11 research outputs found
The LOFAR Transients Pipeline
Current and future astronomical survey facilities provide a remarkably rich
opportunity for transient astronomy, combining unprecedented fields of view
with high sensitivity and the ability to access previously unexplored
wavelength regimes. This is particularly true of LOFAR, a
recently-commissioned, low-frequency radio interferometer, based in the
Netherlands and with stations across Europe. The identification of and response
to transients is one of LOFAR's key science goals. However, the large data
volumes which LOFAR produces, combined with the scientific requirement for
rapid response, make automation essential. To support this, we have developed
the LOFAR Transients Pipeline, or TraP. The TraP ingests multi-frequency image
data from LOFAR or other instruments and searches it for transients and
variables, providing automatic alerts of significant detections and populating
a lightcurve database for further analysis by astronomers. Here, we discuss the
scientific goals of the TraP and how it has been designed to meet them. We
describe its implementation, including both the algorithms adopted to maximize
performance as well as the development methodology used to ensure it is robust
and reliable, particularly in the presence of artefacts typical of radio
astronomy imaging. Finally, we report on a series of tests of the pipeline
carried out using simulated LOFAR observations with a known population of
transients.Comment: 30 pages, 11 figures; Accepted for publication in Astronomy &
Computing; Code at https://github.com/transientskp/tk
The extraordinary radio galaxy MRC B1221-423: probing deeper at radio and optical wavelengths
We present optical spectra and high-resolution multi-wavelength radio
observations of the compact steep-spectrum radio source MRC B1221-423
(z=0.1706). MRC B1221-423 is a very young (~10^5 yr), powerful radio source
which is undergoing a tidal interaction with a companion galaxy. We find strong
evidence of interaction between the AGN and its environment. The radio
morphology is highly distorted, showing a dramatic interaction between the
radio jet and the host galaxy, with the jet being turned almost back on itself.
H I observations show strong absorption against the nucleus at an infall
velocity of ~250 km/s compared to the stellar velocity, as well as a second,
broader component which may represent gas falling into the nucleus. Optical
spectra show that star formation is taking place across the whole system. Broad
optical emission lines in the nucleus show evidence of outflow. Our
observations confirm that MRC B1221-423 is a young radio source in a gas-rich
nuclear environment, and that there was a time delay of a few x 100 Myr between
the onset of star formation and the triggering of the AGN.Comment: 15 pages, 13 figures, to appear in MNRA
Radio lobes and X-ray hot spots in the microquasar S26
We have studied the structure and energetics of the powerful
microquasar/shock-ionized nebula S26 in NGC 7793, with particular focus on its
radio and X-ray properties. Using the Australia Telescope Compact Array, we
have resolved for the first time the radio lobe structure and mapped the
spectral index of the radio cocoon. The steep spectral index of the radio lobes
is consistent with optically-thin synchrotron emission; outside the lobes, the
spectral index is flatter, suggesting an additional contribution from free-free
emission, and perhaps ongoing ejections near the core. The radio core is not
detected, while the X-ray core has a 0.3-8 keV luminosity ~6 x 10^{36} erg/s.
