137 research outputs found
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
LOFAR observations of 4C+19.44. On the discovery of low frequency spectral curvature in relativistic jet knots
We present the first LOFAR observations of the radio jet in the quasar
4C+19.44 (a.k.a. PKS 1354+19) obtained with the long baselines. The achieved
resolution is very well matched to that of archival Jansky Very Large Array
(JVLA) observations at higher radio frequencies as well as the archival X-ray
images obtained with {\it Chandra}. We found that, for several knots along the
jet, the radio flux densities measured at hundreds of MHz lie well below the
values estimated by extrapolating the GHz spectra. This clearly indicates the
presence of spectral curvature. Radio spectral curvature has been already
observed in different source classes and/or extended radio structures and it
has been often interpreted as due to intrinsic processes, as a curved particle
energy distribution, rather than absorption mechanisms ({ Razin-Tsytovich}
effect, free-free or synchrotron self absorption to name a few). Here we
discuss our results according to the scenario where particles undergo
stochastic acceleration mechanisms also in quasar jet knots.Comment: 13 pages, 4 tables, 4 figures, pre-proof version, published on the
Astrophysical Journal (Harris, et al. 2019 ApJ, 873, 21
Search for radio pulsations in LS I +61 303
Context. LS I +61 303 is a member of the select group of gamma-ray binaries:
galactic binary systems that contain a massive star and a compact object, show
a changing milliarcsecond morphology and a similar broad spectral energy
distribution (SED) that peaks at MeV-TeV energies and is modulated by the
orbital motion. The nature of the compact object is unclear in LS I +61 303, LS
5039 and HESS J0632+057, whereas PSR B1259-63 harbours a 47.74 ms radio pulsar.
Aims. A scenario in which a young pulsar wind interacts with the stellar wind
has been proposed to explain the very high energy (VHE, E > 100 GeV) gamma-ray
emission detected from LS I +61 303, although no pulses have been reported from
this system at any wavelength. We aim to find evidence of the pulsar nature of
the compact object. Methods. We performed phased array observations with the
Giant Metrewave Radio Telescope (GMRT) at 1280 MHz centred at phase 0.54.
Simultaneous data from the multi-bit phased array (PA) back-end with a sampling
time of tsamp = 128 microsec and from the polarimeter (PMT) back-end with tsamp
= 256 microsec where taken. Results. No pulses have been found in the data set,
with a minimum detectable mean flux density of \sim 0.38 mJy at 8-sigma level
for the pulsed emission from a putative pulsar with period P >2 ms and duty
cycle D = 10% in the direction of LS I +61 303. Conclusions. The detection of
posible radio pulsations will require deep and sensitive observations at
frequencies \sim0.5-5 GHz and orbital phases 0.6-0.7. However, it may be
unfeasible to detect pulses if the putative pulsar is not beamed at the Earth
or if there is a strong absorption within the binary system.Comment: 8 pages. 5 figures. Accepted for publication in Astronomy and
Astrophysic
Multi-wavelength observations of the binary system PSR B1259-63/LS 2883 around the 2010-2011 periastron passage
We report on broad multi-wavelength observations of the 2010-2011 periastron
passage of the gamma-ray loud binary system PSR B1259-63. High resolution
interferometric radio observations establish extended radio emission trailing
the position of the pulsar. Observations with the Fermi Gamma-ray Space
Telescope reveal GeV gamma-ray flaring activity of the system, reaching the
spin-down luminosity of the pulsar, around 30 days after periastron. There are
no clear signatures of variability at radio, X-ray and TeV energies at the time
of the GeV flare. Variability around periastron in the H emission line,
can be interpreted as the gravitational interaction between the pulsar and the
circumstellar disk. The equivalent width of the H grows from a few days
before periastron until a few days later, and decreases again between 18 and 46
days after periastron. In near infrared we observe the similar decrease of the
equivalent width of Br line between the 40th and 117th day after the
periastron. For the idealized disk, the variability of the H line
represents the variability of the mass and size of the disk. We discuss
possible physical relations between the state of the disk and GeV emission
under assumption that GeV flare is directly related to the decrease of the disk
size.Comment: accepted to MNRA
The geometric distance and binary orbit of PSR B1259-63
