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Lundman, ChristofferORCID iD iconorcid.org/0000-0002-0642-1055
Alternative names
Publications (10 of 18) Show all publications
Ryde, F., Yu, H.-F., Dereli-Begue, H., Lundman, C., Pe'er, A. & Li, L. (2019). On the alpha-intensity correlation in gamma-ray bursts: subphotospheric heating with varying entropy. Monthly notices of the Royal Astronomical Society, 484(2), 1912-1925
Open this publication in new window or tab >>On the alpha-intensity correlation in gamma-ray bursts: subphotospheric heating with varying entropy
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2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 484, no 2, p. 1912-1925Article in journal (Refereed) Published
Abstract [en]

The emission mechanism during the prompt phase in gamma-ray bursts (GRBs) can be investigated through correlations between spectral properties. Here, we revisit the correlation relating the instantaneous flux, F, and the photon index below the spectral break, alpha, in individual emission pulses, by studying the 38 most prominent pulses in the Fermi/Gamma-ray Burst Monitor GRB catalogue. First, we search for signatures of the bias in the determination of alpha due to the limited spectral coverage (window effect) expected in the synchrotron case. The absence of such a characteristic signature argues against the simplest synchrotron models. We instead find that the observed correlation between F and alpha can, in general, be described by the relation F(t) proportional to e(k alpha(t)), for which the median k = 3. We suggest that this correlation is a manifestation of subphotospheric heating in a flow with a varying entropy. Around the peak of the light curve, a large entropy causes the photosphere to approach the saturation radius, leading to an intense emission with a narrow spectrum. As the entropy decreases the photosphere secedes from the saturation radius, and weaker emission with a broader spectrum is expected. This simple scenario naturally leads to a correlated variation of the intensity and spectral shape, covering the observed range.

Place, publisher, year, edition, pages
Oxford University Press, 2019
Keywords
gamma-ray burst: general, gamma-ray burst: individual, radiation mechanism: thermal, methods: data analysis
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-249863 (URN)10.1093/mnras/stz083 (DOI)000462302600034 ()2-s2.0-85063384317 (Scopus ID)
Note

QC 20190426

Available from: 2019-04-26 Created: 2019-04-26 Last updated: 2019-04-26Bibliographically approved
Lundman, C. & Beloborodov, A. M. (2019). Radiation-mediated Shocks in Gamma-Ray Bursts: Subshock Photon Production. Astrophysical Journal, 879(2), Article ID 83.
Open this publication in new window or tab >>Radiation-mediated Shocks in Gamma-Ray Bursts: Subshock Photon Production
2019 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 879, no 2, article id 83Article in journal (Refereed) Published
Abstract [en]

Internal shocks provide a plausible heating mechanism in the jets of gamma-ray bursts (GRBs). Shocks occurring below the jet photosphere are mediated by radiation. It was previously found that radiation-mediated shocks (RMSs) inside GRB jets are inefficient photon producers, and the photons that mediate the RMS must originate from an earlier stage of the explosion. We show that this conclusion is valid only for nonmagnetized jets. RMSs that propagate in moderately magnetized plasma develop a collisionless subshock that locally heats the plasma to a relativistic temperature, and the hot electrons emit copious synchrotron photons inside the RMS. We find that this mechanism is effective for mildly relativistic shocks and may be the main source of photons observed in GRBs. We derive a simple analytical estimate for the generated photon number per proton, Z, which gives Z = 10(5)-10(6), consistent with observations. The number is controlled by two main factors: (1) the abundance of electron-positron pairs created in the shock, which is self-consistently calculated, and (2) the upper limit on the brightness temperature of soft radiation set by induced Compton scattering. The photons are initially injected with low energies that are well below the observed GRB peak. The injected soft photons that survive induced downscattering and free-free absorption gain energy in the RMS via bulk Comptonization and shape its nonthermal spectrum.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019
Keywords
gamma-ray burst: general, magnetohydrodynamics (MHD), radiation mechanisms: thermal, radiative transfer, scattering, shock waves
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-255375 (URN)10.3847/1538-4357/ab229f (DOI)000474750300004 ()
Note

