchrotron photons emitted around and above the photosphere are predicted to dominate the low-energy part of the GRB spectrum, starting from roughly a decade in energy below the MeV peak. We show that this leads to a unique polarization signature: a rise in GRB polarization toward lower energies. We compute the polarization degree of GRB radiation as a function of photon energy for a generic jet model, and show the predictions for GRBs 990123, 090902B, and 110721A. The expected polarization is significant in the X-ray band, in particular for bursts similar to GRB 090902B. The model predicts that radiation in the MeV peak (and at higher energies) is unpolarized as long as the jet is approximately uniform on angular scales delta theta greater than or similar to Gamma(-1) where G is the bulk Lorentz factor of the jet.

  • 11.
    McGlynn, Sinéad
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Lundman, Christoffer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Pe'er, A.
    Thermal and non-thermal emission in gamma-ray bursts: GRB090902B as a case study2010In: Proceedings of Science, 2010Conference paper (Refereed)
    Abstract [en]

    Photospheric (thermal) emission is inherent to the gamma-ray burst (GRB) "fireball" model. Inclusion of this component in the analysis of the GRB prompt emission explains some of the prompt GRB spectra seen by the Fermi satellite over its entire energy band. The sub-MeV peak corresponds to multi-color black body emission, and the high energy tail results from similar contributions of synchrotron emission, synchrotron self Compton (SSC) and Comptonization of the thermal photons by energetic electrons originating after dissipation of the kinetic energy above the photosphere. We study the connection between the thermal and non-thermal parts of the spectrum, and deduce the values of the free model parameters from the data. We demonstrate our analysis method on GRB 090902B.

  • 12.
    Nymark, Tanja
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Axelsson, Magnus
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Lundman, Christoffer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Moretti, Elena
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Pe’er, A.
    Subphotospheric heating in GRBs: analysis and modeling of GRB090902B as observed by Fermi2011In: 2011 Fermi Symposium proceedings: eConf C110509, 2011Conference paper (Other academic)
    Abstract [en]

    We analyze the spectral evolution of GRB 090902B and show that subphotospheric dissipation can explain both the spectra and the spectral evolution. The emission from a GRB photosphere can give rise to a variety of spectral shapes. The spectrum can have a shape close to that of a Planck function (as is observed during the first half of GRB090902B) or be broadened, resembling a typical Band function (as is observed during the second half of GRB090902B). The shape mainly depends on the strength and location of the dissipation in the jet, the ratio of the energy densities of thermal photons and of the electrons at the dissipation site, as well as on the strength of the magnetic field. We further discuss numerical models of the dissipation and relate these to the observed spectra.

  • 13.
    Pearce, Mark
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Eliasson, Linda
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Iyer, Nirmal
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Kiss, Mózsi
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Kushwah, Rakhee
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Larsson, Josefin
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Lundman, Christoffer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Mikhalev, Victor
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Stana, Theodor-Adrian
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Takahashi, H.
    Xie, Fei
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Science prospects for SPHiNX – A small satellite GRB polarimetry mission2019In: Astroparticle physics, ISSN 0927-6505, E-ISSN 1873-2852, Vol. 104, p. 54-63Article in journal (Refereed)
    Abstract [en]

    Gamma-ray bursts (GRBs) are exceptionally bright electromagnetic events occurring daily on the sky. The prompt emission is dominated by X-/γ-rays. Since their discovery over 50 years ago, GRBs are primarily studied through spectral and temporal measurements. The properties of the emission jets and underlying processes are not well understood. A promising way forward is the development of missions capable of characterising the linear polarisation of the high-energy emission. For this reason, the SPHiNX mission has been developed for a small-satellite platform. The polarisation properties of incident high-energy radiation (50–600 keV) are determined by reconstructing Compton scattering interactions in a segmented array of plastic and Gd3Al2Ga3O12(Ce) (GAGG(Ce)) scintillators. During a two-year mission, ∼ 200 GRBs will be observed, with ∼ 50 yielding measurements where the polarisation fraction is determined with a relative error ≤ 10%. This is a significant improvement compared to contemporary missions. This performance, combined with the ability to reconstruct GRB localisation and spectral properties, will allow discrimination between leading classes of emission models.

  • 14.
    Pearce, Mark
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden..
    Eliasson, Linda
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden..
    Iyer, Nirmal
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden..
    Kiss, Mózsi
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden..
    Kushwah, Rakhee
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden..
    Larsson, Josefin
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden..
    Lundman, Christoffer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden..
    Mikhalev, Victor
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden..
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden..
    Stana, Theodor-Adrian
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Takahashi, H.
    Hiroshima Univ, Dept Phys Sci, Hiroshima 7398526, Japan..
    Xie, Fei
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden..
    Science prospects for SPHiNX - A small satellite GRB polarimetry mission2019In: Astroparticle physics, ISSN 0927-6505, E-ISSN 1873-2852, Vol. 104, p. 54-63Article in journal (Refereed)
    Abstract [en]

