25 research outputs found
Dust Echos from Gamma Ray Bursts
The deviation from the power-law decline of the optical flux observed in GRB
970228 and GRB 980326 has been used recently to argue in favor of the
connection between GRBs and supernovae. We consider an alternative explanation
for this phenomenon, based on the scattering of a prompt optical burst by 0.1
solar masses of dust located beyond its sublimation radius 0.1-1 pc from the
burst. In both cases, the optical energy observed at the time of the first
detection of the afterglow suffices to produce an echo after 20-30 days, as
observed. Prompt optical monitoring of future bursts and multiband photometry
of the afterglows will enable quantitative tests of simple models of dust
reprocessing and a prediction of the source redshift.Comment: 4 pages including 3 postscript figures, LaTeX (emulateapj.sty;
newapa.sty for bibliography definitions); accepted for publication in ApJ
On The Synchrotron Self-Compton Emission from Relativistic Shocks and Its Implications for Gamma-Ray Burst Afterglows
We consider the effects of inverse Compton scattering of synchrotron photons
from relativistic electrons in GRB afterglows. We compute the spectrum of the
inverse Compton emission and find that it can dominate the total cooling rate
of the afterglow for several months or even years after the initial explosion.
We demonstrate that the presence of strong inverse Compton cooling can be
deduced from the effect it has on the time-evolution of the cooling break in
the synchrotron spectral component, and therefore on the optical and X-ray
afterglow lightcurves. We then show how the physical interpretation of the
observed characteristics of the synchrotron spectrum must be modified to take
into consideration this extra source of cooling, and give a revised
prescription for computing physical parameters characterizing the expanding
shock wave from the observed quantities. We find that for a given set of
observables (synchrotron break frequencies and fluxes) there is either no
consistent physical interpretation or two of them. Finally we discuss the
prospects of directly detecting the inverse Compton emission with Chandra. We
argue that such a detection is possible for GRBs exploding in a reasonably
dense (n>1 cm^-3) medium.Comment: 21 pages, ApJ submitte
Heating and Cooling of Hot Accretion Flows by Non Local Radiation
We consider non-local effects which arise when radiation emitted at one
radius of an accretion disk either heats or cools gas at other radii through
Compton scattering. We discuss three situations:
1. Radiation from the inner regions of an advection-dominated flow Compton
cooling gas at intermediate radii and Compton heating gas at large radii.
2. Soft radiation from an outer thin accretion disk Compton cooling a hot
one- or two-temperature flow on the inside.
3. Soft radiation from an inner thin accretion disk Compton cooling hot gas
in a surrounding one-temperature flow.
We describe how previous results are modified by these non-local
interactions. We find that Compton heating or cooling of the gas by the
radiation emitted in the inner regions of a hot flow is not important.
Likewise, Compton cooling by the soft photons from an outer thin disk is
negligible when the transition from a cold to a hot flow occurs at a radius
greater than some minimum . However, if the hot flow terminates at
, non-local cooling is so strong that the hot gas is cooled to
a thin disk configuration in a runaway process. In the case of a thin disk
surrounded by a hot one-temperature flow, we find that Compton cooling by soft
radiation dominates over local cooling in the hot gas for \dot{M} \gsim
10^{-3} \alpha \dot{M}_{Edd}, and R \lsim 10^4 R_{Schw}. As a result, the
maximum accretion rate for which an advection-dominated one-temperature
solution exists, decreases by a factor of , compared to the value
computed under an assumption of local energy balance.Comment: LaTeX aaspp.sty, 25 pages, and 6 figures; to appear in Ap
BVRI Light Curves for 22 Type Ia Supernovae
We present 1210 Johnson/Cousins B,V,R, and I photometric observations of 22
recent type Ia supernovae (SNe Ia): SN 1993ac, SN 1993ae, SN 1994M, SN 1994S,
SN 1994T, SN 1994Q, SN 1994ae, SN 1995D, SN 1995E, SN 1995al, SN 1995ac, SN
1995ak, SN 1995bd, SN 1996C, SN 1996X, SN 1996Z, SN 1996ab, SN 1996ai, SN
1996bk, SN 1996bl, SN 1996bo, and SN 1996bv. Most of the photometry was
obtained at the Fred Lawrence Whipple Observatory (FLWO) of the
Harvard-Smithsonian Center for Astrophysics in a cooperative observing plan
aimed at improving the data base for SN Ia. The redshifts of the sample range
from =1200 to 37000 km s with a mean of =7000 km s.Comment: Accepted to the Astronomical Journal, 41 pages, 8 figure
Modeling the Low State Spectrum of the X-Ray Nova XTE J1118+480
Based on recent multiwavelength observations of the new X-ray nova XTE
J1118+480, we can place strong constraints on the geometry of the accretion
