| 1. |
- Ackermann, M., et al.
(författare)
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Constraining The High-Energy Emission From Gamma-Ray Bursts With Fermi
- 2012
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 754:2, s. 121-
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Tidskriftsartikel (refereegranskat)abstract
- We examine 288 gamma-ray bursts (GRBs) detected by the Fermi Gamma-ray Space Telescope's Gamma-ray Burst Monitor (GBM) that fell within the field of view of Fermi's Large Area Telescope (LAT) during the first 2.5 years of observations, which showed no evidence for emission above 100 MeV. We report the photon flux upper limits in the 0.1-10 GeV range during the prompt emission phase as well as for fixed 30 s and 100 s integrations starting from the trigger time for each burst. We compare these limits with the fluxes that would be expected from extrapolations of spectral fits presented in the first GBM spectral catalog and infer that roughly half of the GBM-detected bursts either require spectral breaks between the GBM and LAT energy bands or have intrinsically steeper spectra above the peak of the nu F-nu spectra (E-pk). In order to distinguish between these two scenarios, we perform joint GBM and LAT spectral fits to the 30 brightest GBM-detected bursts and find that a majority of these bursts are indeed softer above E-pk than would be inferred from fitting the GBM data alone. Approximately 20% of this spectroscopic subsample show statistically significant evidence for a cutoff in their high-energy spectra, which if assumed to be due to gamma gamma attenuation, places limits on the maximum Lorentz factor associated with the relativistic outflow producing this emission. All of these latter bursts have maximum Lorentz factor estimates that are well below the minimum Lorentz factors calculated for LAT-detected GRBs, revealing a wide distribution in the bulk Lorentz factor of GRB outflows and indicating that LAT-detected bursts may represent the high end of this distribution.
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| 2. |
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| 3. |
- Ackermann, M., et al.
(författare)
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Fermi Detection of γ-Ray Emission from the M2 Soft X-Ray Flare on 2010 June 12
- 2012
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 745:2, s. 144-
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Tidskriftsartikel (refereegranskat)abstract
- The Geostationary Operational Environmental Satellite (GOES) M2-class solar flare, SOL2010-06-12T00: 57, was modest in many respects yet exhibited remarkable acceleration of energetic particles. The flare produced an similar to 50 s impulsive burst of hard X-and gamma-ray emission up to at least 400 MeV observed by the Fermi Gamma-ray Burst Monitor and Large Area Telescope experiments. The remarkably similar hard X-ray and high-energy gamma-ray time profiles suggest that most of the particles were accelerated to energies greater than or similar to 300 MeV with a delay of similar to 10 s from mildly relativistic electrons, but some reached these energies in as little as similar to 3 s. The gamma-ray line fluence from this flare was about 10 times higher than that typically observed from this modest GOES class of X-ray flare. There is no evidence for time-extended >100 MeV emission as has been found for other flares with high-energy gamma-rays.
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| 4. |
- Ackermann, M., et al.
(författare)
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Multiwavelength observations of GRB 110731A : GeV emission from onset to afterglow
- 2013
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 763:2, s. 71-
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Tidskriftsartikel (refereegranskat)abstract
- We report on the multiwavelength observations of the bright, long gamma-ray burst GRB 110731A, by the Fermi and Swift observatories, and by the MOA and GROND optical telescopes. The analysis of the prompt phase reveals that GRB 110731A shares many features with bright Large Area Telescope bursts observed by Fermi during the first three years on-orbit: a light curve with short time variability across the whole energy range during the prompt phase, delayed onset of the emission above 100 MeV, extra power-law component and temporally extended high-energy emission. In addition, this is the first GRB for which simultaneous GeV, X-ray, and optical data are available over multiple epochs beginning just after the trigger time and extending for more than 800 s, allowing temporal and spectral analysis in different epochs that favor emission from the forward shock in a wind-type medium. The observed temporally extended GeV emission is most likely part of the high-energy end of the afterglow emission. Both the single-zone pair transparency constraint for the prompt signal and the spectral and temporal analysis of the forward-shock afterglow emission independently lead to an estimate of the bulk Lorentz factor of the jet Γ ∼ 500-550.
