1,458 research outputs found
Supernova Fallback onto Magnetars and Propeller-powered Supernovae
We explore fallback accretion onto newly born magnetars during the supernova of massive stars. Strong magnetic fields (~10^(15) G) and short spin periods (~1-10 ms) have an important influence on how the magnetar interacts with the infalling material. At long spin periods, weak magnetic fields, and high accretion rates, sufficient material is accreted to form a black hole, as is commonly found for massive progenitor stars. When B ≾ 5 × 10^(14) G, accretion causes the magnetar to spin sufficiently rapidly to deform triaxially and produces gravitational waves, but only for ≈50-200 s until it collapses to a black hole. Conversely, at short spin periods, strong magnetic fields, and low accretion rates, the magnetar is in the "propeller regime" and avoids becoming a black hole by expelling incoming material. This process spins down the magnetar, so that gravitational waves are only expected if the initial protoneutron star is spinning rapidly. Even when the magnetar survives, it accretes at least ≈0.3 M_☉, so we expect magnetars born within these types of environments to be more massive than the 1.4 M_☉ typically associated with neutron stars. The propeller mechanism converts the ~10^(52)erg of spin energy in the magnetar into the kinetic energy of an outflow, which shock heats the outgoing supernova ejecta during the first ~10-30 s. For a small ~5 M_☉ hydrogen-poor envelope, this energy creates a brighter, faster evolving supernova with high ejecta velocities ~(1-3) × 10^4 km s^(–1) and may appear as a broad-lined Type Ib/c supernova. For a large ≳ 10 M_☉ hydrogen-rich envelope, the result is a bright Type IIP supernova with a plateau luminosity of ≳ 10^(43)erg s^(–1) lasting for a timescale of ~60-80 days
Optical and X-ray emission from stable millisecond magnetars formed from the merger of binary neutron stars
The coalescence of binary neutron stars (NSs) may in some cases produce a
stable massive NS remnant rather than a black hole. Due to the substantial
angular momentum from the binary, such a remnant is born rapidly rotating and
likely acquires a strong magnetic field (a `millisecond magnetar'). Magnetic
spin-down deposits a large fraction of the rotational energy from the magnetar
behind the small quantity of mass ejected during the merger. This has the
potential for creating a bright transient that could be useful for determining
whether a NS or black hole was formed in the merger. We investigate the
expected signature of such an event, including for the first time the important
impact of electron/positron pairs injected by the millisecond magnetar into the
surrounding nebula. These pairs cool via synchrotron and inverse Compton
emission, producing a pair cascade and hard X-ray spectrum. A fraction of these
X-rays are absorbed by the ejecta walls and re-emitted as thermal radiation,
leading to an optical/UV transient peaking at a luminosity of ~1e43-1e44 erg/s
on a timescale of several hours to days. This is dimmer than predicted by
simpler analytic models because the large optical depth of electron/positron
pairs across the nebula suppresses the efficiency with which the magnetar spin
down luminosity is thermalized. Nevertheless, the optical/UV emission is more
than two orders of magnitude brighter than a radioactively powered `kilonova.'
In some cases nebular X-rays are sufficiently luminous to re-ionize the ejecta,
in which case non-thermal X-rays escape the ejecta unattenuated with a similar
peak luminosity and timescale as the optical radiation. We discuss the
implications of our results for the temporally extended X-ray emission that is
observed to follow some short gamma-ray bursts (GRBs), including the kilonova
candidates GRB 080503 and GRB 130603B.Comment: 13 pages, 8 figures, 2 appendices, submitted to MNRA
Can the jet steepen the light curves of GRB afterglow?
