Browsing by Subject "gamma-ray"
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Item Cluster Merger Shock Constraints On Particle Acceleration And Nonthermal Pressure In The Intracluster Medium(2008-03) Nakar, Ehud; Milosavljevic, Milos; Nagai, Daisuke; Milosavljevic, MilosX-ray observations of galaxy cluster merger shocks can be used to constrain nonthermal processes in the intracluster medium (ICM). The presence of nonthermal pressure components in the ICM, as well as the shock acceleration of particles and their escape, all affect shock jump conditions in distinct ways. Therefore, these processes can be constrained using X-ray surface brightness and temperature maps of merger shock fronts. Here we use these observations to place constraints on particle acceleration efficiency in intermediate Mach number ( M approximate to 2-3) shocks and explore the potential to constrain the contribution of nonthermal components ( e. g., cosmic rays, magnetic field, and turbulence) to ICM pressure in cluster outskirts. We model the hydrodynamic jump conditions in merger shocks discovered in the galaxy clusters A520 ( M approximate to 2) and 1E 0657-56 ( M approximate to 3) using a multifluid model comprising a thermal plasma, a nonthermal plasma, and a magnetic field. Based on the published X-ray spectroscopic data alone, we find that the fractional contribution of cosmic rays accelerated in these shocks is less than or similar to 10% of the shock downstream pressure. Current observations do not constrain the fractional contribution of nonthermal components to the pressure of the undisturbed shock upstream. Future X-ray observations, however, have the potential to either detect particle acceleration in these shocks through its effect on the shock dynamics, or place a lower limit on the nonthermal pressure contributions in the undisturbed ICM. We briefly discuss implications formodels of particle acceleration in collisionless shocks and the estimates of galaxy cluster masses derived from X-ray and Sunyaev-Zel'dovich effect observations.Item The Herschel Exploitation Of Local Galaxy Andromeda (HELGA). II. Dust And Gas In Andromeda(2012-09) Smith, Mathew W. L.; Eales, Steve A.; Gomez, H. L.; Roman-Duval, J.; Fritz, J.; Braun, R.; Baes, Maarten; Bendo, George J.; Blommaert, Jadl; Boquien, Mederic; Boselli, Alessandro; Clements, D. L.; Cooray, Asantha R.; Cortese, L.; de Looze, I.; Ford, G. P.; Gear, W. K.; Gentile, G.; Gordon, K. D.; Kirk, J.; Lebouteiller, Vianney; Madden, S.; Mentuch, E.; O'Halloran, B.; Page, M. J.; Schulz, B.; Spinoglio, L.; Verstappen, J.; Wilson, Christine D.; Thilker, D. A.; Mentuch, E.We present an analysis of the dust and gas in Andromeda, using Herschel images sampling the entire far-infrared peak. We fit a modified-blackbody model to similar to 4000 quasi-independent pixels with spatial resolution of similar to 140 pc and find that a variable dust-emissivity index (beta) is required to fit the data. We find no significant long-wavelength excess above this model, suggesting there is no cold dust component. We show that the gas-to-dust ratio varies radially, increasing from similar to 20 in the center to similar to 70 in the star-forming ring at 10 kpc, consistent with the metallicity gradient. In the 10 kpc ring the average beta is similar to 1.9, in good agreement with values determined for the Milky Way (MW). However, in contrast to the MW, we find significant radial variations in beta, which increases from 1.9 at 10 kpc to similar to 2.5 at a radius of 3.1 kpc and then decreases to 1.7 in the center. The dust temperature is fairly constant in the 10 kpc ring (ranging from 17 to 20 K), but increases strongly in the bulge to similar to 30 K. Within 3.1 kpc we find the dust temperature is highly correlated with the 3.6 mu m flux, suggesting the general stellar population in the bulge is the dominant source of dust heating there. At larger radii, there is a weak correlation between the star formation rate and dust temperature. We find no evidence for "dark gas" in M31 in contrast to recent results for the MW. Finally, we obtained an estimate of the CO X-factor by minimizing the dispersion in the gas-to-dust ratio, obtaining a value of (1.9 +/- 0.4) x 10(20) cm(-2) [K km s(-1)](-1).Item Supernova Spectra Below Strong Circumstellar Interaction(2015-02) Leloudas, G.; Hsiao, E. Y.; Johansson, J.; Maeda, K.; Moriya, T. J.; Nordin, J.; Petrushevska, T.; Silverman, J. M.; Sollerman, J.; Stritzinger, M. D.; Taddia, F.; Xu, D.; Silverman, J. M.We construct spectra of supernovae (SNe) interacting strongly with a circumstellar medium (CSM) by adding SN templates, a black-body continuum, and an emission-line spectrum. In a Monte Carlo simulation we vary a large number of parameters, such as the SN type, brightness and phase, the strength of the CSM interaction, the extinction, and the signal to noise ratio (S/N) of the observed spectrum. We generate more than 800 spectra, distribute them to ten different human classifiers, and study how the different simulation parameters affect the appearance of the spectra and their classification. The SNe IIn showing some structure over the continuum were characterized as >SNe IInS> to allow for a better quantification. We demonstrate that the flux ratio of the underlying SN to the continuum f(v) is the single most important parameter determining whether a spectrum can be classified correctly. Other parameters, such as extinction, S/N, and the width and strength of the emission lines, do not play a significant role. Thermonuclear SNe get progressively classified as Ia-CSM, IInS, and IIn as f(v) decreases. The transition between Ia-CSM and IInS occurs at f(v) similar to 0.2-0.3. It is therefore possible to determine that SNe Ia-CSM are found at the (un-extincted) magnitude range -19.5 > M > -21.6, in very good agreement with observations, and that the faintest SN IIn that can hide a SN Ia has M = -20.1. The literature sample of SNe Ia-CSM shows an association with 91T-like SNe Ia. Our experiment does not support that this association can be attributed to a luminosity bias (91T-like being brighter than normal events). We therefore conclude that this association has real physical origins and we propose that 91T-like explosions result from single degenerate progenitors that are responsible for the CSM. Despite the spectroscopic similarities between SNe Ibc and SNe Ia, the number of misclassifications between these types was very small in our simulation and mostly at low S/N. Combined with the SN luminosity function needed to reproduce the observed SN Ia-CSM luminosities, it is unlikely that SNe Ibc constitute an important contaminant within this sample. We show how Type II spectra transition to IIn and how the H alpha profiles vary with f(v). SNe IIn fainter than M = -17.2 are unable to mask SNe IIP brighter than M = -15. A more advanced simulation, including radiative transfer, shows that our simplified model is a good first order approximation. The spectra obtained are in good agreement with real data.