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| 5. |
- Ackermann, M., et al.
(författare)
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THE FIRST FERMI-LAT GAMMA-RAY BURST CATALOG
- 2013
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 209:1
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Tidskriftsartikel (refereegranskat)abstract
- In three years of observations since the beginning of nominal science operations in 2008 August, the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope has observed high-energy (greater than or similar to 20 MeV) gamma-ray emission from 35 gamma-ray bursts (GRBs). Among these, 28 GRBs have been detected above 100 MeV and 7 GRBs above similar to 20 MeV. The first Fermi-LAT catalog of GRBs is a compilation of these detections and provides a systematic study of high-energy emission from GRBs for the first time. To generate the catalog, we examined 733 GRBs detected by the Gamma-Ray Burst Monitor (GBM) on Fermi and processed each of them using the same analysis sequence. Details of the methodology followed by the LAT collaboration for the GRB analysis are provided. We summarize the temporal and spectral properties of the LAT-detected GRBs. We also discuss characteristics of LAT-detected emission such as its delayed onset and longer duration compared with emission detected by the GBM, its power-law temporal decay at late times, and the fact that it is dominated by a power-law spectral component that appears in addition to the usual Band model.
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| 6. |
- Ahlgren, Björn, et al.
(författare)
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Confronting GRB prompt emission with a model for subphotospheric dissipation
- 2015
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Ingår i: Monthly Notices of the Royal Astronomical Society: Letters. - : Oxford University Press. - 1745-3925 .- 1745-3933. ; 454:1, s. L31-L35
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Tidskriftsartikel (refereegranskat)abstract
- The origin of the prompt emission in gamma-ray bursts (GRBs) is still an unsolved problem and several different mechanisms have been suggested. Here, we fit Fermi GRB data with a photospheric emission model which includes dissipation of the jet kinetic energy below the photosphere. The resulting spectra are dominated by Comptonization and contain no significant contribution from synchrotron radiation. In order to fit to the data, we span a physically motivated part of the model's parameter space and create DREAM (Dissipation with Radiative Emission as A table Model), a table model for XSPEC. We show that this model can describe different kinds of GRB spectra, including GRB 090618, representing a typical Band function spectrum, and GRB 100724B, illustrating a double peaked spectrum, previously fitted with a Band+blackbody model, suggesting they originate from a similar scenario. We suggest that the main difference between these two types of bursts is the optical depth at the dissipation site.
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| 7. |
- Axelsson, Magnus, et al.
(författare)
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GRB110721A : AN EXTREME PEAK ENERGY AND SIGNATURES OF THE PHOTOSPHERE
- 2012
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Ingår i: Astrophysical Journal Letters. - 2041-8205. ; 757:2
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Tidskriftsartikel (refereegranskat)abstract
- 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 similar to 24.5 s (in the GBM) and had a peak flux of (5.7 +/- 0.2) x 10(-5) erg s(-1) 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 +/- 2 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 +/- 0.10. The temperature of the blackbody component also decreased, starting from similar to 80 keV, and the decay showed a significant break after similar to 2 s. The spectrum provides strong constraints on the standard synchrotron model, indicating that alternative mechanisms may give rise to the emission at these energies.
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| 8. |
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| 9. |
- Chevalier, Roger, et al.
