42 research outputs found
Gamma-Ray Emission from Microquasars
Microquasars, X-ray binary systems that generate relativistic jets, were
discovered in our Galaxy in the last decade of the XXth century. Their name
indicates that they are manifestations of the same physics as quasars but on a
completely different scale. Parallel to this discovery, the EGRET instrument on
board of the Compton Gamma Ray Observatory detected 271 point like gamma-ray
sources 170 of which were not clearly identified with known objects. This
marked the beginning of gamma-ray source population studies in the Galaxy. We
present in this thesis models for gamma-ray production in microquasars with the
aim to propose them as possible parent populations for different groups of
EGRET unidentified sources. These models are developed for a variety of
scenarios taking into account several possible combinations, i.e. black holes
or neutron stars as the compact object, low mass or high mass stellar
companions, as well as leptonic or hadronic gamma-ray production processes. We
also show that the presented models for gamma-rays emitting microquasars can be
used to explain observations from well known sources that are detected in
energy ranges other than EGRET's. Finally, we include an alternative gamma-ray
producing situation that does not involve microquasars but a specific
unidentified EGRET source possibly linked to a magnetized accreting pulsar.Comment: PhD Thesis supervised by G.E. Romero and presented at the University
of Buenos Aires (UBA) on December 15th, 200
Hadronic gamma-ray emission from windy microquasars
The jets of microquasars with high-mass stellar companions are exposed to the
dense matter field of the stellar wind. We present estimates of the gamma-ray
emission expected from the jet-wind hadronic interaction and we discuss the
detectability of the phenomenon at high energies. The proposed mechanism could
explain some of the unidentified gamma-ray sources detected by EGRET instrument
on the galactic plane.Comment: Accepted for publication in Astronomy and Astrophysics as a Letter to
the Edito
The dust-enshrouded microquasar candidate AX J1639.0-4642 = IGR J16393-4643
We present a multiwavelength study of the field containing the unidentified
X-ray source AX J1639.0-4642, discovered with the ASCA observatory and recently
detected with the IBIS telescope, onboard the INTEGRAL satellite, dubbed IGR
J16393-4643. The huge hydrogen column density towards the source, the hard
spectral index in the 0.7-10 keV band and its flux variability suggest that the
source is a High Mass X-ray Binary (HMXB) enshrouded by dust. Our search
reveals the presence of a non-thermal radio counterpart within the X-ray error
box. After a study of the broadband emission from X-rays to the radio domain,
we propose that AX J1639.0-4642 is a dust-enshrouded Microquasar (MQ)
candidate. In addition, the X-ray source is well within the 95% location
contour of the unidentified gamma-ray source 3EG J1639-4702. The main
properties of AX J1639.0-4642/3EG J1639-4702 are consistent with those of two
other MQs previously proposed to display high-energy gamma-ray emission.Comment: 7 pages, 5 figures. Accepted for publication in A&A. Title and
discussion on the possible NIR counterpart have been modifie
Precessing microblazars and unidentified gamma-ray sources
The recent discovery by Paredes et al. (2000) of a persistent microquasar
that is positionally coincident with an unidentified gamma-ray source has open
the possibility that other sources in the Third EGRET Catalog could be
interpreted as microquasars as well. In this letter we show that some variable
unidentified EGRET sources in the galactic plane could be produced by faint,
otherwise undetected microquasars with precessing jets. When the jet points
towards the observer, gamma-ray emission resulting from upscattered stellar
photons could be detectable yielding a variable source with weak or
undetectable counterpart at longer wavelengths. Strategies for detecting
these``microblazars'' with forthcoming satellites are briefly discussed.Comment: 4 pages, 3 figures, Astronomy & Astrophysics Letters in press, typing
errors correctio
Radio Spectral Index Analysis and Classes of Ejection in LS I +61 303
LS I +61303 is a gamma-ray binary with periodic radio outbursts coincident
with the orbital period of P=26.5 d. The origin of the radio emission is
unclear,it could be due either to a jet, as in microquasars, or to the shock
boundary between the Be star and a possible pulsar wind. We here analyze the
radio spectral index over 6.7 yr from Green Bank Interferometer data at 2.2 GHz
and 8.3 GHz. We find two new characteristics in the radio emission. The first
characteristic is that the periodic outbursts indeed consist of two consecutive
outbursts; the first outburst is optically thick, whereas the second outburst
is optically thin. The spectrum of LS I +61 303 is well reproduced by the
