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| 2. |
- Valan, Vlasta, 1990-
(författare)
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Lessons learned from time-resolved X-ray spectra of gamma-ray bursts
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gamma-ray bursts (GRBs) are among the brightest and most energetic phenomena in the Universe, observed up to great distances. The early X-ray emission ofgamma-ray bursts are usually well described by absorbed power laws. However,in some cases, additional thermal components have been identified. The origin ofthis emission is debated, with proposed explanations including supernova shockbreakout, emission from a cocoon surrounding the jet, and emission from the jetitself. A larger sample of detections is needed to constrain these different models.In this thesis, a time-resolved analysis of 199 GRBs observed by Swift XRT wasperformed to search for thermal components, which revealed 19 cases where thisthermal component is needed to describe GRB X-ray spectra fully. Due to the largespan of blackbody properties, the origin of the blackbody component is thought tobe connected with both the jet itself and the cocoon that surrounds it with nounique origin in all GRBs.This thesis also tackles the question of where the excess X-ray absorption inGRBs originates from. There is an excess absorption above the Galactic valuein the X-ray spectra of all GRBs, and it increases with redshift. However, theexact location where it originates from is still a puzzle, with two possible, but notmutually exclusive, explanations: the host galaxy of the GRB and the inter-galacticmedium and intervening objects. I have analyzed the same sample of 199 GRBsto gain more insights into the origin of this absorption. One possible way to probewhere this excess absorption arises is to search for time variability of the excessabsorption in the X-ray spectra, which would point to the absorber being close tothe GRB. I found that this time decrease of excess absorption is a rare effect. In theGRBs that showed this time decrease, it cannot be excluded that an alternativemodel can explain the observed decrease. This implies that, in the majority ofGRBs, the absorption is happening at a larger scale in the host galaxy and/or inthe intergalactic medium.
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| 3. |
- Davies, Melvyn B, et al.
(författare)
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The ultimate outcome of black hole - neutron star mergers
- 2005
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 356:1, s. 54-58
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Tidskriftsartikel (refereegranskat)abstract
- We present a simple, semi--analytical description for the final stagesof mergers of black hole (BH) -- neutron star (NS) systems. Such systemsare of much interest as gravitational wave sources and gamma--ray burstprogenitors. Numerical studies show that in general the neutron star isnot disrupted at the first phase of mass transfer. Instead, what remainsof the neutron star is left on a wider, eccentric, orbit. We considerthe evolution of such systems as they lose angular momentum viagravitational radiation and come into contact for further phases of masstransfer. During each mass transfer event the neutron star mass isreduced until a critical value where mass loss leads to a rapid increasein the stellar radius. At this point Roche lobe overflow shreds whatremains of the neutron star, most of the mass forming a disc around theblack hole. Such a disc may be massive enough to power a gamma--rayburst. The mass of the neutron star at the time of disruption (andtherefore the disc mass) is largely independent of the initial masses ofthe black hole and neutron star, indicating that BH--NS star mergers maybe standard candles.
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| 4. |
- Larsson, Josefin, et al.
(författare)
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A new constraint for gamma-ray burst progenitor mass
- 2007
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 376:3, s. 1285-1290
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Tidskriftsartikel (refereegranskat)abstract
- Recent comparative observations of long-duration gamma-ray bursts (LGRBs) and core collapse supernova (cc SN) host galaxies demonstrate that these two, highly energetic transient events are distributed very differently upon their hosts. LGRBs are much more concentrated on their host galaxy light than cc SN. Here we explore the suggestion that this differing distribution reflects different progenitor masses for LGRBs and cc SN. Using a simple model we show that, assuming cc SN arise from stars with main-sequence masses > 8 M-circle dot, GRBs are likely to arise from stars with initial masses > 20 M-circle dot. This difference can naturally be explained by the requirement that stars which create a LGRB must also create a black hole.
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| 5. |
- Levan, Andrew J., et al.
(författare)
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A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy
- 2023
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Ingår i: Nature Astronomy. - 2397-3366. ; 7:8, s. 976-985
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Tidskriftsartikel (refereegranskat)abstract
- The majority of long-duration (>2 s) gamma-ray bursts (GRBs) arise from the collapse of massive stars, with a small proportion created from the merger of compact objects. Most of these systems form via standard stellar evolution pathways. However, a fraction of GRBs may result from dynamical interactions in dense environments. These channels could also contribute substantially to the samples of compact object mergers detected as gravitational wave sources. Here we report the case of GRB 191019A, a long GRB (a duration of T90 = 64.4 ± 4.5 s), which we pinpoint close (⪅100 pc projected) to the nucleus of an ancient (>1 Gyr old) host galaxy at z = 0.248. The lack of evidence for star formation and deep limits on any supernova emission disfavour a massive star origin. The most likely route for progenitor formation is via dynamical interactions in the dense nucleus of the host. The progenitor, in this case, could be a compact object merger. These may form in dense nuclear clusters or originate in a gaseous disc around the supermassive black hole. Identifying, to the best of our knowledge, a first example of a dynamically produced GRB demonstrates the role that such bursts may have in probing dense environments and constraining dynamical fractions in gravitational wave populations.
