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Transient Sources
LOFAR's large instantaneous beam will make it
uniquely suited to efficiently monitor a large fraction of the sky,
allowing a sensitive unbiased survey of radio transients for the
first time. Averaging of the data will provide information on a
variety of time scales, ranging from seconds to many days. The resolution
attained will be sufficient for the crucial task of rapid optical
and X-ray identifications. The table below gives an overview of
the classes of object known or expected to exhibit variable radio
emission. Also indicated are the variability time-scales, the number
of objects/events that are expected to be observed per year and
an estimate of the distances to which these objects can be seen.
A brief review of the importance of LOFAR observations for several
of these classes of objects will now follow.
 |
One concept of LOFAR as an all-sky
monitor. Four `virtual core' beams observe the zenith as the
sky tracks past, monitoring up to two-thirds of the sky daily.
A newly-identified event can then be localised with arcsecond
accuracy using the full array. |
Predicted detection numbers of transient
sources :
| Class of object |
Time-scale |
Expected / year |
Maximum Distance |
| GRB afterglows (extragalactic) |
months |
~100 |
Observable universe |
| LIGO Events |
msec / hours |
a few ? |
Observable universe |
| Radio Supernovae |
days / months |
~ 3 |
100 Mpc |
| Intermediate mass BH |
days |
1-5 |
30 Mpc |
| Flare Stars |
msec / hours |
100-1000 |
1 kpc |
| Exo-planets |
min / hours |
10-100 |
30 pc |
Gamma-Ray Bursts and Galactic
Black-Hole/Neutron-Star Systems
The primary instruments for detecting Gamma-ray bursts and galactic
black-hole/neutron-star systems have been satellites observing in
the X-ray and Gamma-ray part of the spectrum. One of the spectacular
successes was the localisation of Gamma-ray bursts and their subsequent
identification with galaxies at cosmological distances. From the
empirical relation between radio and X-ray emission for these systems
it is clear that the all-sky monitoring with LOFAR will be a factor
of 5-10 more effective in discovering such events than previous
all-sky-monitors. Furthermore, it will allow much more accurate
localisation of these events, facilitating follow-ups at other wavelengths.
It is therefore anticipated that LOFAR will be the primary source
of triggers for the high-energy community utilising target-of-opportunity
projects on e.g. HST/VLT/Chandra/XMM, a close cooperation with the
GLAST X- and gamma-ray all sky monitor team is planned. Discovery
and monitoring of these variable high-energy sources at low radio
frequencies will provide us with an unprecedented insight into the
process of explosive particle acceleration throughout the Universe.
(unlarge image) |
Simulation of a single
synchrotron `bubble' event -- a generic representation
of the processes underlying the radio emission from GRBs, SNE,
AGN and XRBs. A shock produces a phase of particle acceleration
of finite, typically short, duration. Subsequently the relativistic
gas expands until it becomes optically thin at progressively
lower frequencies. In this simulation the timescales are typical
for GRBs or XRBs -- at GHz frequencies the emission peaks within
a few days and begins to decay. At LOFAR frequencies the source
evolution is slower, due to the greater internal optical depth,
and the source peaks about one month after particle injection,
but from this point onwards is stronger than at higher frequencies
and may remain visible for a year or more subsequently. This
model is based on van der Laan (1966). |
|
| Radio observations of
the X-ray transient source CI Cam. The time and frequency
evolution of the event is very similar to that predicted from
the simple model (left). Furthermore, note that at the lowest
frequencies (in this case 300 MHz) the rising phase of the event
was detectable within days of the high frequency emission (which
was already peaking). An expanding emitting region (right) was
found to be associated with this transient event, which was
accompanied by a bright X-ray flare and attracted world-wide
attention. |
Exo-Planets
Jupiter exhibits very luminous bursts of decametre radio emission,
making it the brightest source in the sky at frequencies below 20
MHz. Presently, one of the most active areas in astronomical research
is the detection and characterisation of exo-planets. Simple scaling
relations suggest that the intensity of radio bursts of some of
these exo-planets will be one or two orders of magnitude higher
than that of Jupiter. This indicates that LOFAR might detect a number
of exo-planets out to distances of tens of parsecs, providing important
information on magnetic field strengths and rotation rates.
Flare Stars and “LIGO Events”
In addition to the aforementioned high-energy phenomena and exo-planets,
LOFAR will detect radio emission from active and binary stars up
to distances of the order of a kpc. Furthermore, several models
for strong ‘LIGO events’, for example the coalescence
of two neutron stars, not only predicted strong gravitational wave
emission, but also an associated strong burst of coherent radio
emission.
More information:
see NL
science case for LOFAR (pdf)
<
In case of questions or
comments regarding LOFAR, or about these web pages, please contact
lofar@astron.nl
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