| 1. |
- Alp, Dennis, MSc, 1992-
(författare)
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An X-Ray View of Core-collapse Supernovae
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A core-collapse supernova (CCSN) is an astronomical explosion that indicates the death of a massive star. From observations, it is clear that a large fraction of all massive stars undergoes supernova (SN) explosions, but describing how SNe explode has remained a challenge for many decades. A key piece of the puzzle is the properties of the progenitor star.The attached papers focus on comparing theoretical predictions with observations, primarily observations of SN 1987A. It is the closest observed SN in more than four centuries, allowing for more detailed studies than for any other SN. The papers investigate different aspects of the SN phenomenon. These individual studies are observationally diverse, but all attempt to answer different questions that are important for our understanding of the SN process.The properties of the progenitor star set the stage for the SN. Paper III compares SN models based on different progenitor stars with early X-ray and gamma-ray observations of SN 1987A. The results help constrain the evolution of the progenitor. In Paper IV, we searched for SN shock breakouts (SBOs), which are the first electromagnetic signals from CCSNe. The discovered candidates convey information about the progenitors, test the SBO theory, and indicate the presence of other types of X-ray transients.The SN explosion mechanism itself is also integral to the analysis in Paper III. The explosion models used in Paper III rely on some of the most recent three-dimensional neutrino-driven SN models. The results lend further support to the hypothesis that delayed neutrino heating is sufficient to explode the vast majority of all CCSNe.Much can also be learned about SNe by studying their remnants. The remains of the core, the compact remnant, in SN 1987A has not yet been detected. We have investigated how a compact object can remain hidden in the ejecta in Paper I, using an absorption model from Paper II. We favor a scenario where the compact object is a neutron star that is quiescent, dust-obscured, and only emitting thermal emission. Paper V is another study of SN 1987A, but focuses on the X-ray emission from the ongoing interactions between the ejecta and circumstellar medium (CSM). The X-ray emission is primarily generated by thermal processes in shocks produced by collisions between the ejecta and the CSM. We found no evidence for any contribution from relativistic particles or a neutron star.Our description of CCSNe continues to improve but many questions remain unanswered. Future observations will further our knowledge and the models we have studied can be used for continued analyses. The next generation of X-ray missions is very promising and a Galactic SN, which would greatly accelerate the entire research field, could occur at any time.
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| 2. |
- Alp, Dennis, MSc, 1992-, et al.
(författare)
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Blasts from the Past : Supernova Shock Breakouts among X-Ray Transients in the XMM-Newton Archive
- 2020
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 896:1
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Tidskriftsartikel (refereegranskat)abstract
- The first electromagnetic signal from a supernova (SN) is released when the shock crosses the progenitor surface. This shock breakout (SBO) emission provides constraints on progenitor and explosion properties. Observationally, SBOs appear as minute- to hour-long extragalactic X-ray transients. They are challenging to detect and only one SBO has been observed to date. Here, we search the XMM-Newton archive and find 12 new SN SBO candidates. We identify host galaxies to nine of these at estimated redshifts of 0.1-1. The SBO candidates have energies of similar to 10(46)erg, timescales of 30-3000 s, and temperatures of 0.1-1 keV. They are all consistent with being SN SBOs, but some may be misidentified Galactic foreground sources or other extragalactic objects. SBOs from blue supergiants agree well with most of the candidates. However, a few could be SBOs from Wolf-Rayet stars surrounded by dense circumstellar media, whereas two are more naturally explained as SBOs from red supergiants. The observations tentatively support non-spherical SBOs and are in agreement with asymmetries predicted by recent three-dimensional SN explosion simulations. eROSITA may detect similar to 2 SBOs per year, which could be detected in live analyses and promptly followed up.