(författare)
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Radio and X-Ray Emission as Probes of Type IIP Supernovae and Red Supergiant Mass Loss
- 2006
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Ingår i: The Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 641:2, s. 1029-1038
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Tidskriftsartikel (refereegranskat)abstract
- Type IIP (plateau) supernovae are thought to come from stars with initial mass ~8-25 Msolar that end their lives as red supergiants. The expected stellar endpoints can be found from evolutionary calculations, and the corresponding mass-loss properties at these points can be estimated from typical values for Galactic stars. The mass-loss densities of observed supernovae can be estimated from observations of the thermal X-ray and radio synchrotron emission that result from the interaction of the supernova with the surrounding wind. Type IIP supernovae are expected to have energy-conserving interaction during typical times of observation. Because Type IIP supernovae have an extended period of high optical luminosity, Compton cooling could affect the radio-emitting electrons, giving rise to a relatively flat radio light curve in the optically thin regime. Alternatively, a high efficiency of magnetic field production results in synchrotron cooling of the radio-emitting electrons. Both the X-ray and radio luminosities are sensitive to the mass loss and initial masses of the progenitor stars, although the turn-on of radio emission is probably the best estimator of circumstellar density. Both the mass-loss density and the variation of density with stellar mass are consistent with expectations for the progenitor stars deduced from direct observations of recent supernovae. Current observations are consistent with mass being the only parameter; observations of supernovae in metal-poor regions could show how the mass loss depends on metallicity.
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| 10. |
- Gröningsson, Per, et al.
(författare)
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Coronal emission from the shocked circumstellar ring of SN 1987A
- 2006
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 456:2, s. 581-589
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Tidskriftsartikel (refereegranskat)abstract
- High resolution spectra with UVES/VLT of SN 1987A from December 2000 until November 2005 show a number of high ionization lines from gas with velocities of ± 350 km s-1, emerging from the shocked gas formed by the ejecta-ring collision. These include coronal lines from [Fe X], [Fe XI] and [Fe XIV] which have increased by a factor of 20 during the observed period. The evolution of the lines is similar to that of the soft X-rays, indicating that they arise in the same component. The line ratios are consistent with those expected from radiative shocks with velocity 310{-}390 km s-1, corresponding to a shock temperature of (1.6{-}2.5)× 106 K. A fraction of the coronal emission may, however, originate in higher velocity adiabatic shocks.
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| 11. |
- Gumaelius, Lena, et al.
(författare)
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The role of ‘Teknikåttan’– a competition aimed at increasing interest in science and technology for grade 8 students.
- 2017
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Ingår i: NorDiNa. - : University of Oslo Library. - 1504-4556 .- 1894-1257. ; 13:2, s. 197-217
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Tidskriftsartikel (refereegranskat)abstract
- Since 1993, Swedish technical universities have engaged 15-year-old students and their teachers in the annual tournament “Teknikåttan” (technology for students in school year eight), which is aimed at increasing students’ interest in STEM (science, technology, engineering and mathematics) subjects, as well as at making them aware of career possibilities within these areas. Given its large number of participating students, Teknikåttan offers a unique opportunity to study students’ understanding of and interest in STEM subjects by analysing the results collected for the participating students. This paper gives a description of the Teknikåttan tournament and presents an analysis of the results from the first round of the 2014 tournament. The data collected came from the answers of students in the Stockholm region. All questions were characterised according to three parameters, which were used to analyse answers to high-score and low-score questions and differences in answers according to gender. The analysis indicates that a difference exists in answers according to gender, such that boys scored higher than girls overall, but that girls scored higher in questions related to the subject of biology. Finally, a possible expansion of the analysis involving future tournaments is discussed.
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| 12. |
- Iyyani, Shabnam, et al.
(författare)
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Variable jet properties in GRB 110721A : time resolved observations of the jet photosphere
- 2013
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 433:4, s. 2739-2748
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 13. |
- Nolan, P. L., et al.
