6,725 research outputs found
Did GW170817 harbor a pulsar?
If the progenitor of GW170817 harbored a pulsar, then a Poynting flux
dominated bow-shock cavity would have been expected to form around the
traveling binary. The characteristic size of this evacuated region depends
strongly on the spin-down evolution of the pulsar companion, which in turn
depends on the merging timescale of the system. If this evacuated region is
able to grow to a sufficiently large scale, then the deceleration of the jet,
and thus the onset of the afterglow, would be noticeably delayed. The first
detection of afterglow emission, which was uncovered 9.2 days after the
-ray burst trigger, can thus be used to constrain the size of a
pre-existing pulsar-wind cavity. We use this information, together with a model
of the jet to place limits on the presence of a pulsar in GW170817 and discuss
the derived constraints in the context of the observed double neutron star
binary population. We find that the majority of Galactic systems that are close
enough to merge within a Hubble time would have carved a discernibly large
pulsar-wind cavity, inconsistent with the onset timescale of the X-ray
afterglow of GW170817. Conversely, the recently detected system J1913+1102,
which host a low-luminosity pulsar, provides a congruous Milky Way analog of
GW170817's progenitor model. This study highlights the potential of the
proposed observational test for gaining insight into the origin of double
neutron star binaries, in particular if the properties of Galactic systems are
representative of the overall merging population.Comment: Accepted for publication in ApJL, 6 pages, 5 figure
Events in the life of a cocoon surrounding a light, collapsar jet
According to the collapsar model, gamma-ray bursts are thought to be produced
in shocks that occur after the relativistic jet has broken free from the
stellar envelope. If the mass density of the collimated outflow is less than
that of the stellar envelope, the jet will then be surrounded by a cocoon of
relativistic plasma. This material would itself be able to escape along the
direction of least resistance, which is likely to be the rotation axis of the
stellar progenitor, and accelerate in approximately the same way as an
impulsive fireball. We discuss how the properties of the stellar envelope have
a decisive effect on the appearance of a cocoon propagating through it. The
relativistic material that accumulated in the cocoon would have enough kinetic
energy to substantially alter the structure of the relativistic outflow, if not
in fact provide much of the observed explosive power. Shock waves within this
plasma can produce gamma-ray and X-ray transients, in addition to the standard
afterglow emission that would arise from the deceleration shock of the cocoon
fireball.Comment: 16 pages, 5 figures, slightly revised version, accepted for
publication in MNRA
Quiescent times in gamma-ray bursts: II. Dormant periods in the central engine?
Within the framework of the internal-external shocks model for gamma-ray
bursts, we study the various mechanisms that can give rise to quiescent times
in the observed gamma-ray light-curves. In particular, we look for the
signatures that can provide us with evidence as to whether or not the central
engine goes dormant for a period of time comparable to the duration of the
gaps. We show that the properties of the prompt gamma-ray and X-ray emission
can in principle determine whether the quiescent episodes are due to a
modulated relativistic wind or a switching off of the central engine. We
suggest that detailed observations of the prompt afterglow emission from the
reverse shock will strongly constrain the possible mechanisms for the
production of quiescent times in gamma-ray bursts.Comment: 12 pages, 8 figures, with final revisions, MNRAS in pres
The Pulse Scale Conjecture and the Case of BATSE Trigger 2193
The pulses that compose gamma-ray bursts (GRBs) are hypothesized to have the
same shape at all energies, differing only by scale factors in time and
amplitude. This "Pulse Scale Conjecture" is confirmed here between energy
channels of the dominant pulse in GRB 930214c (BATSE trigger 2193), the single
most fluent single-pulsed GRB that occurred before May 1998. Furthermore,
pulses are hypothesized to start at the same time independent of energy. This
"Pulse Start Conjecture" is also confirmed in GRB 930214c. Analysis of GRB
930214c also shows that, in general, higher energy channels show shorter
temporal scale factors. Over the energy range 100 KeV - 1 MeV, it is found that
the temporal scale factors between a pulse measured at different energies are
related to that energy by a power law, possibly indicating a simple
relativistic mechanism is at work. To test robustness, the Pulse Start and
Pulse Scale Conjectures were also tested on the four next most fluent
single-pulse GRBs. Three of the four clearly passed, with a second smaller
pulse possibly confounding the discrepant test. Models where the pulse rise and
decay are created by different phenomena do not typically predict pulses that
satisfy both the Pulse Start Conjecture and the Pulse Scale Conjecture, unless
both processes are seen to undergo common time dilation.Comment: 19 pages, 9 figures, analysis revised and extended, accepted to Ap
Tidal Disruption Event Host Galaxies in the Context of the Local Galaxy Population
We study the properties of tidal disruption event (TDE) host galaxies in the
context of a catalog of ~500,000 galaxies from the Sloan Digital Sky Survey. We
explore whether selection effects can account for the overrepresentation of
TDEs in E+A/post-starburst galaxies by creating matched galaxy samples.