shock-in-jet model commonly used in the context of microquasars and AGNs: the
optically thin spectrum is due to shocks caused by relativistic plasma
("transient jet") traveling through a pre-existing much slower steady flow
("steady jet"). This steady flow is responsible for the preceding optically
thick spectrum. The second characteristic we find is that the observed spectral
evolution, from optically thick to optically thin emission, occurs twice during
the orbital period. We observed this occurrence at the orbital phase of the
main 26.5 d outburst and also at an earlier phase, shifted by 0.3 (i.e almost 8 days before). We show that this result qualitatively
and quantitatively agrees with the two-peak accretion/ejection model proposed
in the past for LS I +61303. We conclude that the radio emission in LS I +61303
originates from a jet and suggest that the variable TeV emission comes from the
usual Compton losses expected as an important by-product in the shock-in-jet
theory.Comment: 27 pages, 7 figures, accepted for publication in Ap
Interacting coronae of two T Tauri stars: first observational evidence for solar-like helmet streamers
Context {The young binary system V773 Tau A exhibits a persistent radio
flaring activity that gradually increases from a level of a few mJy at
apoastron to more than 100 mJy at periastron. Interbinary collisions between
very large (> 15 R) magnetic structures anchored on the two rotating stars of
the system have been proposed to be the origin of these periodic radio flares.
Magnetic structures extended over tens of stellar radii, that can also account
for the observed fast decay of the radio flares, seem to correspond to the
typical solar semi-open quite extended magnetic configurations called helmet
streamers.} Aims {We aim to find direct observational evidence for the
postulated, solar-like, coronal topologies.} Methods {We performed
seven-consecutive-day VLBI observations at 8.4 GHz using an array consisting of
the VLBA and the 100-m Effelsberg telescope.} Results {Two distintive
structures appear in the radio images here presented. They happen to be
associated with the primary and secondary stars of the V773 Tau A system. In
one image (Fig.2-B) the two features are extended up to 18 R each and are
nearly parallel revealing the presence of two interacting helmet streamers. One
image (Fig.2-E) taken a few hours after a flare monitored by the 100-m
Effelsberg telescope shows one elongated fading structure substantially rotated
with respect to those seen in the B run. The same decay scenario is seen in
Fig.2-G for the helmet streamer associated with the other star.} Conclusions
{This is the very first direct evidence revealing that even if the flare origin
is magnetic reconnection due to interbinary collision, both stars independently
emit in the radio range with structures of their own. These structures are
helmet streamers, observed for the first time in stars other than the Sun.}Comment: 7 pages, 3 figures, A&A in pres
VLBA images of the precessing jet of LSI+61303
Context: In 2004, changes in the radio morphology of the Be/X-ray binary
system LSI+61303 suggested that it is a precessing microquasar. In 2006, a set
of VLBA observations performed throughout the entire orbit of the system were
not used to study its precession because the changes in radio morphology could
tentatively be explained by the alternative pulsar model. However, a recent
radio spectral index data analysis has confirmed the predictions of the
two-peak microquasar model, which therefore does apply in LSI+61303. Aims: We
revisit the set of VLBA observations performed throughout the orbit to
determine the precession period and improve our understanding of the physical
mechanism behind the precession. Methods: By reanalyzing the VLBA data set, we
improve the dynamic range of images by a factor of four, using
self-calibration. Different fitting techniques are used and compared to
determine the peak positions in phase-referenced maps. Results: The improved
dynamic range shows that in addition to the images with a one-sided structure,
there are several images with a double-sided structure. The astrometry
indicates that the peak in consecutive images for the whole set of observations
describes a well-defined ellipse, 6-7 times larger than the orbit, with a
period of about 28 d. Conclusions: A double-sided structure is not expected to
be formed from the expanding shocked wind predicted in the pulsar scenario. In
contrast, a precessing microquasar model can explain the double- and one-sided
structures in terms of variable Doppler boosting. The ellipse defined by the
astrometry could be the cross-section of the precession cone, at the distance
of the 8.4 GHz-core of the steady jet, and 28d the precession period.Comment: 7 pages, 5 figures, Accepted for publication in Astronomy and
Astrophysics, added references for sect.