(författare)
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Fermi large area telescope second source catalog
- 2012
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 199:2, s. 31-
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Tidskriftsartikel (refereegranskat)abstract
- We present the second catalog of high-energy gamma-ray sources detected by the Large Area Telescope (LAT), the primary science instrument on the Fermi Gamma-ray Space Telescope (Fermi), derived from data taken during the first 24 months of the science phase of the mission, which began on 2008 August 4. Source detection is based on the average flux over the 24 month period. The second Fermi-LAT catalog (2FGL) includes source location regions, defined in terms of elliptical fits to the 95% confidence regions and spectral fits in terms of power-law, exponentially cutoff power-law, or log-normal forms. Also included are flux measurements in five energy bands and light curves on monthly intervals for each source. Twelve sources in the catalog are modeled as spatially extended. We provide a detailed comparison of the results from this catalog with those from the first Fermi-LAT catalog (1FGL). Although the diffuse Galactic and isotropic models used in the 2FGL analysis are improved compared to the 1FGL catalog, we attach caution flags to 162 of the sources to indicate possible confusion with residual imperfections in the diffuse model. The 2FGL catalog contains 1873 sources detected and characterized in the 100 MeV to 100 GeV range of which we consider 127 as being firmly identified and 1171 as being reliably associated with counterparts of known or likely gamma-ray-producing source classes.
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| 14. |
- Nymark, Tanja K., et al.
(författare)
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X-ray Emission from Radiative Shocks in Type II Supernovae
- 2006
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Ingår i: Proceedings of the X-ray Universe 2005, Vols 1 and 2. - 9789290929154 ; , s. 373-374
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Konferensbidrag (refereegranskat)abstract
- A numerical model is presented which calculates the Xray emission from radiative shocks in supernovae. Type IIL and Type IIn supemovae generally have a sufficiently dense circumstellar medium for the interaction between the supernova ejecta and circumstellar medium to be strong, leading to substantial X-ray emission already a few days after the explosion. Assuming the flow is stationary, our model combines hydrodynamic calculations with time dependent ionization balance and multi-level calculations. We find large differences between the self-consistent, hydrodynamic model and simple one- or two-temperature fits. The dependence of the resulting spectrum on shock velocity and chemical composition are discussed, as well as the difference between spectra produced by this model and single-temperature spectra. The applicability of our model for various types of supernovae is discussed, and examples of applications to observations are given.
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| 15. |
- Nymark, Tanja K., et al.
(författare)
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X-ray emission from radiative shocks in type II supernovae
- 2006
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 449:1, s. 171-192
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Tidskriftsartikel (refereegranskat)abstract
- The X-ray emission from the circumstellar interaction in type II supernovae with a dense circumstellar medium is calculated. In type IIL and type IIn supernovae, mass loss rates are generally high enough for the region behind the reverse shock to be radiative, producing strong radiation, particularly in X-rays. We present a model for the emission from the cooling region in the case of a radiative reverse shock. Under the assumption of a stationary flow, a hydrodynamic model is combined with a time-dependent ionization balance and multilevel calculations. The applicability of the steady state approximation is discussed for various values of the ejecta density gradient and different sets of chemical composition. We show how the emerging spectrum strongly depends on the reverse shock velocity and the composition of the shocked gas. We discuss differences between a spectrum produced by this model and a single-temperature spectrum. Large differences are found especially for the line emission, which can seriously affect abundance estimates. We also illustrate the effects of absorption in the cool shocked ejecta. The applicability of our model to various types of supernovae is discussed.
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| 16. |
- Nymark, Tanja, et al.
(författare)
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Modeling the X-ray emission of SN 1993J
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 494:1, s. 179-189
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Tidskriftsartikel (refereegranskat)abstract
- Aims: We investigate the effects of radiative shocks on the observed X-ray emission from the type II supernova SN 1993J. To this end, the X-ray emission is modeled as a result of the interaction between the supernova ejecta and a dense circumstellar medium at an age of 8 years. Methods: The circumstances under which the reverse shock is radiative are discussed and the observed X-ray emission is analyzed using the numerical code described in Nymark et al. (2006, A&A, 449, 171). We argue that the original analysis of the X-ray observations suffered from the lack of self-consistent models for cooling shocks with high density and velocity, leading to questionable conclusions about the temperatures and elemental abundances. We reanalyze the spectra with our numerical model, and discuss the expected spectra for different explosion models for the progenitors. Results: We find that the spectra of SN 1993J are compatible with a CNO-enriched composition and that the X-ray flux is dominated by the reverse shock.