Accounting for possible selection effects due to black hole (BH) mass, redshift
completeness, strong AGN presence, bulge colors, and surface brightness can
reduce the apparent overrepresentation of TDEs in E+A host galaxies by a factor
of ~4 (from ~100-190 to ~25-48), but cannot fully explain the
preference. We find that TDE host galaxies have atypical photometric properties
compared to similar, "typical" galaxies. In particular, TDE host galaxies tend
to live in or near the "green valley" between star-forming and passive
galaxies, and have bluer bulge colors ( mag), lower
half-light surface brightnesses (by ~1 mag/arcsec), higher Sersic indices
(), and higher bulge-to-total-light ratios () than galaxies with matched BH masses. We find that TDE host
galaxies appear more centrally concentrated and that all have high galaxy
Sersic indices and fractions---on average in the top 10% of galaxies of
the same BH mass---suggesting a higher nuclear stellar density. We identify a
region in Sersic index and BH mass parameter space that contains ~2% of our
reference catalog galaxies but of TDE host galaxies. The unique
photometric properties of TDE host galaxies may be useful for selecting
candidate TDEs for spectroscopic follow-up observations in large transient
surveys.Comment: 26 pages, 11 figures, 5 tables. Published in Ap
r-process enrichment of ultra-faint dwarf galaxies by fast merging double neutron stars
The recent aLIGO/aVirgo discovery of gravitational waves from the neutron
star merger (NSM) GW170817 and the follow up kilonova observations have shown
that NSMs produce copious amount of r-process material. However, it is
difficult to reconcile the large natal kicks and long average merging times of
Double Neutron Stars (DNSs), with the levels of r-process enrichment seen in
ultra faint dwarf (UFD) galaxies such as Reticulum II and Tucana III. Assuming
that such dwarf systems have lost a significant fraction of their stellar mass
through tidal stripping, we conclude that contrary to most current models, it
is the DNSs with rather large natal kicks but very short merging timescales
that can enrich UFD-type galaxies. These binaries are either on highly
eccentric orbits, or form with very short separations due to an additional
mass-transfer between the first-born neutron star and a naked helium star,
progenitor of the second-born neutron star. These DNSs are born with a
frequency that agrees with the statistics of the r-process UFDs, and merge well
within the virial radius of their host halos, therefore contributing
significantly to their r-process enrichment.Comment: Accepted for publication in Ap
GRB990123: Evidence that the Gamma Rays Come from a Central Engine
GRB990123 was a long complex gamma-ray burst with an optical transient that
started early within the gamma-ray phase. The peak and power law decay of the
early optical emission strongly indicates the presence of a decelerating
relativistic shell during that phase. Prior to this burst, it was not known if
the shell decelerated during the burst, so an external shock origin for the
gamma rays was still possible. If the gamma-rays are produced in the external
shock, then the pulse widths should reflect the observed deceleration of the
shell and increase by about 2.3. We analyze the fine time structure observed in
the gamma-ray data from BATSE and determine that the width of the peaks do not
increase as expected for a decelerating shell; the later pulses are, at most, a
factor of 1.15 longer than the earlier pulses. We also analyze the variability
to determine what fraction of the shell's surface could be involved in the
production of the gamma rays, the so-called surface filling factor. For
GRB990123 we find a filling factor of 0.008. The lack of pulse width evolution
eliminates the only remaining kinematically acceptable external shock
explanation for the gamma-ray phase and, thus, the gamma rays must originate at
a central engine.Comment: 14 pages, 3 embedded figues, Latex, Submitted to ApJ
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