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| 6. |
- Levan, Andrew J., et al.
(författare)
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Neutron star binaries and long-duration gamma-ray bursts
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 372:3, s. 1351-1356
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Tidskriftsartikel (refereegranskat)abstract
- Cosmological long-duration gamma-ray bursts (LGRBs) are thought to originate from the core collapse to black holes (BHs) of stripped massive stars. Those with sufficient rotation form a centrifugally supported torus whose collapse powers the GRB. We investigate the role of tidal locking within a tight binary as a source of the necessary angular momentum. We find that the binary orbit must be no wider than a few solar radii for a torus to form upon core collapse. Comparing this criterion to the observed population of binaries containing two compact objects suggests that rotation may have been important in the formation of up to 50 per cent of the observed systems. As these systems created a neutron star and not a BH, they presumably did not produce highly luminous GRBs. We suggest instead that they make the subset of GRBs in the relatively local universe which have much lower luminosity.
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| 7. |
- Agudo, I., et al.
(författare)
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Panning for gold, but finding helium: Discovery of the ultra-stripped supernova SN 2019wxt from gravitational-wave follow-up observations
- 2023
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 675
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Tidskriftsartikel (refereegranskat)abstract
- We present the results from multi-wavelength observations of a transient discovered during an intensive follow-up campaign of S191213g, a gravitational wave (GW) event reported by the LIGO-Virgo Collaboration as a possible binary neutron star merger in a low latency search. This search yielded SN 2019wxt, a young transient in a galaxy whose sky position (in the 80% GW contour) and distance (∼150 Mpc) were plausibly compatible with the localisation uncertainty of the GW event. Initially, the transienta's tightly constrained age, its relatively faint peak magnitude (Mi ∼ -16.7 mag), and the r-band decline rate of ∼1 mag per 5 days appeared suggestive of a compact binary merger. However, SN 2019wxt spectroscopically resembled a type Ib supernova, and analysis of the optical-near-infrared evolution rapidly led to the conclusion that while it could not be associated with S191213g, it nevertheless represented an extreme outcome of stellar evolution. By modelling the light curve, we estimated an ejecta mass of only ∼0.1 M·, with 56Ni comprising ∼20% of this. We were broadly able to reproduce its spectral evolution with a composition dominated by helium and oxygen, with trace amounts of calcium. We considered various progenitor channels that could give rise to the observed properties of SN 2019wxt and concluded that an ultra-stripped origin in a binary system is the most likely explanation. Disentangling genuine electromagnetic counterparts to GW events from transients such as SN 2019wxt soon after discovery is challenging: in a bid to characterise this level of contamination, we estimated the rate of events with a volumetric rate density comparable to that of SN 2019wxt and found that around one such event per week can occur within the typical GW localisation area of O4 alerts out to a luminosity distance of 500 Mpc, beyond which it would become fainter than the typical depth of current electromagnetic follow-up campaigns.
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| 8. |
- Andreoni, Igor, et al.
(författare)
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Target-of-opportunity Observations of Gravitational-wave Events with Vera C. Rubin Observatory
- 2022
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 260:1
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Tidskriftsartikel (refereegranskat)abstract
- The discovery of the electromagnetic counterpart to the binary neutron star (NS) merger GW170817 has opened the era of gravitational-wave multimessenger astronomy. Rapid identification of the optical/infrared kilonova enabled a precise localization of the source, which paved the way to deep multiwavelength follow-up and its myriad of related science results. Fully exploiting this new territory of exploration requires the acquisition of electromagnetic data from samples of NS mergers and other gravitational-wave sources. After GW170817, the frontier is now to map the diversity of kilonova properties and provide more stringent constraints on the Hubble constant, and enable new tests of fundamental physics. The Vera C. Rubin Observatory's Legacy Survey of Space and Time can play a key role in this field in the 2020s, when an improved network of gravitational-wave detectors is expected to reach a sensitivity that will enable the discovery of a high rate of merger events involving NSs (∼tens per year) out to distances of several hundred megaparsecs. We design comprehensive target-of-opportunity observing strategies for follow-up of gravitational-wave triggers that will make the Rubin Observatory the premier instrument for discovery and early characterization of NS and other compact-object mergers, and yet unknown classes of gravitational-wave events.
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| 9. |
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| 10. |
- Axelsson, Magnus, et al.
(författare)
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On the origin of black hole spin in high-mass black hole binaries : Cygnus X-1
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 412:4, s. 2260-2264
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Tidskriftsartikel (refereegranskat)abstract
- To date, there have been several detections of high-mass black hole binaries in both the Milky Way and other galaxies. For some of these, the spin parameter of the black hole has been estimated. As many of these systems are quite tight, a suggested origin of the spin is angular momentum imparted by the synchronous rotation of the black hole progenitor with its binary companion. Using Cygnus X-1, the best studied high-mass black hole binary, we investigate this possibility. We find that such an origin of the spin is not likely, and our results point rather to the spin being the result of processes during the collapse.
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