Pulsar Observing

by Stephan W. Witz last modified Jun 22, 2017 by Emmanuel Momjian

The VLA can be used for several kinds of pulsar observing: phase-binning using the WIDAR correlator, using the phased-array for single-beam pulsar processing in either search or fold modes, or simply standard imaging mode with fast integrations. Any of these types of pulsar observing are considered Resident Shared Risk Observing (RSRO), and participants in this program are expected to work closely with NRAO staff. Of these, phased-array and fast-dump observing are significantly more mature than phase binning. For any questions not addressed here regarding the capabilities of these observing modes, please contact the NRAO Helpdesk.

Phased-array pulsar processing

The "Y" Ultimate Pulsar Processing Instrument (YUPPI) is a suite of software that runs in the correlator backend (CBE) computer cluster and can process a single-beam phased/summed-array data stream for pulsar observations in real time, into either folded profiles or search mode (filterbank) output. Coherent dedispersion can be optionally applied in either mode.

In this mode, the voltage data streams from each antenna are divided into a number of frequency subbands within the correlator, then summed and requantized before being output to the cluster for pulsar processing. The limitation on bandwidth comes primarily from the available network connections between the correlator and cluster. In all cases, a maximum of 64 subbands total can be processed. Depending on the number of bits chosen, this results in the following total bandwidth constraints:

Table 3.18.1: Pulsar Observing Bandwidth Constraints

Subband bandwidth

Subband quantization

Max total bandwidth

Samplers

32 MHz 8 bits 2048 MHz 8-bit
64 MHz 4 bits 4096 MHz 3-bit
128 MHz 2 bits 8192 MHz 3-bit
<32 MHz 8 bits 64*BWsub 8-bit

As described in the VLA Frequency Bands and Tunability section, the 8-bit samplers provide two independently tunable 1 GHz IFs, while the 3-bit samplers provide four tunable 2 GHz IFs. 

The pulsar-specific processing is done in real time using the DSPSR software package and, in principle, any processing option supported by DSPSR can be used, although this will be constrained by the real-time computing power available in the cluster. In general, each subband can be divided into an arbitrary (2n) number of channels; 1 (summed), 2 or 4 detected polarization products can be output; and coherent dedispersion can be enabled or not.

Fold mode

In fold mode, the data are averaged modulo a known pulsar ephemeris (provided via a standard TEMPO/TEMPO2 "par file") into pulse profiles. The data can also be folded at a constant topocentric period, for example at 10 Hz to detect the injected noise cal signal. Fold integration times as short as 1 second have been tested. Up to 16384 profile bins can be used. The data are recorded in PSRFITS format using the standard 16-bit data encoding. This means the final output data rate is given by:

Data rate = 2 bytes × Nsubband × Nchannel × Nbin × Npoln / Tint

If the desired data rate exceeds ~25 MB/s, additional testing ahead of time may be required.

Search mode

In search mode, the data are simply detected and averaged over a specified amount of time before being output to disk, resulting in a filterbank data array (power vs time and frequency). Coherent dedispersion at a known DM can optionally be enabled for this.  Data can be recorded using 2, 4, 8, 16 or 32 bits, resulting in a final data rate of:

Data rate = (Nbit/8) bytes × Nsubband × Nchannel × Npoln / Tint

The maximum sustained output rate in this mode should be kept less than ~400 MB/s.

Subarrays

It is possible to use any of the phased-array pulsar modes listed here in a subarray observation, following the guidelines described in the Subarrays section. In addition, the above constraints on the pulsar processing apply to the total of all simultaneously-used subarrays, rather than each subarray separately. For example, the total number of subbands in use across all subarrays must not be greater than 64; the total (not per-subarray) data rate must meet the above constraints, etc. It is possible to use different parameters such as subband bandwidth, number of bits, processing mode (fold versus seach) in the different subarrays.

VLBI

It is possible to use phased-array pulsar processing as part of a VLBI experiment; see the VLBI Observations section and links therein for additional information about VLBI at the VLA. The main constraint on this type of observation is that a subband that is being recorded for VLBI can not also be sent to the pulsar processing system. However since VLBI recording typically only requires a small number of subbands (2 through 8), any additional subbands produced by WIDAR can be sent to the pulsar system, following the constraints above. This provides a high time resolution data stream covering wider bandwidth than the VLBI data. One typical use case is to detect a pulsar using the VLA data stream, and determine a short-term timing ephemeris covering the observation. This can then be used to gate the VLBI correlation, reducing uncertainties associated with extrapolating existing timing solutions or obtaining time on other telescopes for these purposes.

Gated or binned visibilities

The WIDAR correlator has the capability to internally integrate (fold) visibilities into 1 or more pulse phase bins, following a standard TEMPO-compatible pulsar ephemeris. This mode can be used to image the emission from a pulsar of known period anywhere in the telescope field of view. This provides both higher signal-to-noise ratio on the pulsar than a standard image, and allows the pulsed emission to be separated from continuous emission from other sources in the field.

This mode is significantly less mature than the phased array modes described above, and any use of it will require testing ahead of the observations. Some constraints on what we expect to be possible include:

  • Binning is limited to the case where the pulse period is divided evenly into bins covering the full pulse period.  "Gating" style observations (common in VLBI) where a single on-pulse bin is used are not supported.
  • The maximum number of pulse phase bins is 1000.
  • There is a tradeoff between the total bandwidth and the minimum bin width (pulse period divided by number of bins):
    • With 4 subbands (512 MHz total), the minimum allowed bin width is 12 μs.
    • With 16 subbands (2048 MHz total), the minimum allowed bin width is 50 μs.
    • With 64 subbands (8192 MHz total), the minimum allowed bin width is 200 μs.
  • The number of channels per subband is currently limited to 128 maximum.  Combining recirculation and binning is not allowed.
  • Integration (dump) time must be an integer number of pulse periods.
  • The data rate produced in this mode is the standard VLA data rate (see the Data Rate section) multiplied by the number of bins.  The data rate must be kept less than 60 MB/s.

It should also be noted that there is currently very limited support for binned observations in standard data processing software (e.g., CASA). Development of data analysis procedures is ongoing and users of this mode should be aware that this will likely involve some advanced/low-level manipulation of raw VLA data sets.

Fast-dump visibilities

While not specifically a pulsar mode, standard visibility data can be dumped as fast as 5 ms, which may be sufficient for imaging of slow pulsars. See the Time Resolution and Data Rates section for more details.