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| 17. |
- Nymark, Tanja, et al.
(författare)
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Modeling the X-Rays from SNe 1993J & 1998S
- 2007
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Ingår i: Supernova 1987A. - : American Institute of Physics (AIP). - 9780735404489 ; , s. 430-432
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Konferensbidrag (refereegranskat)abstract
- The X-ray emission from the Type II supernovae SNe 1993J and 1998S is modelled using the numerical code described in Nymark et al. (2006), assuming that most of the observed Xray emission comes from the reverse shock created by the interaction between the supernova ejecta and a dense circumstellar medium. We find that the spectra of both SNe are compatible with a CNO-enriched composition. For SN 1998S we find evidence for a component from both the reverse and forward shocks while the X-ray flux for SN 1993J is dominated by the reverse shock.
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| 18. |
- Nymark, Tanja, et al.
(författare)
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Modeling the X‐rays from SNe 1993J & 1998S
- 2007
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Ingår i: AIP Conference Proceedings. - : AIP. - 0094-243X .- 1551-7616. - 9780735404489 ; 937:430, s. 430-432
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Tidskriftsartikel (refereegranskat)abstract
- TheX-ray emission from the Type II supernovae SNe 1993J and1998S is modelled using the numerical code described in Nymarket al. (2006), assuming that most of the observed X-ray emissioncomes from the reverse shock created by the interaction betweenthe supernova ejecta and a dense circumstellar medium. We findthat the spectra of both SNe are compatible with aCNO-enriched composition. For SN 1998S we find evidence for acomponent from both the reverse and forward shock, while theX-ray flux for SN 1993J is dominated by the reverseshock.
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| 19. |
- Nymark, Tanja, et al.
(författare)
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Subphotospheric heating in GRBs : analysis and modeling of GRB090902B as observed by Fermi
- 2011
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Ingår i: 2011 Fermi Symposium proceedings.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 20. |
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| 21. |
- Nymark, Tanja, 1971-
(författare)
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X-ray emission from supernova shock waves
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A theoretical study of the interaction between supernovae and their surroundings is presented.Supernovae are the endpoint of the life of massive stars, and are the dominant contributors to the chemical evolution of the Universe. During its life a massive star greatly modifies its environment. During and after the explosion of the star it interacts with its surroundings in a number of ways. A study of this interaction yields invaluable information about the late stages of stellar evolution and the physics of supernova explosions. Recent advances in observational facilities have given a wealth of observational data on interacting supernovae, and it is therefore essential to have good theoretical models for interpreting the data.This thesis presents an overview of the physics of supernovae and of their interaction with a circumstellar medium. In particular the reverse shock created by the interaction is investigated. In most Type IIL and Type IIn supernovae this shock is radiative, and due to the high temperature most of the radiation comes out as X-rays. A numerical model is presented which calculates the emission from the cooling region behind the reverse shock in a self-consistent way, by combining a hydrodynamic model with a time-dependent ionization balance and multilevel calculations. This has been applied to some of the best cases of circumstellar interaction.As a further application of the model the radio and X-ray emission from Type IIP supernovae is discussed. We estimate the mass loss rate of the progenitors of Type IIP supernovae, and find that a superwind phase is not required.VLT observations of the ring of SN 1987A show broad optical emission lines coming from a range of ionization stages, in particular optical coronal lines of Fe X-XIV. Models of the line emission indicate that the lines are formed by cooling shocks with shock velocities in the range 310-390 km/s, confirming the picture of shocks striking the protrusions from the ring obliquely.X-ray observations of the Type IIb SN 1993J and Type IIn SN 1998S are analyzed. For SN 1993J we find that the spectrum is best fit with a CNO-enriched composition. For SN 1998S we find that the high metal overabundance that has previously been claimed, is not necessary when a self-consistent model of the cooling region is applied.
