Array beams are calculated from the data streams from one or more stations in order to produce time-series' and dynamic spectra for high time resolution observations.
The minimum integration time is 5.12 μs. The maximum spectral channel
the width of one sub-band (195.3125 kHz with the 200 MHz clock).
Sub-bands may be split into multiples of 16 channels, up to a maximum
of 256 (though more can be specified in Expert Mode), to provide higher
spectral resolution. Please note that increased spectral resolution
comes with a corresponding decrease in time resolution: The time
resolution is calculated by taking the inverse of the frequency
resolution. For example, if 256 channels per sub-band are specified,
the time resolution will decrease to 0.0013s.
In the current implementation, there are three Beam-Formed sub-modes:
1) The Coherent Stokes (CS) sub-mode produces a coherent sum of multiple stations (also known as a “tied-array” beam) by correcting for geometrical and instrumental delays. This produces a beam with restricted field-of-view, but with the full cumulative sensitivity of the combined stations.
This sub-mode is currently only possible for the 6 Superterp stations, which all receive a single clock signal. By autumn 2012, it will be possible to sum all 24 core stations.
Up to 127 (~300 in Expert Mode) simultaneous, full-bandwidth tied-array beams with different pointings can be recorded in this mode. Either Stokes I or Stokes I,Q,U,V can be recorded, with a range of possible frequency (0.76 - 12.21 kHz or single sub-band width) and time (>= 81.92 µs) resolutions.
2) The Incoherent Stokes (IS) sub-mode produces an incoherent combination of the various station beams by summing the powers after correction for only the geometrical delay. This produces beams with the same field-of-view as a station beam, but results in a decrease in sensitivity compared with a coherently-added tied-array beam.
One such incoherent array beam can be formed for each of the beams created at station level - i.e., if all the stations being summed split their recorded bandwidth across say 8 pointing directions, then 8 incoherent array beams can be formed from these.
All LOFAR stations, including the international stations, can be summed in this sub-mode, which can be run in parallel with the Coherent Stokes sub-mode. Either Stokes I or Stokes I,Q,U,V can be recorded, with a range of possible frequency (0.76 - 12.21 kHz) and time (>= 81.92 µs) resolutions.
3) The Fly’s Eye (FE) sub-mode records the individual station beams (one or more per station) without summing. This is useful for diagnostic comparisons of the stations, but can also be used for extremely wide-ﬁeld surveys, if one points each station in a different direction. This survey functionality is however not available in the current call for proposals. In combination with the Complex Voltage sub-mode, Fly’s Eye can also be used to record the separate station voltages as input for oﬄine fast-imaging experiments.
It is also possible to simultaneously record an incoherent sum of all the stations used in this mode.
Polarizations (CS/FE), Polarizations (IS):
For the different modes explained above, polarizations can be selected:
Combining Interferometric and Beam Formed Modes
These modes, and in some cases even a combination of these modes, can be run in parallel with the standard imaging mode described above. This allows one to simultaneously image a field while recording high time resolution dynamic spectra to probe sub-second variations of any source in the field.
Beam-formed data products will be stored in the LOFAR Long-Term Archive and may be retrieved by investigators from there.
On-line processing of these data products via the Known Pulsar Pipeline or to create dynamic spectra will be available later.
Figure 2: A schematic overview of the overall Pulsar Pipeline, as it runs online on the BG/P, followed by offline scientific processing on the offline cluster. Offline pipeline processing can be run on data directly out of the BG/P or on RFI-filtered data.
The offline pulsar processing is shown schematically in Figure 2, and is described in more detail by Stappers et al (2011), where the online beam-formed pipeline and its various sub-modes are also discussed (see also Mol & Romein 2011).
The Beam-Formed data written by BG/P are stored on the LOFAR offline processing cluster in the HDF5 format (Hierarchical Data Format). Several conversion tools have been developed to convert these data into other formats, e.g. PSRFITS, suitable for direct input into to standard pulsar data reduction packages, such as PSRCHIVE, PRESTO, and SIGPROC.
However, the long-term goal is to adapt these packages to natively read HDF5, using classes which exist for interpreting the HDF5 files.
Among other things, these reduction packages allow for RFI masking,
dedispersion, and searching of the data for single pulses and periodic
signals. Already, a test-mode exists to perform coherent dedispersion
online, also for multiple beams/dispersion measures. Likewise, online
RFI excision is also being implemented in order to excise corrupted data
from individual stations before it is added in to form an array beam.
Since the Beam-Formed data serve a much larger community than just pulsar astronomers, a dynamic spectrum tool is under development. This tool will allow for the creation of dynamic spectra from the beam-formed data files, including correction for the beam response across the pass-band, averaging of the data in time and/or frequency and basic plotting of the results. All dynamic spectra, whether processed or not, will be stored in an HDF5 file format. Ultimately it will be possible to run this mode online to return dynamic spectrum data files in place of the raw beam-formed data files.
Table 1 gives an indication of sensitivity and processing speed of typical Beam Formed observations.
Table 1: Sensitivity and Processing Performance Parameters for typical Beam-Formed Observations.
1 This assumes the raw 32-bit floats written out by BG/P. The Known Pulsar Pipeline converts these to 8-bit integers, and hence reduces the data volume by a factor of 4 in the case of Pulsar observations. If the scientific goal is to create dedispersed/folded pulse profiles, then the volume of the resulting data products is over an order of magnitude smaller. The dynamic spectrum pipeline may also do this, once developed.
2 Approximate minimum, period averaged, 100 MHz flux density for a detection with a signal-to-noise ratio > 10. For Coherent Stokes, this assumes the 6 Superterp stations. Once the Single Clock is installed on all Core stations, the raw sensitivity will increase by a factor of 4 (i.e. the minimum flux will become ~3mJy. For Incoherent Stokes, 48 incoherently added HBA sub-stations (24 tiles) are assumed. For Fly's Eye the sensitivity corresponds to that of a single HBA sub-station.
3 Processing time includes: a) conversion of 32-bit to 8-bit data; b) RFI flagging; c) dedispersion; d) folding; e) creation of diagnostics plots. Multiple beams/stations can be run in parallel. Standard observations of known sources often require only 1 beam.