16 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
Structure and Magnetic Fields in the Precessing Jet System SS 433 II. Intrinsic Brightness of the Jets
Deep Very Large Array imaging of the binary X-ray source SS 433, sometimes
classified as a microquasar, has been used to study the intrinsic brightness
distribution and evolution of its radio jets. The intrinsic brightness of the
jets as a function of age at emission of the jet material tau is recovered by
removal of the Doppler boosting and projection effects. We find that
intrinsically the two jets are remarkably similar when compared for equal tau,
and that they are best described by Doppler boosting of the form D^{2+alpha},
as expected for continuous jets. The intrinsic brightnesses of the jets as
functions of age behave in complex ways. In the age range 60 < tau < 150 days,
the jet decays are best represented by exponential functions of tau, but linear
or power law functions are not statistically excluded. This is followed by a
region out to tau ~ 250 days during which the intrinsic brightness is
essentially constant. At later times the jet decay can be fit roughly as
exponential or power law functions of tau.Comment: 30 Pages, 11 Figures, Submitted to Ap
Radio Interferometric Planet Search II: Constraints on sub-Jupiter-Mass Companions to GJ 896A
We present results from the Radio Interferometric Planet (RIPL) search for
compan- ions to the nearby star GJ 896A. We present 11 observations over 4.9
years. Fitting astrometric parameters to the data reveals a residual with
peak-to-peak amplitude of ~ 3 mas in right ascension. This residual is well-fit
by an acceleration term of 0.458 \pm 0.032 mas/y^2. The parallax is fit to an
accuracy of 0.2 mas and the proper motion terms are fit to accuracies of 0.01
mas/y. After fitting astrometric and acceleration terms residuals are 0.26 mas
in each coordinate, demonstrating that stellar jitter does not limit the
ability to carry out radio astrometric planet detection and characterization.
The acceleration term originates in part from the companion GJ 896B but the
amplitude of the acceleration in declination is not accurately predicted by the
orbital model. The acceleration sets a mass upper limit of 0.15 MJ at a
semi-major axis of 2 AU for a planetary companion to GJ 896A. For semi-major
axes between 0.3 and 2 AU upper limits are determined by the maximum angular
separation; the upper limits scale from the minimum value in proportion to the
inverse of the radius. Upper limits at larger radii are set by the acceleration
and scale as the radius squared. An improved solution for the stellar binary
system could improve the exoplanet mass sensitivity by an order of magnitude.Comment: Accepted for publication in Ap