arXiv subjects

Lih-Sin The

Publications and source records attributed to Lih-Sin The.

7 recordsLinked to original sources

arXiv2019

Supermassive black holes at high redshifts

MeV blazars are the most luminous persistent sources in the Universe and emit most of their energy in the MeV band. These objects display very large jet powers and accretion luminosities and are known to host black holes with a mass often exceeding $10^9 M_{\odot}$. An MeV survey, performed by a new generation MeV telescope which will bridge the entire energy and sensitivity gap between the current generation of hard X-ray and gamma-ray instruments, will detect $>$1000 MeV blazars up to a redshift of $z=5-6$. Here we show that this would allow us: 1) to probe the formation and growth mechanisms of supermassive black holes at high redshifts, 2) to pinpoint the location of the emission region in powerful blazars, 3) to determine how accretion and black hole spin interplay to power the jet.

astro-ph.HE↗

arXiv2016

The origin of the cosmic gamma-ray background in the MeV range

There has been much debate about the origin of the diffuse $γ$--ray background in the MeV range. At lower energies, AGNs and Seyfert galaxies can explain the background, but not above $\simeq$0.3 MeV. Beyond $\sim$10 MeV blazars appear to account for the flux observed. That leaves an unexplained gap for which different candidates have been proposed, including annihilations of WIMPS. One candidate are Type Ia supernovae (SNe Ia). Early studies concluded that they were able to account for the $γ$--ray background in the gap, while later work attributed a significantly lower contribution to them. All those estimates were based on SN Ia explosion models which did not reflect the full 3D hydrodynamics of SNe Ia explosions. In addition, new measurements obtained since 2010 have provided new, direct estimates of high-z SNe Ia rates beyond $z\sim$2. We take into account these new advances to see the predicted contribution to the gamma--ray background. We use here a wide variety of explosion models and a plethora of new measurements of SNe Ia rates. SNe Ia still fall short of the observed background. Only for a fit, which would imply $\sim$150\% systematic error in detecting SNe Ia events, do the theoretical predictions approach the observed fluxes. This fit is, however, at odds at the highest redshifts with recent SN Ia rates estimates. Other astrophysical sources such as FSRQs do match the observed flux levels in the MeV regime, while SNe Ia make up to 30--50\% of the observed flux.

astro-ph.HE↗

arXiv2014

Expectations for the Hard X-ray Continuum and Gamma-ray Line Fluxes from the Type Ia supernova SN 2014J in M82

The hard X-ray continuum and gamma-ray lines from a Type Ia supernova dominate its integrated photon emissions and can provide unique diagnostics of the mass of the ejecta, the $^{56}$Ni yield and spatial distribution, its kinetic energy and expansion speed, and the mechanism of explosion. Such signatures and their time behavior "X-ray" the bulk debris field in direct fashion, and do not depend upon the oftimes problematic and elaborate UV, optical, and near-infrared spectroscopy and radiative transfer that have informed the study of these events for decades. However, to date no hard photons have ever been detected from a Type Ia supernova in explosion. With the advent of the supernova SN 2014J in M82, at a distance of $\sim$3.5 Mpc, this situation may soon change. Both NuSTAR and INTEGRAL have the potential to detect SN 2014J, and, if spectra and light curves can be measured, would usefully constrain the various explosion models published during the last $\sim$thirty years. In support of these observational campaigns, we provide predictions for the hard X-ray continuum and gamma-line emissions for fifteen Type Ia explosion models gleaned from the literature. The model set, containing as it does deflagration, delayed detonation, merger detonation, pulsational delayed detonation, and sub-Chandrasekhar helium detonation models, collectively spans a wide range of properties and, hence, signatures. We provide a brief discussion of various diagnostics (with examples), but importantly make the spectral and line results available electronically to aid in the interpretation of the anticipated data.

