The Transients KSP will study variable radio sources via four distinct approaches:
- We will rapidly and regularly scan a large fraction of the entire northern sky in the Radio Sky Monitor mode (see below)
- We will undertake surveys in a phased-array mode, allowing observations of coherent phenomena at the highest time resolution (sub microsecond)
- In piggyback mode we will attempt to seach all LOFAR observations to look for variable and transient sources, by comparing with previous images of that region of sky.
- We will also perform targeted deep / high-resolution observations of specific key astrophysical sources, often in concert with other facilities -- e.g. orbiting X-ray observatories, ground-based optical telescopes.
The Radio Sky Monitor
As an illustration of how the Radio Sky Monitor (RSM) might operate, using seven beams to tile out a hexagonal pattern, operating the LOFAR Radio Sky Monitor from the core (ie. with maximum baselines of ~2 km) will result in an instantaneous field of view of ~ 0.1 steradians at 120 MHz and ~ 0.2 steradians at 30 MHz. These fields of view are extremely large and illustrate the unprecedented monitoring and surveying capabilities of the telescope.
A variety of strategies for operating the RSM mode can be envisaged, e.g.
- Rapid All-Sky Monitoring: Rapid shallow half-sky (hemispherical) surveys could be performed on short timescales in order to survey for rapid transients. The total number of beams required to tile out the whole hemisphere varies from of order 100 at 30 MHz to ~1600 for 120 MHz with international stations (which are larger and therefore have a smaller field of view). This is a very small number of pointings in order to survey half of the entire sky, and means that several minutes could be spent at each point in the sky (if such a scan were carried out over something like one day). This in turn means that ~mJy sensitivity surveys can be carried out daily.
- Zenith monitoring: Staring at the zenith optimises the sensitivity and beam stability of the telescope, whilst providing a sizeable and repeatedly monitored part of the sky. Mapping out the entire field of view which passes the zenith can be achieved in approximately 20 pointings at 30 MHz and 30 at 120 MHz, tracking each field for about an hour, achieving (sub-)mJy sensitivity.
- Galactic plane monitoring: Most of the northern galactic plane is visible from LOFAR and could be monitored for galactic transients. A large fraction of the `known' radio transients, such as those associated with rotation-powered neutron stars and X-ray binary systems, are heavily concentrated towards the galactic plane (note that many sources e.g. flare stars, GRB afterglows, AGN are not). The RSM zenith-monitoring mode will sample the galactic plane between about 90 < l < 160. However, much of the galactic plane will not be well sampled by this program, especially towards the galactic centre. In order to counterbalance this, we propose regular scans of the galactic plane. Since the galactic plane will also, however, suffer most from strong field sources and heavy dispersion at 30 MHz, these surveys will only be conducted at 120 MHz.
Our aim is that the data will be correlated, calibrated and delivered to Amsterdam at a rate of one image per beam per second, corresponding to the LOFAR `standard data products'. Delivery of maps made from longer, logarithmically-spaced time intervals, from the central computing cluster, will result in a total image transfer rate of approximately two per beam per second, i.e. 48 images / second for the RSM. Images will be analysed in real time (via source-detection software currently under development by the TKP) to check both for new transients and for the fluxes of known objects. Of course non-standard modes, such as phase-arrays for pulsar and exoplanet searches, will require different routes to transient discovery and classification.
Any new transients will have their data fed into a classification / alert pipeline which may under certain circumstances trigger follow up observations (such as full-array and frequencies up to 240 MHz). A sketch of how such a pipeline may integrate with the LOFAR central processing is presented below. It will be our policy that alerts will passed to the broader community as well as directly to partner observatories
. The fluxes of known variable sources will be fed into the transients database (currently in the design phase) and may also trigger follow-up observations if there are particularly dramatic changes.
Figure 4. A schematic illustration of how LOFAR data will be searched for transients, and the subsequent steps that will be taken when a bright new event is discovered.