11 research outputs found
On the Anti-Correlation between Duration and Redshift in Gamma-ray Bursts
For gamma-ray bursts (GRBs) with durations greater than two seconds
(so-called long GRBs), the intrinsic prompt gamma-ray emission appears, on
average, to last longer for bursts at lower redshifts. We explore the nature of
this duration-redshift anti-correlation, describing systems and conditions in
which this cosmological evolution could arise. In particular, we explore its
dependence on metallicity of a massive star progenitor, as we can securely
count on the average stellar metallicity to increase with decreasing redshift.
Although higher metallicity/lower redshift stars lose mass and angular momentum
through line-driven winds, in some cases these stars are able to form more
extended accretion disks when they collapse, potentially leading to longer
duration GRBs. We also examine how this duration-redshift trend may show up in
interacting binary models composed of a massive star and compact object
companion, recently suggested to be the progenitors of radio bright GRBs. Under
certain conditions, mass loss and equation of state effects from higher
metallicity, lower redshift massive stars can decrease the binary separation.
This can then lead to spin-up of the massive star and allow for a longer
duration GRB upon the massive star's collapse. Finally, the duration-redshift
trend may also be supported by a relatively larger population of
small-separation binaries born in situ at low redshift.Comment: Accepted to Ap
Conditions for Reionizing the Universe with A Low Galaxy Ionizing Photon Escape Fraction
We explore scenarios for reionizing the intergalactic medium with low galaxy
ionizing photon escape fractions. We combine simulation-based halo-mass
dependent escape fractions with an extrapolation of the observed galaxy
rest-ultraviolet luminosity functions to solve for the reionization history
from z=20 to z=4. We explore the posterior distributions for key unknown
quantities, including the limiting halo mass for star-formation, the ionizing
photon production efficiency, and a potential contribution from active galactic
nuclei (AGN). We marginalize over the allowable parameter space using a Markov
Chain Monte Carlo method, finding a solution which satisfies the most
model-independent constraints on reionization. Our fiducial model can match
observational constraints with an average escape fraction of <5% throughout the
bulk of the epoch of reionization if: i) galaxies form stars down to the atomic
cooling limit before reionization and a photosuppression mass of
log(M_h/Msol)~9 during/after reionization (-13<M_UV,lim<-11); ii) galaxies
become more efficient producers of ionizing photons at higher redshifts and
fainter magnitudes, and iii) there is a significant, but sub-dominant,
contribution by AGN at z -15)
dominate the ionizing emissivity, leading to an earlier start to reionization
and a smoother evolution of the ionized volume filling fraction than models
which assume a single escape fraction at all redshifts and luminosities. The
ionizing emissivity from this model is consistent with observations at z=4-5
(and below, when extrapolated), in contrast to some models which assume a
single escape fraction. Our predicted ionized volume filling fraction at z=7 of
Q_HII=78% (+\- 8%) is in ~1-2 sigma tension with observations of Lya emitters
at z~7 and the damping wing analyses of the two known z>7 quasars, which prefer
Q_HII,z=7~40-50%.Comment: 45 pages, 21 figures, accepted for publication in the Astrophysical
Journa