QC 20190808

Available from: 2019-08-08 Created: 2019-08-08 Last updated: 2019-08-08Bibliographically approved
Pearce, M., Eliasson, L., Iyer, N., Kiss, M., Kushwah, R., Larsson, J., . . . Xie, F. (2019). Science prospects for SPHiNX - A small satellite GRB polarimetry mission. Astroparticle physics, 104, 54-63
Open this publication in new window or tab >>Science prospects for SPHiNX - A small satellite GRB polarimetry mission
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2019 (English)In: Astroparticle physics, ISSN 0927-6505, E-ISSN 1873-2852, Vol. 104, p. 54-63Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2019
Keywords
Polarimetry, X-ray, Gamma-ray burst, Small satellite
National Category
Subatomic Physics
Identifiers
urn:nbn:se:kth:diva-238104 (URN)10.1016/j.astropartphys.2018.08.007 (DOI)000447479300004 ()2-s2.0-85052499332 (Scopus ID)
Note

QC 20190111

Available from: 2019-01-11 Created: 2019-01-11 Last updated: 2019-01-11Bibliographically approved
Ahlgren, B., Larsson, J., Ahlberg, E., Lundman, C., Ryde, F. & Pe'er, A. (2019). Testing a model for subphotospheric dissipation in GRBs: fits to Fermi data constrain the dissipation scenario. Monthly notices of the Royal Astronomical Society, 485, 474-497
Open this publication in new window or tab >>Testing a model for subphotospheric dissipation in GRBs: fits to Fermi data constrain the dissipation scenario
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2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 485, p. 474-497Article in journal (Refereed) Published
Abstract [en]

It has been suggested that the prompt emission in gamma-ray bursts (GRBs) could be described by radiation from the photosphere in a hot fireball. Such models must be tested by directly fitting them to data. In this work we use data from the Fermi Gamma-ray Space Telescope and consider a specific photospheric model, in which the kinetic energy of a low-magnetization outflow is dissipated locally by internal shocks below the photosphere. We construct a table model with a physically motivated parameter space and fit it to time-resolved spectra of the 36 brightest Fermi GRBs with a known redshift. We find that about two-thirds of the examined spectra cannot be described by the model, as it typically underpredicts the observed flux. However, since the sample is strongly biased towards bright GRBs, we argue that this fraction will be significantly lowered when considering the full population. From the successful fits we find that the model can reproduce the full range of spectral slopes present in the sample. For these cases we also find that the dissipation consistently occurs at a radius of ∼1012 cm and that only a few per cent efficiency is required. Furthermore, we find a positive correlation between the fireball luminosity and the Lorentz factor. Such a correlation has been previously reported by independent methods. We conclude that if GRB spectra are due to photospheric emission, the dissipation cannot only be the specific scenario we consider here.

Keywords
gamma-ray burst
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-245227 (URN)10.1093/mnras/stz110 (DOI)000466786400034 ()
Note

QC 20190308

Available from: 2019-03-07 Created: 2019-03-07 Last updated: 2019-05-29Bibliographically approved
Lundman, C., Vurm, I. & Beloborodov, A. M. (2018). Polarization of Gamma-Ray Bursts in the Dissipative Photosphere Model. Astrophysical Journal, 856(2), Article ID 145.
Open this publication in new window or tab >>Polarization of Gamma-Ray Bursts in the Dissipative Photosphere Model
2018 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 856, no 2, article id 145Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2018
Keywords
gamma-ray burst: general, polarization, radiation mechanisms: non-thermal, radiative transfer, relativistic processes, scattering
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-228140 (URN)10.3847/1538-4357/aab3e8 (DOI)000429119800004 ()2-s2.0-85045537662 (Scopus ID)
Note