The pulsar/massive star binary system PSR B1259-63/LS 2883 is one of the best-studied gamma-ray binaries, a class of systems whose bright gamma-ray flaring can provide important insights into high-energy physics. Using the Australian Long Baseline Array, we have conducted very long baseline interferometric observations of PSR B1259-63 over 4.4 years, fully sampling the 3.4-year orbital period. From our measured parallax of 0.38 ± 0.05 maswe use a Bayesian approach to infer a distance of 2.6+0.4-0.3 kpc. We find that the binary orbit is viewed at an angle of 154 ± 3° to the line of sight, implying that the pulsar moves clockwise around its orbit as viewed on the sky. Taking our findings together with previous results from pulsar timing observations, all seven orbital elements for the system are now fully determined. We use our measurement of the inclination angle to constrain the mass of the stellar companion to lie in the range 15-31M?. Our measured distance and proper motion are consistent with the system having originated in the Cen OB1 association and receiving a modest natal kick, causing it to have moved ~8 pc from its birthplace over the past ~3 × 105 years. The orientation of the orbit on the plane of the sky matches the direction of motion of the X-ray synchrotron-emitting knot observed by the Chandra X-ray Observatory to be moving away from the system
Modeling high-energy light curves of the PSR B1259-63/LS 2883 binary based on 3-D SPH simulations
Temporal changes of X-ray to very-high-energy gamma-ray emissions from the
pulsar-Be star binary PSR B1259-63/LS 2883 are studied based on 3-D SPH
simulations of pulsar wind interaction with Be-disk and wind. We focus on the
periastron passage of the binary and calculate the variation of the synchrotron
and inverse-Compton emissions using the simulated shock geometry and pressure
distribution of the pulsar wind. The characteristic double-peaked X-ray light
curve from observations is reproduced by our simulation under a dense Be disk
condition (base density ~10^{-9} g cm^{-3}). We interpret the pre- and
post-periastron peaks as being due to a significant increase in the conversion
efficiency from pulsar spin down power to the shock-accelerated particle energy
at orbital phases when the pulsar crosses the disk before periastron passage,
and when the pulsar wind creates a cavity in the disk gas after periastron
passage, respectively. On the contrary, in the model TeV light curve, which
also shows a double peak feature, the first peak appears around the periastron
phase. The possible effects of cooling processes on the TeV light curve are
briefly discussed.Comment: 32 pages, 6 figues. Accepted for publication in Ap
Anisotropic inverse Compton scattering of photons from the circumstellar disc in PSR B1259-63
The gamma-ray binary system PSR B1259-63 consists of a 48 ms pulsar orbiting
a Be star. The system is particularly interesting because it is the only
gamma-ray binary system where the nature of the compact object is known. The
non-thermal radiation from the system is powered by the spin-down luminosity of
the pulsar and the unpulsed radiation originates from the stand-off shock front
which forms between the pulsar and stellar wind. The Be star/optical companion
in the system produces an excess infrared flux from the associated
circumstellar disc. This infrared excess provides an additional photon source
for inverse Compton scattering. We discuss the effects of the IR excess near
periastron, for anisotropic inverse Compton scattering and associated gamma-ray
production. We determine the infrared excess from the circumstellar disc using
a modified version of a curve of growth method, which takes into account the
changing optical depth through the circumstellar disc during the orbit. The
model is constrained using archive data and additional mid-IR observations
obtained with the VLT during January 2011. The inverse Compton scattering rate
was calculated for three orientations of the circumstellar disc. The predicted
gamma-ray light curves show that the disc contribution is a maximum around
periastron and not around the disc crossing epoch. This is a result of the disc
being brightest near the stellar surface. Additional spectroscopic and
near-infrared observations were obtained of the system and these are discussed
in relation to the possibility of shock heating during the disc crossing epoch.Comment: 15 pages, 14 figures and 4 tables. MNRAS in press. Updated
references, correction of typos in a few of the equations in sec. 5.2 and
appendix, and other minor typo correction
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