The size of the radio cocoon matches that seen in the optical emission lines
and diffuse soft X-ray emission. The total 5.5-GHz flux of cocoon and lobes is
~2.1 mJy, which at the assumed distance of 3.9 Mpc corresponds to about 3 times
the luminosity of Cas A. The total 9.0-GHz flux is ~1.6 mJy. The X-ray hot
spots (combined 0.3-8 keV luminosity ~2 x 10^{37} erg/s) are located ~20 pc
outwards of the radio hot spots (ie, downstream along the jet direction),
consistent with a different physical origin of X-ray and radio emission
(thermal-plasma and synchrotron, respectively). The total particle energy in
the bubble is ~10^{53} erg: from the observed radio flux, we estimate that only
about a few 10^{50} erg are stored in the relativistic electrons; the rest is
in protons, nuclei and non-relativistic electrons. The X-ray-emitting component
of the gas in the hot spots contains ~10^{51} erg, and ~10^{52} erg over the
whole cocoon. We suggest that S26 provides a clue to understand how the ambient
medium is heated by the mechanical power of a black hole near its Eddington
accretion rate.Comment: Accepted by MNRAS on 2010 July 12. Twelve pages, 8 figures, size =
1.3 MB. Contact the authors for higher-res figure
Accretion states of the Galactic microquasar GRS 1758-258
We present the results of a radio and X-ray study of the Galactic microquasar GRS 1758â258, using unpublished archival data and new observations. We focus in particular on the 2000â2002 state transitions, and on its more quiet behaviour in 2008â2009. Our spectral and timing analysis of the XMMâNewton data shows that the source was in the canonical intermediate, soft and hard states in 2000 September 19, 2001 March 22 and 2002 September 28, respectively. We estimate the disc size, luminosity and temperature, which are consistent with a black hole mass ~10 M\astrosun. There is much overlap between the range of total X-ray luminosities (on average ~0.02L Edd) in the hard and soft states, and probably between the corresponding mass accretion rates; in fact, the hard state is often more luminous. The extended radio lobes seen in 1992 and 1997 are still present in 2008â2009. The 5-GHz radio core flux density has shown variability between ~0.1 and 0.5 mJy over the last two decades. This firmly places GRS 1758â258 in the radio-quiet sequence of Galactic black holes, in the radio/X-ray plane. We note that this dichotomy is similar to the dichotomy between the radio/X-ray sequences of Seyfert and radio galaxies. We propose that the different radio efficiency of the two sequences is due to relativistic electron/positron jets in radio-loud black holes, and subrelativistic, thermally dominated outflows in radio-quiet sources
Baryons in the relativistic jets of the stellar-mass black-hole candidate 4U 1630-47
Accreting black holes are known to power relativistic jets, both in stellar-mass binary systems and at the centres of galaxies. The power carried away by the jets, and, hence, the feedback they provide to their surroundings, depends strongly on their composition. Jets containing a baryonic component should carry significantly more energy than electronâpositron jets. Energetic considerations1, 2 and circular-polarization measurements3 have provided conflicting circumstantial evidence for the presence or absence of baryons in jets, and the only system in which they have been unequivocally detected is the peculiar X-ray binary SSâ433 (refs 4, 5). Here we report the detection of Doppler-shifted X-ray emission lines from a more typical black-hole candidate X-ray binary, 4Uâ1630-47, coincident with the reappearance of radio emission from the jets of the source. We argue that these lines arise from baryonic matter in a jet travelling at approximately two-thirds the speed of light, thereby establishing the presence of baryons in the jet. Such baryonic jets are more likely to be powered by the accretion disk6 than by the spin of the black hole7, and if the baryons can be accelerated to relativistic speeds, the jets should be strong sources of Îł-rays and neutrino emission
Multiwavelength Observations of A0620-00 in Quiescence
[Abridged.] We present multiwavelength observations of the black hole binary
system, A0620-00. Using the Cosmic Origins Spectrograph on the Hubble Space
Telescope, we have obtained the first FUV spectrum of A0620-00. The observed
spectrum is flat in the FUV and very faint (with continuum fluxes \simeq 1e -
17 ergs/cm^2/s/A). We compiled the dereddened, broadband spectral energy
distribution of A0620-00 and compared it to previous SEDs as well as
theoretical models. The SEDs show that the source varies at all wavelengths for
which we have multiple samples. Contrary to previous observations, the
optical-UV spectrum does not continue to drop to shorter wavelengths, but
instead shows a recovery and an increasingly blue spectrum in the FUV. We
created an optical-UV spectrum of A0620-00 with the donor star contribution
removed. The non-stellar spectrum peaks at \simeq3000 {\deg}A. The peak can be
fit with a T=10,000 K blackbody with a small emitting area, probably
originating in the hot spot where the accretion stream impacts the outer disk.