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| 3. |
- Alp, Dennis, MSc, 1992-
(författare)
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Core-collapse Supernovae : Theory vs. Observations
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A core-collapse supernova (CCSN) is an astronomical explosion that indicates the death of a massive star. The iron core of the star collapses into either a neutron star or a black hole while the rest of the material is expelled at high velocities. Supernovae (SNe) are important for the chemical evolution of the Universe because a large fraction of the heavier elements such as oxygen, silicon, and iron are liberated by CCSN explosions. Another important role of SNe is that the ejected material seed the next generation of stars and planets. From observations, it is clear that a large fraction of all massive stars undergoes SN explosions, but describing how SNe explode has remained a challenge for many decades.The attached papers focus on comparing theoretical predictions with observations, primarily observations of SN 1987A. The compact remnant in SN 1987A has not yet been detected and we have investigated how a compact object can remain hidden in the ejecta (Paper I and II). Because of the high opacity of the metal-rich ejecta, the direct X-ray observations are not very constraining even for potentially favorable viewing angles. However, the combined observations still strongly constrain fallback accretion and put a limit on possible pulsar wind activity. The thermal surface emission from a neutron star is consistent with the observations if our line of sight is dust-obscured, and only marginally consistent otherwise. Future observations provide promising opportunities for detecting the compact object.We have also compared the most recent three-dimensional neutrino-driven SN models that are based on explosion simulations with early X-ray and gamma-ray observations of SN 1987A (Paper III). The models that are designed to match SN 1987A fit the data well, but not all tensions can be explained by choosing a suitable viewing angle. More generally, the asymmetries do not affect the early emission qualitatively and different progenitors of the same class result in similar early emission. We also find that the progenitor metallicity is important for the low-energy X-ray cuto↵. Current instruments should be able to detect this emission from SNe at distances of 3–10 Mpc, which correspond to distances slightly beyond the Local Group.
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| 4. |
- Alp, Dennis, 1992-
(författare)
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Early X-Ray and Gamma-Ray Emission from 3D Neutrino-Driven SN Simulations and Comparisons With Observations of SN 1987A
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- During the first few hundred days, core-collapse supernovae (CCSNe) strongly emit X-rays and gamma-rays originating from radioactive elements, primarily the 56Ni chain. We use SN models based on three-dimensional (3D) neutrino-driven explosion simulations to compute this early emission and compare the predictions to observations of SN 1987A. The agreement between the models and observations is good but small differences that cannot be matched by a suitable choice of viewing angle are evident. The discrepancies indicate that the models need to be slightly more mixed and the bulk of the 56Ni should be moving away from us at higher velocities than can be found in the models. Asymmetries and 3D structures vary the flux by a factor of a few but do not affect the emission qualitatively. The emission also shows similar properties for qualitatively similar progenitors. The only major difference is that stripped-envelope SNe evolve faster and are more than an order of magnitude more luminous. The soft X-ray cutoff is primarily determined by the metallicity of the progenitor. Future NuSTAR observations should detect the down-scattered continuum and low-energy cutoff of (non-)stripped SNe at distances of (3)10 Mpc. INTEGRAL/SPI can detect the direct line emission at distances of (0.2)2 Mpc.
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| 5. |
- Alp, Dennis, et al.
