10,445 research outputs found
A possible black hole in the gamma-ray microquasar LS 5039
The population of high energy and very high energy gamma-ray sources,
detected with EGRET and the new generation of ground-based Cherenkov
telescopes, conforms a reduced but physically important sample. Most of these
sources are extragalactic (e.g., blazars), while among the galactic ones there
are pulsars and SN remnants. The microquasar LS 5039, previously proposed to be
associated with an EGRET source by Paredes et al. (2000), has recently been
detected at TeV energies, confirming that microquasars should be regarded as a
class of high energy gamma-ray sources. To model and understand how the
energetic photons are produced and escape from LS 5039 it is crucial to unveil
the nature of the compact object, which remains unknown. Here we present new
intermediate-dispersion spectroscopy of this source which, combined with values
reported in the literature, provides an orbital period of 3.90603+/-0.00017 d,
a mass function f(M)=0.0053+/-0.0009 M_sun, and an eccentricity e=0.35+/-0.04.
Atmosphere model fitting to the spectrum of the optical companion, together
with our new distance estimate of d=2.5+/-0.1 kpc, yields R_opt=9.3+0.7-0.6
R_sun, log (L_opt/L_sun)=5.26+/-0.06, and M_opt=22.9+3.4-2.9 M_sun. These,
combined with our dynamical solution and the assumption of
pseudo-synchronization, yield an inclination i=24.9+/-2.8 degree and a compact
object mass M_X=3.7+1.3-1.0 M_sun. This is above neutron star masses for most
of the standard equations of state and, therefore, we propose that the compact
object in LS 5039 is a black hole. We finally discuss about the implications of
our orbital solution and new parameters of the binary system on the CNO
products, the accretion/ejection energetic balance, the SN explosion scenario,
and the behaviour of the TeV emission with the new orbital period.Comment: 10 pages, 8 figures. Accepted for publication in MNRAS. Minor changes
according to referee repor
A Multiwavelength Investigation of the Relationship Between 2CG135+1 and LSI+61o 303
We present the results of a multiwavelength monitoring campaign targeting the
gamma-ray source 2CG 135+1 in an attempt to confirm the association of this
object with the radio/Be/X-ray binary system LSI +61o 303. The campaign
included simultaneous radio, optical, infrared, and hard x-ray/gamma-ray
observations carried out with a variety of instruments, covering (not
continously) almost three binary cycles of LSI +61o 303 during the period
April-July 1994. Three separate OSSE observations of the gamma-ray source were
carried out, covering different phases of the radio lightcurve. Hard
X-ray/gamma-ray emission was detected from the direction of 2CG 135+1 during
the first of these OSSE observations. The signal to noise ratio of the OSSE
observations was insufficient to establish a spectral or intensity correlation
of the high-energy emission with simultaneous radio, optical and infrared
emission of LSI +61o 303. We briefly discuss the theoretical implications of
our observations.Comment: 17 pages, 9 figures, 6 tables to be published in Astrophysical
Journal, 10 April 199
Periodic morphological changes in the radio structure of the gamma-ray binary LS 5039
Gamma-ray binaries allow us to study physical processes such as particle
acceleration up to TeV energies and VHE gamma-ray emission and absorption with
changing geometrical configurations on a periodic basis. These sources produce
outflows of radio-emitting particles whose structure can be imaged with VLBI.
LS 5039 is a gamma-ray binary that has shown variable VLBI structures in the
past. We aim to characterise the radio morphological changes of LS 5039 and
discriminate if they are either repeatable or erratic. We observed LS 5039 with
the VLBA at 5 GHz during five consecutive days to cover the 3.9-day orbit and
an extra day to disentangle between orbital or secular variability. We also
compiled the available high-resolution radio observations of the source to
study its morphological variability at different orbital phases. We used a
simple model to interpret the obtained images. The new observations show that
the morphology of LS 5039 up to projected distances of 10 milliarcseconds
changes in 24 h. The observed radio morphological changes display a periodic
orbital modulation. Multifrequency and multiepoch VLBI observations confirm
that the morphological periodicity is stable on timescales of years. Using a
simple model we show that the observed behaviour is compatible with the
presence of a young non-accreting pulsar with an outflow behind it. The
morphology is reproduced for inclinations of the orbit of 60-75 deg. For masses
of the companion star in the range 20-50 Msun, this range of inclinations
implies a mass of the compact object of 1.3-2.7 Msun. The periodic orbital
modulation of the radio morphology of LS 5039 suggests that all gamma-ray
binaries are expected to show a similar behaviour. The changes in the radio
structure of LS 5039 are compatible with the presence of a young non-accreting
neutron star, which suggests that the known gamma-ray binaries contain young
pulsars.Comment: 15 pages, 7 figures. Accepted for publication in Astronomy and
Astrophysic
Noncommutative Field Theory and the Dynamics of Quantum Hall Fluids
We study the spectrum of density fluctuations of Fractional Hall Fluids in
the context of the noncommutative hidrodynamical model of Susskind. We show
that, within the weak-field expansion, the leading correction to the
noncommutative Chern--Simons Lagrangian (a Maxwell term in the effective
action,) destroys the incompressibility of the Hall fluid due to strong UV/IR
effects at one loop. We speculate on possible relations of this instability
with the transition to the Wigner crystal, and conclude that calculations
within the weak-field expansion must be carried out with an explicit
ultraviolet cutoff at the noncommutativity scale. We point out that the
noncommutative dipoles exactly match the spatial structure of the
Halperin--Kallin quasiexcitons. Therefore, we propose that the noncommutative
formalism must describe accurately the spectrum at very large momenta, provided
no weak-field approximations are made. We further conjecture that the
noncommutative open Wilson lines are `vertex operators' for the quasiexcitons.Comment: 20 pages, harvma
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