Beaming of relativistic ejecta in GRBs has been postulated by many authors in
order to reduce the total GRB energy, thus it is very important to look for the
observational evidence of beaming. Rhoads (1999) has pointed out that the
dynamics of the blast wave, which is formed when the beamed ejecta sweeping the
external medium, will be significantly modified by the sideways expansion due
to the increased swept up matter. He claimed that shortly after the bulk
Lorentz factor () of the blast wave drops below the inverse of the
initial opening angle () of the beamed ejecta, there will be a
sharp break in the afterglow light curves. However, some other authors have
performed numerical calculations and shown that the break of the light curve is
weaker and much smoother than the one analytically predicted. In this paper we
reanalyse the dynamical evolution of the jet blast wave, calculate the jet
emission analytically, we find that the sharp break predicted by Rhoads will
actually not exist, and for most cases the afterglow light curve will almost
not be affected by sideways expansion unless the beaming angle is extremely
small. We demonstrate that only when , the afterglow light
curves may be steepened by sideways expansion, and in fact there cannot be two
breaks as claimed before. We have also constructed a simple numerical code to
verify our conclusion.Comment: 12 pages, 2 figures, accepted by ApJ, added numerical calculation
Numerical Modeling of the Early Light Curves of Type IIP Supernovae
The early rise of Type IIP supernovae (SN IIP) provides important information
for constraining the properties of their progenitors. This can in turn be
compared to pre-explosion imaging constraints and stellar models to develop a
more complete picture of how massive stars evolve and end their lives. Using
the SuperNova Explosion Code (SNEC), we model the first 40 days of SNe IIP to
better understand what constraints can be derived from their early light
curves. We use two sets of red supergiant progenitor models with zero-age main
sequence masses in the range between 9 Msol and 20 Msol. We find that the early
properties of the light curve depend most sensitively on the radius of the
progenitor, and thus provide a relation between the g-band rise time and the
radius at the time of explosion. This relation will be useful for deriving
constraints on progenitors from future observations, especially in cases where
detailed modeling of the entire rise is not practical. When comparing to
observed rise times, the radii we find are a factor of a few larger than
previous semi-analytic derivations and generally in better agreement with what
is found with current stellar evolution calculations.Comment: 8 pages, 7 figure
General properties of X-Ray Riches and X-Ray Flashes in comparison with Gamma-Ray Bursts
The X-Ray Flashes (XRFs) and X-Ray Riches (XRRs) are two
subclasses of Gamma-Ray Bursts (GRBs), which have respectively no detection in the gamma-ray energy and very faint gamma to X-ray fluence. To investigate their nature we compiled a sample of 54 events observed by BeppoSAX and HETE-2, available in literature and from the web. To classify XRRs/XRFs for those two experiments, we adopted the same spectral hardness ratio. We studied their prompt
emission in the X and γ range and their spectral parameters and compared them with those of GRBs. We find XRRs/XRFs are characterized by a significantly smaller value of Epeak while the spectral slopes α and β are quite similar. We analysed also the optical and X-ray afterglow fluxes and their ratio and compared them with that obtained for GRBs. We find that the distribution of X-ray flux of XRR/XRF
afterglow is consistent with that of GRBs, which is incompatible with the off-axis model. For example, in the inhomogeneous jet model it implies that the observer
anglefor an XRR/XRF is at most 2◦. It is also not explained by the high Redshift scenario
On Spectral and Temporal Variability in Blazars and Gamma Ray Bursts
A simple model for variability in relativistic plasma outflows is studied, in
which nonthermal electrons are continuously and uniformly injected in the
comoving frame over a time interval dt. The evolution of the electron
distribution is assumed to be dominated by synchrotron losses, and the energy-
and time-dependence of the synchrotron and synchrotron self-Compton (SSC)
fluxes are calculated for a power-law electron injection function with index s
= 2. The mean time of a flare or pulse measured at photon energy E with respect
to the onset of the injection event varies as E^{-1/2} and E^{-1/4} for
synchrotron and SSC processes, respectively, until the time approaches the
limiting intrinsic mean time (1+z)dt/(2 D), where z is the redshift and D is
the Doppler factor. This dependence is in accord with recent analyses of blazar
and GRB emissions, and suggests a method to discriminate between external
Compton and SSC models of high-energy gamma radiation from blazars and GRBs.
