25 research outputs found
Bolometric light curves of supernovae and post-explosion magnetic fields
The various effects leading to diversity in the bolometric light curves of
supernovae are examined: nucleosynthesis, kinematic differences, ejected mass,
degree of mixing, and configuration and intensity of the magnetic field are
discussed. In Type Ia supernovae, a departure in the bolometric light curve
from the full-trapping decline of Co can occur within the two and a half
years after the explosion, depending on the evolutionary path followed by the
WD during the accretion phase. If convection has developed in the WD core
during the presupernova evolution, starting several thousand years before the
explosion, a tangled magnetic field close to the equipartition value should
have grown in the WD. Such an intense magnetic field would confine positrons
where they originate from the Co decays, and preclude a strong departure
from the full-trapping decline, as the supernova expands. This situation is
expected to occur in C+O Chandrasekhar WDs as opposed to edge-lit detonated
sub-Chandrasekhar WDs. If the pre-explosion magnetic field of the WD is less
intense than 10G, a lack of confinement of the positrons emitted in the
Co decay and a departure from full-trapping decline would occur. The
time at which it takes place can provide estimates of the original magnetic
field of the WD, its configuration, and also of the mass of the supernova
ejecta. In SN 1991bg, the bolometric light curve suggests absence of a
significant tangled magnetic field (intensity lower than G).
Chandrasekhar-mass models do not reproduce the bolometric light curve of this
supernova. For SN 1972E, on the contrary, there is evidence for a tangled
configuration of the magnetic field and its light curve is well reproduced by a
Chandrasekhar WD explosion.Comment: 54 pages, including 8 figures. To appear in Ap
Multiwavelength observation of a new black hole candidate: EXS 1737.9-2952
We report a multiwavelength analysis of an unusual high-energy transient: EXS 1737.9-2952. Due to the features this source exhibited in the hard X-ray domain similar to another source in the Galactic center region (1E1740.9 - 2952), and in order to study the molecular gas toward this X-ray source, we performed an observation of the EXS region in 1993 August, using the Swedish-ESO Submillimeter Telescope (SEST) located in La Silla (Chilean Andes). We observed a cloud, at the "forbidden" velocity of 135 km s-1, using 12CO (1-0) transitions, giving a maximum column density of 8 × 1021 cm-2. In 1994 we conducted other observations to search for higher density regions inside the cloud, using HCO+ and CS lines, but they were unsuccessful: we concluded that the cloud could be associated with the X-ray source and its mean density is of the order of ≈103 cm-3. In 1994 April, we performed a set of observations of the field containing EXS, at 20 cm and 6 cm, using the VLA in its A configuration, and found four possible radio candidates for an association with the EXS X-ray source, one of them (source 3) being extended, exhibiting a head-tail morphology, and a having a thermal spectrum with a spectral index ≈-0.7. We reproduced our observation in 1994 November and December, using the C configuration at 6 cm, in order to investigate on possible variability and extension of these sources and found a marginal indication in the 20 cm image that source 3 may have a weak second component displaced about 15". Nevertheless, this indication is too faint to associate this source definitely with EXS since no significant variation was detected. In addition, during the 1994 November-December observation, two more extended sources were detected but their association with EXS is unlikely. We also analyzed the Einstein/IPC image of the 5 σ EXS error box which does not exhibit, at the time of the observation, any significant low-energy X-ray counterpart to EXS. A nearby pulsar PSR 1737-30 in the ROSAT catalog is outside this error box. Finally, IRAS maps of the EXS region do not show any IR contribution at the location of the radio sources. We conclude that (1) EXS 1737.9-2952 is a high-energy transient, (2) a persistent counterpart at other wavelengths is not demonstrated, and (3) EXS, when flaring, as well as other GC gamma-ray sources, could possibly contribute to the 511 keV bulge emission
The Distance of the Soft Gamma Repeater SGR 1806-20
We present CO(J=1-0) observations in the direction of the Soft Gamma Repeater
SGR 1806-20 with the SEST telescope. We detected several molecular clouds, and
we discuss in this paper the implications of these observations for the
distance to the X-ray counterpart AX 1805.7-2025, the supernova remnant
G10.0-0.3 and the very luminous O9-B2 star detected in the line of sight. The
distance of SGR 1806-20 is estimated to be 14.5 +/- 1.4 kpc and this Soft Gamma
Repeater is very likely associated with one of the brightest HII regions in the
Galaxy, W31. The large size of G10.0-0.3 (25 x 38 pc) for a young supernova
remnant possibly powered by a central pulsar (AX 1805.7-2025) indicates that
G10.0-0.3 could be expanding in the very low density region produced by the
wind of the blue star.Comment: 20 pages, four postscript figures, LaTeX. To appear in Astrophysical
Journal, 1997, in pres
The lives and deaths of positrons in the interstellar medium
We reexamine in detail the various processes undergone by positrons in the
ISM from their birth to their annihilation using the most recent results of
positron interaction cross sections with H, H2 and He. The positrons' lives are
divided into two phases: the 'in-flight' phase and the thermal phase. The first
phase is treated with a Monte Carlo simulation that allows us to determine the
fraction of positrons that form positronium and annihilate as well as the
characteristics of the annihilation emission as a function of the medium
conditions. The second phase is treated with a binary reaction rate approach,
with cross sections adopted from experimental measurement or theoretical
calculations. An extensive search and update of the knowledge of positron
processes was thus undertaken. New reaction rates and line widths have been
obtained. We investigate the treatment of the complicated interactions between
positrons and interstellar dust grains. New reaction rates and widths of the
line resulting from the annihilation inside and outside of the grain have been
obtained. The final results of our calculations showed that dust is only
important in the hot phase of the ISM, where it dominates all other processes.
Combining the new calculations, we have constructed annihilation spectra for
each phase of the ISM, considering various grain contents, as well as an
overall combined spectrum for the ISM as a whole.Comment: 16 pages, 6 figures. accepted in Astronomy and Astrophysic
Positron transport in the interstellar medium
We seek to understand the propagation mechanisms of positrons in the
interstellar medium (ISM). This understanding is a key to determine whether the
spatial distribution of the annihilation emission observed in our Galaxy
reflects the spatial distribution of positron sources and, therefore, makes it
possible to place constraints on the origin of positrons. We review the
different processes that are likely to affect the transport of positrons in the
ISM. These processes fall into three broad categories: scattering off
magnetohydrodynamic waves, collisions with particles of the interstellar gas
and advection with large-scale fluid motions. We assess the efficiency of each
process and describe its impact on the propagation of positrons. We also
develop a model of positron propagation, based on Monte-Carlo simulations,
which enable us to estimate the distances traveled by positrons in the
different phases of the ISM. We find that low-energy (< 10 MeV) positrons
generally have negligible interactions with magnetohydrodynamic waves, insofar
as these waves are heavily damped. Positron propagation is mainly controlled by
collisions with gas particles. Under these circumstances, positrons can travel
huge distances (up to 30 kpc/n_H for 1 MeV positrons) along magnetic field
lines before annihilating.Comment: 18 pages, 12 figures, accepted in Astronomy and Astrophysic