QC 20180518

Available from: 2018-05-18 Created: 2018-05-18 Last updated: 2018-05-18Bibliographically approved
Iyyani, S., Ryde, F., Ahlgren, B., Burgess, J. M., Larsson, J., Pe'er, A., . . . McGlynn, S. (2015). Extremely narrow spectrum of GRB110920A: further evidence for localized, subphotospheric dissipation. Monthly notices of the Royal Astronomical Society, 450(2), 1651-1663
Open this publication in new window or tab >>Extremely narrow spectrum of GRB110920A: further evidence for localized, subphotospheric dissipation
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2015 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 450, no 2, p. 1651-1663Article in journal (Refereed) Published
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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-170964 (URN)10.1093/mnras/stv636 (DOI)000356338500039 ()
Note

QC 20150713

Available from: 2015-07-13 Created: 2015-07-13 Last updated: 2017-12-04Bibliographically approved
Lundman, C., Pe'er, A. & Ryde, F. (2014). Polarization properties of photospheric emission from relativistic, collimated outflows. Monthly notices of the Royal Astronomical Society, 440(4), 3292-3308
Open this publication in new window or tab >>Polarization properties of photospheric emission from relativistic, collimated outflows
2014 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 440, no 4, p. 3292-3308Article in journal (Refereed) Published
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.

Keywords
Gamma-ray burst: general, Polarization, Radiation mechanisms: thermal, Radiative transfer, Scattering
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-146938 (URN)10.1093/mnras/stu457 (DOI)000336213800031 ()2-s2.0-84899881224 (Scopus ID)
Note

QC 20140623

Available from: 2014-06-23 Created: 2014-06-18 Last updated: 2017-12-05Bibliographically approved
Lundman, C., Pe'er, A. & Ryde, F. (2013). A theory of photospheric emission from relativistic, collimated outflows. Monthly notices of the Royal Astronomical Society, 428(3), 2430-2442
Open this publication in new window or tab >>A theory of photospheric emission from relativistic, collimated outflows
2013 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 428, no 3, p. 2430-2442Article in journal (Refereed) Published
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 &lt; 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 &gt; few) observed at Θv &gt; 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.

Keywords
Gamma-ray burst: general., Plasmas, Radiation mechanisms: thermal, Radiative transfer, Scattering
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-119684 (URN)10.1093/mnras/sts219 (DOI)000318230700043 ()2-s2.0-84873463810 (Scopus ID)
Note

QC 20130320

Available from: 2013-03-20 Created: 2013-03-20 Last updated: 2017-12-06Bibliographically approved
Lundman, C. (2013). Photospheric emission from structured, relativistic jets: applications to gamma-ray burst spectra and polarization. (Doctoral dissertation). Stockholm: KTH Royal Institute of Technology
Open this publication in new window or tab >>Photospheric emission from structured, relativistic jets: applications to gamma-ray burst spectra and polarization
2013 (English)Doctoral 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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. p. viii, 151
Series
Trita-FYS, ISSN 0280-316X ; 2013:68
Keywords
gamma-ray bursts, photospheric emission, radiative processes, polarization, special relativity
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-136178 (URN)978-91-7501-967-3 (ISBN)
Public defence
2013-12-20, FB54, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20131204

Available from: 2013-12-04 Created: 2013-12-04 Last updated: 2013-12-04Bibliographically approved
Iyyani, S., Ryde, F., Axelsson, M., Burgess, J. M., Guiriec, S., Larsson, J., . . . Rosquist, K. (2013). Variable jet properties in GRB 110721A: time resolved observations of the jet photosphere. Monthly notices of the Royal Astronomical Society, 433(4), 2739-2748
Open this publication in new window or tab >>Variable jet properties in GRB 110721A: time resolved observations of the jet photosphere
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2013 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 433, no 4, p. 2739-2748Article in journal (Refereed) Published
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.

Keywords
gamma-ray burst: individual: GRB 110721A
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-127493 (URN)10.1093/mnras/stt863 (DOI)000322405900004 ()2-s2.0-84881118688 (Scopus ID)
Note

QC 20130902

Available from: 2013-09-02 Created: 2013-08-30 Last updated: 2017-06-22Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-0642-1055

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