    Gamma-ray bursts (GRBs) are exceptionally bright electromagnetic events occurring daily on the sky. The prompt emission is dominated by X-/gamma-rays. Since their discovery over 50 years ago, GRBs are primarily studied through spectral and temporal measurements. The properties of the emission jets and underlying processes are not well understood. A promising way forward is the development of missions capable of characterising the linear polarisation of the high-energy emission. For this reason, the SPHiNX mission has been developed for a small-satellite platform. The polarisation properties of incident high-energy radiation (50-600 keV) are determined by reconstructing Compton scattering interactions in a segmented array of plastic and Gd3Al2Ga3O12(Ce) (GAGG(Ce)) scintillators. During a two-year mission, similar to 200 GRBs will be observed, with similar to 50 yielding measurements where the polarisation fraction is determined with a relative error <= 10%. This is a significant improvement compared to contemporary missions. This performance, combined with the ability to reconstruct GRB localisation and spectral properties, will allow discrimination between leading classes of emission models.

  • 15.
    Ryde, Felix
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Axelsson, M.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Zhang, B. B.
    McGlynn, S.
    Pe'er, A.
    Lundman, Christoffer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Larsson, S.
    Battelino, Milan
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Zhang, B.
    Bissaldi, E.
    Bregeon, J.
    Briggs, M. S.
    Chiang, J.
    de Palma, F.
    Guiriec, S.
    Larsson, J.
    Longo, F.
    McBreen, S.
    Omodei, N.
    Petrosian, V.
    Preece, R.
    van der Horst, A. J.
    IDENTIFICATION AND PROPERTIES OF THE PHOTOSPHERIC EMISSION IN GRB090902B2010In: ASTROPHYSICAL JOURNAL LETTERS, ISSN 2041-8213, Vol. 709, no 2, p. L172-L177Article in journal (Refereed)
    Abstract [en]

    The Fermi Gamma-ray Space Telescope observed the bright and long GRB090902B, lying at a redshift of z = 1.822. Together the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) cover the spectral range from 8 keV to >300 GeV. Here we show that the prompt burst spectrum is consistent with emission from the jet photosphere combined with nonthermal emission described by a single power law with photon index -1.9. The photosphere gives rise to a strong quasi-blackbody spectrum which is somewhat broader than a single Planck function and has a characteristic temperature of similar to 290 keV. We model the photospheric emission with a multicolor blackbody, and its shape indicates that the photospheric radius increases at higher latitudes. We derive the averaged photospheric radius R-ph = (1.1 +/- 0.3) x 10(12) Y-1/4 cm and the bulk Lorentz factor of the flow, which is found to vary by a factor of 2 and has a maximal value of Gamma = 750 Y-1/4. Here, Y is the ratio between the total fireball energy and the energy emitted in the gamma rays. We find that during the first quarter of the prompt phase the photospheric emission dominates, which explains the delayed onset of the observed flux in the LAT compared to the GBM. We interpret the broadband emission as synchrotron emission at R similar to 4 x 10(15) cm. Our analysis emphasizes the importance of having high temporal resolution when performing spectral analysis on gamma-ray bursts, since there is strong spectral evolution.

  • 16.
    Ryde, Felix
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, Sweden.
    Pe'er, Asaf
    Harvard-Smithsonian Center for Astrophysics, United States.
    Nymark, Tanja
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, Sweden.
    Axelsson, Magnus
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, Sweden.
    Moretti, Elena
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, Sweden.
    Lundman, Christoffer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, Sweden.
    Battelino, Milan
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, Sweden.
    Bissaldi, Elisabetta
    Leopold-Franzens-Universität Innsbruck, Austria.
    Chiang, James
    Stanford University, United States.
    Jackson, Miranda S.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, Sweden.
    Larsson, Stefan
    Stockholm University, Sweden.
    Longo, Francesco
    Sezione di Trieste, Italy; Università di Trieste, Italy.
    McGlynn, Sinead
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Technische Universität München, Germany.
    Omodei, Nicola
    Stanford University, United States.
    Observational evidence of dissipative photospheres in gamma-ray bursts2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 415, no 4, p. 3693-3705Article in journal (Refereed)
    Abstract [en]

    The emission from a gamma-ray burst (GRB) photosphere can give rise to a variety of spectral shapes. The spectrum can retain the shape of a Planck function or it can be broadened and have the shape of a Band function. This fact is best illustrated by studying GRB090902B. The main gamma-ray spectral component is initially close to a Planck function, which can only be explained by emission from the jet photosphere. Later, the same component evolves into a broader Band function. This burst thus provides observational evidence that the photosphere can give rise to a non-thermal spectrum. We show that such a broadening is most naturally explained by subphotospheric dissipation in the jet. The broadening mainly depends on the strength and location of the dissipation, the magnetic field strength and the relation between the energy densities of thermal photons and electrons. We suggest that the evolution in spectral shape observed in GRB090902B is due to a decrease in the bulk Lorentz factor of the flow, leading to the main dissipation becoming subphotospheric. Such a change in the flow parameters can also explain the correlation observed between the peak energy of the spectrum and low-energy power-law slope, a, a correlation commonly observed in GRBs. We conclude that photospheric emission could indeed be a ubiquitous feature during the prompt phase in GRBs and play a decisive role in creating the diverse spectral shapes and spectral evolutions that are observed.

1 - 16 of 16
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