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| 22. |
- Preece, R., et al.
(författare)
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The First Pulse of the Extremely Bright GRB 130427A : A Test Lab for Synchrotron Shocks
- 2014
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 343:6166, s. 51-54
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Tidskriftsartikel (refereegranskat)abstract
- Gamma-ray burst (GRB) 130427A is one of the most energetic GRBs ever observed. The initial pulse up to 2.5 seconds is possibly the brightest well-isolated pulse observed to date. A fine time resolution spectral analysis shows power-law decays of the peak energy from the onset of the pulse, consistent with models of internal synchrotron shock pulses. However, a strongly correlated power-law behavior is observed between the luminosity and the spectral peak energy that is inconsistent with curvature effects arising in the relativistic outflow. It is difficult for any of the existing models to account for all of the observed spectral and temporal behaviors simultaneously.
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| 23. |
- Quirola-Vasquez, J., et al.
(författare)
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The exceptional X-ray evolution of SN 1996cr in high resolution
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 490:4, s. 4536-4564
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Tidskriftsartikel (refereegranskat)abstract
- We present X-ray spectra spanning 18 yr of evolution for SN 1996cr, one of the five nearest SNe detected in the modern era. Chandra HETG exposures in 2000, 2004, and 2009 allow us to resolve spectrally the velocity profiles of Ne, Mg, Si, S, and Fe emission lines and monitor their evolution as tracers of the ejecta-circumstellar medium interaction. To explain the diversity of X-ray line profiles, we explore several possible geometrical models. Based on the highest signal-to-noise 2009 epoch, we find that a polar geometry with two distinct opening angle configurations and internal obscuration can successfully reproduce all of the observed line profiles. The best-fitting model consists of two plasma components: (1) a mildly absorbed (2x10(21) cm(-2)), cooler (approximate to 2 keV) with high Ne, Mg, Si, and S abundances associated with a wide polar interaction region (half-opening angle approximate to 58 degrees); (2) a moderately absorbed (2 x 10(22) cm(-2)), hotter (greater than or similar to 20 keV) plasma with high Fe abundances and strong internal obscuration associated with a narrow polar interaction region (half-opening angle approximate to 20 degrees). We extend this model to seven further epochs with lower signal-to-noise ratio and/or lower spectral-resolution between 2000 and 2018, yielding several interesting trends in absorption, flux, geometry, and expansion velocity. We argue that the hotter and colder components are associated with reflected and forward shocks, respectively, at least at later epochs. We discuss the physical implications of our results and plausible explosion scenarios to understand the X-ray data of SN1996cr.
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| 24. |
- Ryde, Felix, et al.
(författare)
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Observational evidence of dissipative photospheres in gamma-ray bursts
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 415:4, s. 3693-3705
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 25. |
- Ryde, Nils, et al.
(författare)
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Lumen Accipe et Imperti - Föreläsning och motivation
- 2011
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Ingår i: Lednings- och kompetensutbildning/CED. - 9789197797467 ; , s. 24-30
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Konferensbidrag (refereegranskat)abstract
- Abstract in Undetermined Vi problematiserar den kritik mot föreläsningsbaserat lärande vid universitet och högskolor, som är vanlig i dagens pedagogiska diskussioner. Vi baserar vårt arbete på enkätundersökningar och på djupintervjuer av studenter på grundutbildningskurser i fysik vid Lunds universitet och Kungliga Tekniska högskolan. Vi anser att den rådande bilden av föreläsningens värde bör nyanseras så att man inte bortser från det unikt positiva hos en bra föreläsning. Man bör därför inte förkasta denna pedgogiska metod, utan, väl förberedd, har föreläsningen en viktig och uppskattad roll i inlärningen.
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