However, one or more components in addition to the blackbody are needed to fit
the FUV upturn and the red optical fluxes in the optical-UV spectrum. By
comparing the mass accretion rate determined from the hot spot luminosity to
the mean accretion rate inferred from the outburst history, we find that the
latter is an order of magnitude smaller than the former, indicating that
\sim90% of the accreted mass must be lost from the system if the predictions of
the disk instability model and the estimated interoutburst interval are
correct. The mass accretion rate at the hot spot is 10^5 the accretion rate at
the black hole inferred from the X-ray luminosity. To reconcile these requires
that outflows carry away virtually all of the accreted mass, a very low rate of
mass transfer from the outer cold disk into the inner hot region, and/or
radiatively inefficient accretion.Comment: ApJ, accepte
The nature and likely redshift of GLEAM J0917-0012
International audienceWe previously reported a putative detection of a radio galaxy at , selected from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. The redshift of this source, GLEAM J0917-0012, was based on three weakly detected molecular emission lines observed with the Atacama Large Millimetre Array (ALMA). In order to confirm this result, we conducted deep spectroscopic follow-up observations with ALMA and the Karl Jansky Very Large Array (VLA). The ALMA observations targeted the same CO lines previously reported in Band 3 (84-115 GHz) and the VLA targeted the CO(4-3) and [CI(1-0)] lines for an independent confirmation in Q-band (41 and 44 GHz). Neither observation detected any emission lines, removing support for our original interpretation. Adding publicly available optical data from the Hyper Suprime-Cam survey, Widefield Infrared Survey Explorer (WISE), and Herschel Space Observatory in the infrared, as well as 10 GHz polarisation and 162 MHz inter-planetary scintillation observations, we model the physical and observational characteristics of GLEAM J0917-0012 as a function of redshift. Comparing these predictions and observational relations to the data, we are able to constrain its nature and distance. We argue that if GLEAM J0917-0012 is at then it has an extremely unusual nature, and that the more likely solution is that the source lies above
A LOFAR DETECTION of the LOW-MASS YOUNG STAR T TAU at 149 MHz
Radio observations of young stellar objects (YSOs) enable the study of
ionised plasma outflows from young protostars via their free-free radiation.
Previous studies of the low-mass young system T Tau have used radio
observations to model the spectrum and estimate important physical properties
of the associated ionised plasma (local electron density, ionised gas content
and emission measure). However, without an indication of the low-frequency
turnover in the free-free spectrum, these properties remain difficult to
constrain. This paper presents the detection of T Tau at 149 MHz with the Low
Frequency Array (LOFAR) - the first time a YSO has been observed at such low
frequencies. The recovered total flux indicates that the free-free spectrum may
be turning over near 149 MHz. The spectral energy distribution is fitted and
yields improved constraints on local electron density ( cm), ionised gas mass (
M) and emission measure ( pc cm).Comment: 10 pages, accepted for publication in Ap
The ASKAP Variables and Slow Transients (VAST) pilot survey
The Variables and Slow Transients Survey (VAST) on the Australian Square
Kilometre Array Pathfinder (ASKAP) is designed to detect highly variable and
transient radio sources on timescales from 5 seconds to years. In this
paper, we present the survey description, observation strategy and initial
results from the VAST Phase I Pilot Survey. This pilot survey consists of hours of observations conducted at a central frequency of 888~MHz between
2019 August and 2020 August, with a typical rms sensitivity of
0.24~mJy~beam and angular resolution of arcseconds. There are
113 fields, \red{each of which was observed for 12 minutes integration time},
with between 5 and 13 repeats, with cadences between 1 day and 8 months. The
total area of the pilot survey footprint is 5\,131 square degrees, covering six
distinct regions of the sky. An initial search of two of these regions,
totalling 1\,646 square degrees, revealed 28 highly variable and/or transient
sources. Seven of these are known pulsars, including the millisecond pulsar
J2039--5617. Another seven are stars, four of which have no previously reported
radio detection (SCR~J0533--4257, LEHPM~2-783, UCAC3~89--412162 and 2MASS
J22414436--6119311). Of the remaining 14 sources, two are active galactic
nuclei, six are associated with galaxies and the other six have no
multiwavelength counterparts and are yet to be identified