669 research outputs found
Repeated Relativistic Ejections in GRS 1915+105
In 1994 February-August we observed with the VLA four ejection events of
radio emitting clouds from the high energy source GRS 1915+105. These events
are all consistent with anti-parallel ejections of twin pairs of clouds moving
away from the compact source at 0.92 of the speed of light and angles of
70 with respect to the line of sight. The flux ratios and time
evolution of the expelled clouds are consistent with actual motions of the
radiating matter rather than with the simple propagation of pulses in a medium
moving at slower velocities. The large kinetic power of the sudden, short, and
rather discontinuous ejections exceeds by more than an order of magnitude the
maximum steady photon luminosity of the source, suggesting that in GRS 1915+105
a radiation acceleration mechanism of the ejecta is unlikely. As in other
galactic and extragalactic radio sources, the decrease in flux density as a
function of angular separation from the central source shows a steepening with
distance. Additional ejection events have been observed in 1995 and 1997 and we
compare them with the 1994 events.Comment: 20 pages, 7 figures. To appear in The Astrophysical Journal, January
10, 1999 issu
New evidence on the origin of the microquasar GRO J1655-40
Aims. Motivated by the new determination of the distance to the microquasar
GRO J1655-40 by Foellmi et al. (2006), we conduct a detailed study of the
distribution of the atomic and molecular gas, and dust around the open cluster
NGC 6242, the possible birth place of the microquasar. The proximity and
relative height of the cluster on the galactic disk provides a unique
opportunity to study SNR evolution and its possible physical link with
microquasar formation. Methods. We search in the interstellar atomic and
molecular gas around NGC 6242 for traces that may have been left from a
supernova explosion associated to the formation of the black hole in GRO
J1655-40. Furthermore, the 60/100 mu IR color is used as a tracer of
shocked-heated dust. Results. At the kinematical distance of the cluster the
observations have revealed the existence of a HI hole of 1.5*1.5 degrees in
diameter and compressed CO material acumulated along the south-eastern internal
border of the HI cavity. In this same area, we found extended infrared emission
with characteristics of shocked-heated dust. Based on the HI, CO and FIR
emissions, we suggest that the cavity in the ISM was produced by a supernova
explosion occured within NGC 6242. The lower limit to the kinematic energy
transferred by the supernova shock to the surrounding interstellar medium is ~
10^{49} erg and the atomic and molecular mass displaced to form the cavity of ~
16.500 solar masses. The lower limit to the time elapsed since the SN explosion
is ~ 2.2*10^{5} yr, which is consistent with the time required by GRO J1655-40
to move from the cluster up to its present position. The observations suggest
that GRO J1655-40 could have been born inside NGC 6242, being one of the
nearest microquasars known so far.Comment: 6 pages, 6 figures. Accepted for publication in Astronomy &
Astrophysic
Search for interactions between ejections of GRS 1915+105 and its environment
To unravel the effect of likely interactions between the energetic ejections
of the galactic superluminal source GRS 1915+105 and its surrounding
interstellar medium, we observed its environment. Two IRAS sources are
symmetrically placed with respect to GRS 1915+105, and are aligned with the
sub-arcsec ejections of this source.
We analyzed these two sources IRAS 19124+1106 and IRAS 19132+1035 through
near-infrared, millimeter and centimeter wavelengths. The evidence for these
regions being interaction zones seems inconclusive.Comment: 4 pages, 3 figures, to appear in Proceedings of X-ray Astronomy '999
- Stellar Endpoints, AGN and the Diffuse Background, 2000. G. Malaguti, G.
Palumbo & N. White (eds), Gordon & Breach (Singapore
Gamma-ray binaries
Recent observations have shown that some compact stellar binaries radiate the
highest energy light in the universe. The challenge has been to determine the
nature of the compact object and whether the very high energy gamma-rays are
ultimately powered by pulsar winds or relativistic jets. Multiwavelength
observations have shown that one of the three gamma-ray binaries known so far,
PSR B1259-63, is a neutron star binary and that the very energetic gamma-rays
from this source and from another gamma-ray binary, LS I +61 303, may be
produced by the interaction of pulsar winds with the wind from the companion
star. At this time it is an open question whether the third gamma-ray binary,
LS 5039, is also powered by a pulsar wind or a microquasar jet, where
relativistic particles in collimated jets would boost the energy of the wind
from the stellar companion to TeV energies.Comment: 4 pages, 3 figures. Invited talk to appear in Proceedings of the
conference "The Multi-Messenger Approach to High-Energy Gamma-ray Sources",
Barcelona, 4-7 July 200
Identification of the optical and near-infrared counterpart of GRS 1758-258
Context. Understood to be a microquasar in the Galactic center region, GRS
1758-258 has not yet been unambiguously identified to have an
optical/near-infrared counterpart, mainly because of the high absorption and
the historic lack of suitable astrometric stars, which led to the use of
secondary astrometric solutions. Although it is considered with 1E 1740.7-2942
as the prototypical microquasar in the Galactic center region, the Galactic
origin of both sources has not yet been confirmed. Aims. We attempt to improve
previous astrometry to identify a candidate counterpart to GRS 1758-258. We
present observations with the Gran Telescopio de Canarias (GTC), in which we
try to detect any powerful emission lines that would infer an extragalactic
origin of this source. Methods. We use modern star catalogues to reanalyze
archival images of the GRS 1758-258 field in the optical and near-infrared
wavelengths, and compute a new astrometric solution. We also reanalyzed
archival radio data of GRS 1758-258 to determine a new and more accurate radio
position. Results. Our improved astrometric solution for the GRS 1758-258 field
represents a significant advancement on previous works and allows us to
identify a single optical/near-infrared source, which we propose as the
counterpart of GRS 1758-258. The GTC spectrum of this source is however of low
signal-to-noise ratio and does not rule out a Galactic origin. Hence, new
spectral observations are required to confirm or discard a Galactic nature.Comment: 4 pages, 3 figures, accepted by Astronomy and Astrophysic
What Fraction of the Young Clusters in the Antennae Galaxies are "Missing"?