(författare)
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Refraction in exoplanet atmospheres Photometric signatures, implications for transmission spectroscopy, and search in Kepler data
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 609
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Tidskriftsartikel (refereegranskat)abstract
- Context. Refraction deflects photons that pass through atmospheres, which affects transit light curves. Refraction thus provides an avenue to probe physical properties of exoplanet atmospheres and to constrain the presence of clouds and hazes. In addition, an effective surface can be imposed by refraction, thereby limiting the pressure levels probed by transmission spectroscopy. Aims. The main objective of the paper is to model the effects of refraction on photometric light curves for realistic planets and to explore the dependencies on atmospheric physical parameters. We also explore under which circumstances transmission spectra are significantly affected by refraction. Finally, we search for refraction signatures in photometric residuals in Kepler data. Methods. We use the model of Hui & Seager (2002, ApJ, 572, 540) to compute deflection angles and refraction transit light curves, allowing us to explore the parameter space of atmospheric properties. The observational search is performed by stacking large samples of transit light curves from Kepler. Results. We find that out-of-transit refraction shoulders are the most easily observable features, which can reach peak amplitudes of similar to 10 parts per million (ppm) for planets around Sun-like stars. More typical amplitudes are a few ppm or less for Jovians and at the sub-ppm level for super-Earths. In-transit, ingress, and egress refraction features are challenging to detect because of the short timescales and degeneracies with other transit model parameters. Interestingly, the signal-to-noise ratio of any refraction residuals for planets orbiting Sun-like hosts are expected to be similar for planets orbiting red dwarfs and ultra-cool stars. We also find that the maximum depth probed by transmission spectroscopy is not limited by refraction for weakly lensing planets, but that the incidence of refraction can vary significantly for strongly lensing planets. We find no signs of refraction features in the stacked Kepler light curves, which is in agreement with our model predictions.
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| 6. |
- Alp, Dennis, et al.
(författare)
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The 30 Year Search for the Compact Object in SN 1987A
- 2018
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 864:2
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Tidskriftsartikel (refereegranskat)abstract
- Despite more than 30 years of searching, the compact object in Supernova (SN) 1987A has not yet been detected. We present new limits on the compact object in SN 1987A using millimeter, near-infrared, optical, ultraviolet, and X-ray observations from ALMA, VLT, HST, and Chandra. The limits are approximately 0.1 mJy (0.1 x 10(-26) erg s(-1) cm(-2) Hz(-1)) at 213 GHz, 1 L-circle dot (6 x 10(-29) erg s(-1) cm(-2) Hz(-1)) in the optical if our line of sight is free of ejecta dust, and 10(36) erg s(-1) (2 x 10(-30) erg s(-1) cm(-2) Hz(-1) ) in 2-10 keV X-rays. Our X-ray limits are an order of magnitude less constraining than previous limits because we use a more realistic ejecta absorption model based on three-dimensional neutrino-driven SN explosion models. The allowed bolometric luminosity of the compact object is 22 L-circle dot if our line of sight is free of ejecta dust, or 138L(circle dot) if dust-obscured. Depending on assumptions, these values limit the effective temperature of a neutron star (NS) to <4-8 MK and do not exclude models, which typically are in the range 3-4 MK. For the simplest accretion model, the accretion rate for an efficiency 77 is limited to <10(-11) eta(-1) M-circle dot yr(-1), which excludes most predictions. For pulsar activity modeled by a rotating magnetic dipole in vacuum, the limit on the magnetic field strength (B) for a given spin period (P) is B less than or similar to 10(14) P-2 G s(-2), which firmly excludes pulsars comparable to the Crab. By combining information about radiation reprocessing and geometry, we infer that the compact object is a dust-obscured thermally emitting NS, which may appear as a region of higher-temperature ejecta dust emission.
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| 7. |
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| 8. |
- Alp, Dennis, PhD, 1992-, et al.
(författare)
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Thermal Emission and Radioactive Lines, but No Pulsar, in the Broadband X-Ray Spectrum of Supernova 1987A
- 2021
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 916:2
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Tidskriftsartikel (refereegranskat)abstract
- Supernova 1987A offers a unique opportunity to study an evolving supernova in unprecedented detail over several decades. The X-ray emission is dominated by interactions between the ejecta and the circumstellar medium, primarily the equatorial ring (ER). We analyze 3.3.Ms of NuSTAR data obtained between 2012 and 2020, and two decades of XMM-Newton data. Since similar to 2013, the flux below 2.keV has declined, the 3-8.keV flux has increased but has started to flatten, and the emission above 10.keV has remained nearly constant. The spectra are well described by a model with three thermal shock components. Two components at 0.3 and 0.9.keV are associated with dense clumps in the ER, and a 4.keV component may be a combination of emission from diffuse gas in the ER and the surrounding low-density H II region. We disfavor models that involve nonthermal X-ray emission and place constraints on nonthermal components, but cannot firmly exclude an underlying power law. Radioactive lines show a Ti-44 redshift of 670(+520) (-380) km s(-1), Ti-44 mass of ' 1.73(-0.29)(+0.27) 10(-4) M-circle dot, and Fe-55Y mass of <4.2 10(-4) M-circle dot. The 35-65.keV luminosity limit on the compact object is 2 ' 1034.erg.s-1, and < 15% of the 10-20.keV flux is pulsed. Considering previous limits, we conclude that there are currently no indications of a compact object, aside from a possible hint of dust heated by a neutron star in recent ALMA images.
