22 research outputs found
VLBI search for the radio counterpart of HESS J1943+213
HESS J1943+213, a TeV point source close to the Galactic plane recently
discovered by the H.E.S.S. collaboration, was proposed to be an extreme BL
Lacertae object, though a pulsar wind nebula (PWN) nature could not be
completely discarded. To investigate its nature, we performed high-resolution
radio observations with the European Very Long Baseline Interferometry Network
(EVN) and reanalyzed archival continuum and H {\sc i} data. The EVN
observations revealed a compact radio counterpart of the TeV source. The low
brightness temperature and the resolved nature of the radio source are
indications against the beamed BL Lacertae hypothesis. The radio/X-ray source
appears immersed in a 1\arcmin elliptical feature suggesting a possible
galactic origin (PWN nature) for the HESS source. We found that HESS\,J1943+213
is located in the interior of a \sim1\degr diameter H {\sc i} feature, and
explored the possibility of they being physically related.Comment: Significantly revised and extended. Accepted for publication in ApJ
(ApJ, 762, 63). (4 figures.
The SFXC software correlator for Very Long Baseline Interferometry: Algorithms and Implementation
In this paper a description is given of the SFXC software correlator,
developed and maintained at the Joint Institute for VLBI in Europe (JIVE). The
software is designed to run on generic Linux-based computing clusters. The
correlation algorithm is explained in detail, as are some of the novel modes
that software correlation has enabled, such as wide-field VLBI imaging through
the use of multiple phase centres and pulsar gating and binning. This is
followed by an overview of the software architecture. Finally, the performance
of the correlator as a function of number of CPU cores, telescopes and spectral
channels is shown.Comment: Accepted by Experimental Astronom
Persistent Non-Gaussian Structure in the Image of Sagittarius A* at 86 GHz
Observations of the Galactic Center supermassive black hole Sagittarius A*
(Sgr A*) with very long baseline interferometry (VLBI) are affected by
interstellar scattering along our line of sight. At long radio observing
wavelengths (cm), the scattering heavily dominates image
morphology. At 3.5 mm (86 GHz), the intrinsic source structure is no longer
sub-dominant to scattering, and thus the intrinsic emission from Sgr A* is
resolvable with the Global Millimeter VLBI Array (GMVA). Long-baseline
detections to the phased Atacama Large Millimeter/submillimeter Array (ALMA) in
2017 provided new constraints on the intrinsic and scattering properties of Sgr
A*, but the stochastic nature of the scattering requires multiple observing
epochs to reliably estimate its statistical properties. We present new
observations with the GMVA+ALMA, taken in 2018, which confirm non-Gaussian
structure in the scattered image seen in 2017. In particular, the ALMA-GBT
baseline shows more flux density than expected for an anistropic Gaussian
model, providing a tight constraint on the source size and an upper limit on
the dissipation scale of interstellar turbulence. We find an intrinsic source
extent along the minor axis of as both via extrapolation of
longer wavelength scattering constraints and direct modeling of the 3.5 mm
observations. Simultaneously fitting for the scattering parameters, we find an
at-most modestly asymmetrical (major-to-minor axis ratio of )
intrinsic source morphology for Sgr A*.Comment: 18 pages, 10 figures, submitted to Ap
Beyond the myth of the supernova-remnant origin of cosmic rays
The origin of Galactic cosmic-ray ions has remained an enigma for almost a
century. Although it has generally been thought that they are accelerated in
the shock waves associated with powerful supernova explosions-for which there
have been recent claims of evidence-the mystery is far from resolved. In fact,
we may be on the wrong track altogether in looking for isolated regions of
cosmic-ray acceleration.Comment: Nature Progress Revie
Radio imaging observations of PSR J1023+0038 in an LMXB state
The transitional millisecond pulsar (MSP) binary system PSR J1023+0038 re-entered an accreting state in 2013 June in which it bears many similarities to low-mass X-ray binaries (LMXBs) in quiescence or near-quiescence. At a distance of just 1.37 kpc, PSR J1023+0038 offers an unsurpassed ability to study low-level accretion onto a highly magnetized compact object. We have monitored PSR J1023+0038 intensively using radio imaging with the Karl G. Jansky Very Large Array, the European VLBI Network and the Low Frequency Array, seeing rapidly variable, flat spectrum emission that persists over a period of six months. The flat spectrum and variability are indicative of synchrotron emission originating in an outflow from the system, most likely in the form of a compact, partially self-absorbed jet, as is seen in LMXBs at higher accretion rates. The radio brightness, however, greatly exceeds extrapolations made from observations of more vigorously accreting neutron star LMXB systems. We postulate that PSR J1023+0038 is undergoing radiatively inefficient "propeller-mode" accretion, with the jet carrying away a dominant fraction of the liberated accretion luminosity. We confirm that the enhanced ?-ray emission seen in PSR J1023+0038 since it re-entered an accreting state has been maintained; the increased ?-ray emission in this state can also potentially be associated with propeller-mode accretion. Similar accretion modes can be invoked to explain the radio and X-ray properties of the other two known transitional MSP systems XSS J12270-4859 and PSR J1824-2452I (M28I), suggesting that radiatively inefficient accretion may be a ubiquitous phenomenon among (at least one class of) neutron star binaries at low accretion rates
An 86 GHz search for pulsars in the galactic center with the Atacama Large Millimeter / submillimeter Array
High Energy Astrophysic
A High-Resolution Study of Two Galactic H I Halo Clouds in the Ophiuchus Superbubble
Two compact H I clouds which seem to belong to the Ophiuchus superbubble were studied at ∼30′′ resolution using the Very Large Array (VLA) in C and D configurations together with the Green Bank Telescope (GBT) providing the short-spacing flux. Here we present preliminary results of the data analysis
A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr
Original article can be found at: http://www.nature.com/nature/index.html Copyright Nature Publishing Group.Type Ic supernovae have drawn attention since 1998 owing to their sparse association with long duration γ-ray bursts (GRBs). Although the GRB central engine generates ultra-relativistic jets, no relativistic outflows have yet been found in type Ib/c supernovae explosions. Here, radio observations reveal a mildly relativistic expansion in a nearby type Ic supernova, SN 2007gr.Peer reviewe