A reexamination of the correspondence between 6 cm radio continuum sources
and young star clusters in the Antennae galaxies indicates that 85 % of the
strong thermal sources have optical counterparts, once the optical image is
shifted 1.2 arcsec to the southwest. A sample of 37 radio-optical matches are
studied in detail showing correlations between radio properties and a variety
of optical characteristics. There is a strong correlation between the radio
flux and the intrinsic optical brightness. In particular, the brightest radio
source is also the intrinsically brightest optical cluster (WS80). It is also
the most extincted cluster in the sample, the strongest CO source and the
strongest 15 micron source . Furthermore, the brightest ten radio sources are
all amongst the youngest clusters with ages in the range 0 - 4 Myr and
extinctions from A_V = 0.5 to 7.6 mag (with a median value of 2.6 mag). Only a
few of the very red clusters originally discovered by Whitmore & Schweizer are
radio sources, contrary to earlier suggestions. Finally, a new hybrid method of
determining cluster ages has been developed using both UBVI colors and H_alpha
equivalent widths to break the age-reddening degeneracy.Comment: 51 pages, 13 postscript figures, LaTex. To appear in the Astronomical
Journal, 124, 2002, Septembe
Metallicity dependence of HMXB populations
High-mass X-ray binaries (HMXBs) might have contributed a non-negligible
fraction of the energy feedback to the interstellar and intergalactic media at
high redshift, becoming important sources for the heating and ionization
history of the Universe. However, the importance of this contribution depends
on the hypothesized increase in the number of HMXBs formed in low-metallicity
galaxies and in their luminosities. In this work we test the aforementioned
hypothesis, and quantify the metallicity dependence of HMXB population
properties. We compile from the literature a large set of data on the sizes and
X-ray luminosities of HMXB populations in nearby galaxies with known
metallicities and star formation rates. We use Bayesian inference to fit simple
Monte Carlo models that describe the metallicity dependence of the size and
luminosity of the HMXB populations. We find that HMXBs are typically ten times
more numerous per unit star formation rate in low-metallicity galaxies (12 +
log(O/H) < 8, namely < 20% solar) than in solar-metallicity galaxies. The
metallicity dependence of the luminosity of HMXBs is small compared to that of
the population size. Our results support the hypothesis that HMXBs are more
numerous in low-metallicity galaxies, implying the need to investigate the
feedback in the form of X-rays and energetic mass outflows of these high-energy
sources during cosmic dawn.Comment: 9 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Long-term Multi-wavelength Observations of GRS 1758-258 and the ADAF Model
We present a long-term multi-wavelength light curve of Galactic black hole
candidate GRS 1758-258 by combining previously published and archival data from
GRANAT, ROSAT, CGRO, RXTE, SAX, ASCA, EXOSAT, and the VLA. In addition we
include first spectral results from the balloon-borne Gamma-ray Arcminute
Telescope Imaging System (GRATIS). In light of divergent analyses of the
1991-1993 ROSAT observations, we have re-analyzed these data; we find the soft
X-rays track the hard X-rays, and that the fits require no black-body
component-- indicating that GRS 1758-258 did not go to the high state in 1993.
We offer an interpretation based on the ADAF model for a system with mdot
\ltsim mdot_crit. We find the 1990-1993 coeval hard and soft X-ray observations
support the ADAF predictions. We discuss a new way to constrain black-hole mass
with spectral data and the ADAF theory, and apply this technique to GRS
1758-258 to find M_1 \gtsim 8--9 M_sol at an assumed distance of 8.5 kpc.
Further investigations of the ADAF model allow us to evaluate the model
critically against the data and flux-flux diagram of Barret, McClintock &
Grindlay (1996) and to understand the limits of the latter's ``X-ray burster
box.''Comment: 32 pp., AASTEX, 8 ps figures, accepted to Ap
Structure du marche gazier americain, reglementation et tarification de l'acces des tiers au reseau.
L'ouverture de la concurrence du marche americain du gaz naturel s'accompagne de profondes modifications des reglementations mises en place pour une organisation et un fonctionnement efficace de l'industrie gaziere. L'object principal de notre papier est d'essayer de transcrire les faits stylises du marche du gaz naturel en Amerique du Nord et de modeliser la structure de l'industrie dans ce secteur ouvert en aval a la concurrence (ou en voie de l'etre).STRUCTURES DES MARCHES ; ENERGIE
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