flow in which a low/hard state spectrum, characteristic of an accreting black
hole binary, is produced. We argue that the absence of any soft blackbody-like
component in the X-ray band implies the existence of an extended hot
optically-thin region, with the optically-thick cool disk truncated at some
radius R_{tr} > 55 R_{Schw}. We show that such a model can indeed reproduce the
main features of the observed spectrum: the relatively high optical to X-ray
ratio, the sharp downturn in the far UV band and the hard X-ray spectrum. The
absence of the disk blackbody component also underscores the requirement that
the seed photons for thermal Comptonization be produced locally in the hot
flow, e.g. via synchrotron radiation. We attribute the observed spectral break
at 2 keV to absorption in a warm, partially ionized gas.Comment: 6 pages, including 1 figure; LaTeX (emulateapj5.sty), to appear in
Ap
Electron-Positron Pairs in Hot Accretion Flows and Thin Disk Coronae
We investigate equilibrium accretion flows dominated by pairs. We
consider one- and two-temperature accretion disk coronae above a thin disk, as
well as hot optically thin two-temperature accretion flows without an
underlying thin disk; we model the latter in the framework of
advection-dominated accretion flows (ADAFs). In all three cases we include
equipartition magnetic fields. We confirm the previous result that the
equilibrium density of pairs in two-temperature ADAFs is negligible; and show
that the inclusion of magnetic fields and the corresponding synchrotron cooling
reduces the pair density even further. Similarly, we find that pairs are
unimportant in two-temperature coronae. Even when the corona has significantly
enhanced heating by direct transfer of viscous dissipation in the thin disk to
the corona, the inefficient Coulomb coupling between protons and electrons acts
as a bottleneck and prevents the high compactness required for pair-dominated
solutions. Only in the case of a one-temperature corona model do we find
pair-dominated thermal equilibria. These pair-dominated solutions occur over a
limited range of optical depth and temperature.Comment: 38 pages, including 10 figures, LaTeX; to appear in Ap
Black Hole and Neutron Star Transients in Quiescence
We consider the X-ray luminosity difference between neutron star and black
hole soft X-ray transients (NS and BH SXTs) in quiescence. The current
observational data suggest that BH SXTs are significantly fainter than NS SXTs.
The luminosities of quiescent BH SXTs are consistent with the predictions of
binary evolution models for the mass transfer rate if (1) accretion occurs via
an ADAF in these systems and (2) the accreting compact objects have event
horizons. The luminosities of quiescent NS SXTs are not consistent with the
predictions of ADAF models when combined with binary evolution models, unless
most of the mass accreted in the ADAF is prevented from reaching the neutron
star surface. We consider the possibility that mass accretion is reduced in
quiescent NS SXTs because of an efficient propeller and develop a model of the
propeller effect that accounts for the observed luminosities. We argue that
modest winds from ADAFs are consistent with the observations while strong winds
are probably not.Comment: LateX, 37 pages, 7 figures; Accepted for publication in The
Astrophysical Journa
Advection-Dominated Accretion and the Spectral States of Black Hole X-Ray Binaries: Application to Nova Muscae 1991
We present a self-consistent model of accretion flows which unifies four
distinct spectral states observed in black hole X-ray binaries: quiescent, low,
intermediate and high states. In the quiescent, low and intermediate states,
the flow consists of an inner hot advection-dominated part extending from the
black hole horizon to a transition radius and an outer thin disk. In the high
state the thin disk is present at all radii. The model is essentially
parameter-free and treats consistently the dynamics of the accretion flow, the
thermal balance of the ions and electrons, and the radiation processes in the
accreting gas. With increasing mass accretion rate, the model goes through a
sequence of stages for which the computed spectra resemble very well
observations of the four spectral states; in particular, the low-to-high state
transition observed in black hole binaries is naturally explained as resulting
from a decrease in the transition radius. We also make a tentative proposal for
the very high state, but this aspect of the model is less secure.
We test the model against observations of the soft X-ray transient Nova
Muscae during its 1991 outburst. The model reproduces the observed lightcurves
and spectra surprisingly well, and makes a number of predictions which can be
tested with future observations.Comment: 68 pages, LaTeX, includes 1 table (forgotten in the previous version)
and 14 figures; submitted to The Astrophysical Journa