The qualititative behavior of the X-ray spectral index/flux relation observed
from BL Lac objects can be explained with this model. This demonstrates that
synchrotron losses are primarily responsible for the X-ray variability behavior
and strengthens a new test for beaming from correlated hard X-ray/TeV
observations.Comment: 10 pages, 2 figures, accepted for publication in Astrophysical
Journal Letters; uses aaspp4.sty, epsf.st
Fireballs Loading and the Blast Wave Model of Gamma Ray Bursts
A simple function for the spectral power
is proposed to model, with 9 parameters, the spectral and temporal evolution of
the observed nonthermal synchrotron power flux from GRBs in the blast wave
model. Here mc is the observed dimensionless photon
energy and is the observing time. Assumptions and an issue of lack of
self-consistency are spelled out. The spectra are found to be most sensitive to
the baryon loading, expressed in terms of the initial bulk Lorentz factor
, and an equipartition term which is assumed to be constant in
time and independent of . Expressions are given for the peak spectral
power at the photon energy of the spectral power peak. A general rule is that the total
fireball particle kinetic energy , where is the deceleration time scale and is the maximum measured bolometric
power output in radiation, during which it is carried primarily by photons with
energy .Comment: 26 pages, including 4 figures, uses epsf.sty, rotate.sty; submitted
to ApJ; revised version with extended introduction, redrawn figures, and
correction
Delayed soft X-ray emission lines in the afterglow of GRB 030227
Strong, delayed X-ray line emission is detected in the afterglow of GRB
030227, appearing near the end of the XMM-Newton observation, nearly twenty
hours after the burst. The observed flux in the lines, not simply the
equivalent width, sharply increases from an undetectable level (<1.7e-14
erg/cm^2/s, 3 sigma) to 4.1e-14 erg/cm^2/s in the final 9.7 ks. The line
emission alone has nearly twice as many detected photons as any previous
detection of X-ray lines. The lines correspond well to hydrogen and/or
helium-like emission from Mg, Si, S, Ar and Ca at a redshift z=1.39. There is
no evidence for Fe, Co or Ni--the ultimate iron abundance must be less than a
tenth that of the lighter metals. If the supernova and GRB events are nearly
simultaneous there must be continuing, sporadic power output after the GRB of a
luminosity >~5e46 erg/s, exceeding all but the most powerful quasars.Comment: Submitted to ApJL. 14 pages, 3 figures with AASLaTe
The Non-Relativistic Evolution of GRBs 980703 and 970508: Beaming-Independent Calorimetry
We use the Sedov-Taylor self-similar solution to model the radio emission
from the gamma-ray bursts (GRBs) 980703 and 970508, when the blastwave has
decelerated to non-relativistic velocities. This approach allows us to infer
the energy independent of jet collimation. We find that for GRB 980703 the
kinetic energy at the time of the transition to non-relativistic evolution,
t_NR ~ 40 d, is E_ST ~ (1-6)e51 erg. For GRB 970508 we find E_ST ~ 3e51 erg at
t_NR ~ 100 d, nearly an order of magnitude higher than the energy derived in
Frail, Waxman and Kulkarni (2000). This is due primarily to revised
cosmological parameters and partly to the maximum likelihood fit we use here.
Taking into account radiative losses prior to t_NR, the inferred energies agree
well with those derived from the early, relativistic evolution of the
afterglow. Thus, the analysis presented here provides a robust,
geometry-independent confirmation that the energy scale of cosmological GRBs is
about 5e51 erg, and additionally shows that the central engine in these two
bursts did not produce a significant amount of energy in mildly relativistic
ejecta at late time. Furthermore, a comparison to the prompt energy release
reveals a wide dispersion in the gamma-ray efficiency, strengthening our
growing understanding that E_gamma is a not a reliable proxy for the total
energy.Comment: Submitted to ApJ; 13 pages, 6 figures, 1 table; high-resolution
figures can be found at: http://www.astro.caltech.edu/~ejb/NR
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