astro-ph.HE↗

arXiv2006

s-Process Nucleosynthesis in Advanced Burning Phases of Massive Stars

We present a detailed study of s-process nucleosynthesis in massive stars of solar-like initial composition and masses 15, 20,25, and 30 Msun. We update our previous results of s-process nucleosynthesis during the core He-burning of these stars and then focus on an analysis of the s-process under the physical conditions encountered during the shell-carbon burning. We show that the recent compilation of the Ne22(alpha,n)Mg25 rate leads to a remarkable reduction of the efficiency of the s-process during core He-burning. In particular, this rate leads to the lowest overproduction factor of Kr80 found to date during core He-burning in massive stars. The s-process yields resulting from shell carbon burning turn out to be very sensitive to the structural evolution of the carbon shell. This structure is influenced by the mass fraction of C12 attained at the end of core helium burning, which in turn is mainly determined by the C12(alpha,gamma)O16 reaction. The still present uncertainty in the rate for this reaction implies that the s-process in massive stars is also subject to this uncertainty. We identify some isotopes like Zn70 and Rb87 as the signatures of the s-process during shell carbon burning in massive stars. In determining the relative contribution of our s-only stellar yields to the solar abundances, we find it is important to take into account the neutron exposure of shell carbon burning. When we analyze our yields with a Salpeter Initial Mass Function, we find that massive stars contribute at least 40% to s-only nuclei with mass A <= 87. For s-only nuclei with mass A >90, massive stars contribute on average ~7%, except for Gd152, Os187, and Hg198 which are ~14%, \~13%, and ~11%, respectively.

astro-ph↗

arXiv2004

Evolution of Massive Stars Up to the End of Central Oxygen Burning

We present a detailed study of the evolution of massive stars of masses 15, 20, 25 and 30 $\msun$ assuming solar-like initial chemical composition. The stellar sequences were evolved through the advanced burning phases up to the end of core oxygen burning. We present a careful analysis of the physical characteristics of the stellar models. In particular, we investigate the effect of the still unsettled reaction $^{12}$C($α$,$γ$)$^{16}$O on the advanced evolution by using recent compilations of this rate. We find that this rate has a significant impact on the evolution not only during the core helium burning phase, but also during the late burning phases, especially the shell carbon-burning. We have also considered the effect of different treatment of convective instability based on the Ledoux criterion in regions of varying molecular weight gradient during the hydrogen and helium burning phases. We compare our results with other investigations whenever available. Finally, our present study constitutes the basis of analyzing the nucleosynthesis processes in massive stars. In particular we will present a detail analysis of the {\it s}-process in a forthcoming paper.

astro-ph↗

arXiv2001

Late Light Curves of Type Ia Supernovae

We extend earlier efforts to determine whether the late (t$\geq$60d) light-curves of type Ia SNe are better explained by the escape of positrons from the ejecta or by the complete deposition of positron kinetic energy in a trapping magnetic field We find that applying a filter efficiency correction, derived from measured spectra, to B, V, R, and I light-curves after day 50 can produce a consistent bolometric light-curve. The V band is an accurate indicator of total emission in the 3500$Å$ - 9700$Å$ range, with a constant fraction ($\sim$25%) appearing in the V band after day 50. This suggests that the V band scales with the bolometric luminosity, and that the deposited energy is instantaneously recycled into optical emission during this epoch. Varying bolometric corrections for the other bands are derived. We see significant evolution of the colors of SNe Ia between day 50 and day 170. We suggest that this may be due to the transition from spectra dominated by emission lines from the radioactive nucleus, $^{56}$Co, to those from the stable daughter nucleus, $^{56}$Fe. We show that the B, V, R, and I band light-curves of SNe Ia after t$\geq$60d can be completely explained with energy deposition from $^{56}$Co decay photons and positrons if substantial positron escape occurs.

astro-ph↗

arXiv1997

Is Positron Escape Seen In the Late-time Light Curves of Type Ia Supernovae?

At times later than 200 days, Type Ia SN light curves are dominated by the kinetic energy deposition from positrons created in the decay of $^{56}$Co. In this paper, the transport of positrons after emission are simulated, for deflagration and delayed detonation models, assuming various configurations of the magnetic field. We find the light curve due to positron kinetic energy has a different shape for a radially combed magnetic field than it does for a tangled field that traps positrons. The radial field light curves fit the observed light curves better than do the trapping field light curves for the four SNe used in this study. The radial field permits a much larger fraction of the positrons to escape, perhaps enough to explain a large percentage of the 511 keV annihilation radiation from the Galactic plane observed by OSSE.

astro-ph↗