Spectral and variability properties of LS 5039 from radio to very high-energy gamma-rays
Microquasars are X-ray binaries with relativistic jets. The microquasar LS
5039 turned out to be the first high-energy gamma-ray microquasar candidate due
to its likely association with the EGRET source 3EG J1824-1514. Further
theoretical studies supported this association, which could be extended to
other EGRET sources. Very recently, Aharonian et al. (2005) have communicated
the detection of the microquasar LS 5039 at TeV energies. This fact confirms
the EGRET source association and leaves no doubt about the gamma-ray emitting
nature of this object. The aim of the present work is to show that, applying a
cold-matter dominated jet model to LS 5039, we can reproduce many of the
spectral and variability features observed in this source. Jet physics is
explored, and some physical quantities are estimated as a by-product of the
performed modeling. Although at the moment only LS 5039 has been detected on
the entire electromagnetic spectrum, it does not seem unlikely that other
microquasars will show similar spectral properties. Therefore, an in-depth
study of the first gamma-ray microquasar, on theoretical grounds supported by
observations, can render a useful knowledge applicable elsewhere.Comment: 3 pages, 3 figures, 1 Table. Proceedings of the conference:
Relativistic Astrophysics and Cosmology: Einstein's Legac
Magnetic Field Upper Limits for Jet Formation
Context: Very high magnetic fields at the surface of neutron stars or in the
accretion disk of black holes inhibit the production of jets. Aims: We quantify
here the magnetic field strength for jet formation. Methods: By using the
Alfven Radius, R_A, we study what we call {\it the basic condition}, R_A/R_*=1
or R_A/R_{LSO}=1 (LSO, last stable orbit), in its dependency on the magnetic
field strength and the mass accretion rate, and we analyse these results in 3-D
and 2-D plots in the case of neutron star and black hole accretor systems,
respectively. For this purpose, we did a systematic search of all available
observational data for magnetic field strength and the mass accretion rate.
Results: The association of a classical X-ray pulsar (i.e. B ~10^{12} G) with
jets is excluded even if accreting at the Eddington critical rate. Z-sources
may develop jets for B \simlt 10^{8.2} G, whereas Atoll-sources are potential
sources of jets if B \simlt 10^{7.7} G. It is not ruled out that a millisecond
X-ray pulsar could develop jets, at least for those sources where B \simlt
10^{7.5} G. In this case the millisecond X-ray pulsar could switch to a
microquasar phase during its maximum accretion rate. For stellar-mass black
hole X-ray binaries, the condition is that B \simlt 1.35 x 10^8 G and B \simlt
5 x 10^8 G at the last stable orbit for a Schwarzschild and a Kerr black hole,
respectively. For active galactic nuclei (AGNs), it reaches B \simlt 10^{5.9} G
for each kind of black hole. These theoretical results are in complete
agreement with available observational data.Comment: 7 pages, 6 figures, A&A in press. Version 2 with minor correction
A numerical model for the gamma-ray emission of the microquasar LS 5039
The possible association between the microquasar LS 5039 and the EGRET source
3EG J1824-1514 suggests that microquasars could also be sources of high energy
gamma-rays. In this paper, we explore, with a detailed numerical model, if this
system can produce the emission detected by EGRET (>100 MeV) through inverse
Compton (IC) scattering. Our numerical approach considers a population of
relativistic electrons entrained in a cylindrical inhomogeneous jet,
interacting with both the radiation and the magnetic fields, taking into
account the Thomson and Klein-Nishina regimes of interaction. The computed
spectrum reproduces the observed spectral characteristics at very high energy.Comment: 14 pages, 4 figures, 3 tables, LaTeX, uses aa.cls. published in A&A,
final published version (referee format)mnkdi