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| 9. |
- Alp, Dennis, et al.
(författare)
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X-Ray Absorption in Young Core-collapse Supernova Remnants
- 2018
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 864:2
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Tidskriftsartikel (refereegranskat)abstract
- The material expelled by core-collapse supernova (SN) explosions absorbs X-rays from the central regions. We use SN models based on three-dimensional neutrino-driven explosions to estimate optical depths to the center of the explosion, compare different progenitor models, and investigate the effects of explosion asymmetries. The optical depths below 2 keV for progenitors with a remaining hydrogen envelope are expected to be high during the first century after the explosion due to photoabsorption. A typical optical depth is 100 t(4)(-2 )E(-2), where t(4) is the time since the explosion in units of 10,000 days (similar to 27 years) and E is the energy in units of keV. Compton scattering dominates above 50 keV, but the scattering depth is lower and reaches unity at similar to 1000 days at 1 MeV. The optical depths are approximately an order of magnitude lower for hydrogen-stripped progenitors. The metallicity of the SN ejecta is much higher than that in the interstellar medium, which enhances photoabsorption and makes absorption edges stronger. These results are applicable to young SN remnants in general, but we explore the effects on observations of SN 1987A and the compact object in Cas A in detail. For SN 1987A, the absorption is high and the X-ray upper limits of similar to 100 L-circle dot on a compact object are approximately an order of magnitude less constraining than previous estimates using other absorption models. The details are presented in an accompanying paper. For the central compact object in Cas A, we find no significant effects of our more detailed absorption model on the inferred surface temperature.
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| 10. |
- Alp, Dennis, MSc, 1992-, et al.
(författare)
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X-Ray and Gamma-Ray Emission from Core-collapse Supernovae : Comparison of Three-dimensional Neutrino-driven Explosions with SN 1987A
- 2019
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 882:1
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Tidskriftsartikel (refereegranskat)abstract
- During the first few hundred days after the explosion, core-collapse supernovae (SNe) emit down-scattered X-rays and gamma-rays originating from radioactive line emissions, primarily from the Ni-56 -> Co-56 -> Fe-56 chain. We use supernova (SN) models based on three-dimensional neutrino-driven explosion simulations of single stars and mergers to compute this emission and compare the predictions with observations of SN 1987A. A number of models are clearly excluded, showing that high-energy emission is a powerful way of discriminating between models. The best models are almost consistent with the observations, but differences that cannot be matched by a suitable choice of viewing angle are evident. Therefore, our self-consistent models suggest that neutrino-driven explosions are able to produce, in principle, sufficient mixing, although remaining discrepancies may require small changes to the progenitor structures. The soft X-ray cutoff is primarily determined by the metallicity of the progenitor envelope. The main effect of asymmetries is to vary the flux level by a factor of similar to 3. For the more asymmetric models, the shapes of the light curves also change. In addition to the models of SN 1987A, we investigate two models of SNe II-P and one model of a stripped-envelope SN IIb. The Type II-P models have observables similar to those of the models of SN 1987A, but the stripped-envelope SN model is significantly more luminous and evolves faster. Finally, we make simple predictions for future observations of nearby SNe.
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