ALFA Pulsar Studies

Data acquisition

With ALFA, we need about 47 pointings to cover one square degree, compared to about 330 pointings needed to cover one square degree with similar density with a single-pixel feed. Until 2009, we were using the Wideband Arecibo Pulsar Processors (WAPPs) to detect the signal from ALFA's seven beams. These cover 100 MHz of band (with dual polarization capability), initially centered at 1420 MHz and later at 1440 MHz. In 2009, the survey transitioned to new and improved back-ends, the Mock polyphase filterbank spectrometers, which are capable of covering 300 MHz (from 1225 MHz to 1525 MHz, the bandwidth covered by ALFA) for each of the seven beams (see detailed technical specifications here). This has resulted in greatly increased search sensitivity, but only for pointing where we can effectively deal with all the radio frequency interference.
From August 1 to October 8 2004, we conducted a preliminary survey that covered the two regions closest to the Galactic plane (|b| < 1 degrees) visible from Arecibo: the "Inner Galaxy" (40 < l < 75 degrees) and the "Anti-center" (170 < l < 210 degrees). Each pointing was 134 seconds for the Inner Galaxy and 67 seconds for the Anti-center. This was done in sparse mode, where we do only 1/3 of the pointings needed to cover the whole region. This preliminary survey found a total of 11 new pulsars, and detected 30 previously known pulsars. For a detailed description of this survey, and the strategy of the present survey, see Cordes et al. (2006).

The survey will cover the Galactic plane (|b| < 5 degrees) visible with the Arecibo 305-m radio telescope (35 < l < 75 degrees, see Fig. 1). Each pointing lasts about 268 seconds in the Inner Galaxy and 134 seconds in the Anti-center.

Data processing and storage

It is expected that, over the next several years, this survey will generate over 1000 Terabytes of data. The data is stored at the Cornell University Center for Advanced Computing, the publicly available data can be retrieved here. These are processed by three independent software pipelines:

• The first pipeline is the ``Quicklook'', where pulsars are found almost in real time. This is made possible by reducing the spectral and time resolution by a factor of 16, and using a computer cluster, the Arecibo Signal Processor to search for pulsars in the data. This is a nice and quick way of detecting slow pulsars, but the sensitivity to fast pulsars is severely degraded. This was the pipeline that discovered the compact, highly relativistic pulsar - neutron star system J1946+2052 and many other pulsars. After this, the full-resolution data are processed by the pipelines below.
• The PRESTO pipeline is based on PRESTO, a large suite of pulsar search and analysis software. It employs a Fourier-Domain acceleration search technique, which compensates for the loss of detection sensitivity in a traditional periodicity search due to a rapidly changing frequency of the periodic pulsar signal. Such frequency modulation can occur, for instance, due to a pulsar's orbital motion in a compact binary. The PRESTO pipeline also includes a single-pulse search pipeline and a newly introduced Fast Folding Algorithm. The PRESTO pipeline is run on the Guillimin supercomputer operated by McGill University in Montreal, Canada, producing several million signal candidates so far.
• The third pipeline is based on distributed computing. Since March 2009, part of the Einstein@Home computing power is used to analyze PALFA data. The Einstein@Home algorithm is particularly sensitive to radio pulsars in tight binary systems, with a phase-space coverage that is complementary to that of the PRESTO pipeline. To date (August 2016), it has discovered 30 previously unknown pulsars. This was the pipeline that discovered the pulsar - neutron star system PSR J1913+1102.
  

The data processed thus far has revealed that the radio frequency interference (RFI) environment at Arecibo significantly affects the detection threshold of the survey, creating unforseen challenges in identifying the many weak pulsars that are likely lurking in the data. To address this, the PALFA consortium is actively developing novel techniques for identification, mitigation, and excision of RFI. We are also implementing a variety of heuristics as well as machine learning algorithms for identifying real pulsars among the millions of signal candidates, most of which appear to be due to RFI. The inevitable growth in the incidence and variety of man-made RFI suggests that this problem will likely be important for all future radio pulsar surveys.

Outreach Efforts

The Arecibo Remote Command Center (ARCC) at the University of Texas at Brownsville and the University of Wisconsin at Miwaukee is currently engaged in searching for radio pulsars in ALFA data. ARCC is an integrated research/education facility that allows students at the high school and undergraduate level to be directly involved with the research at the Arecibo telescope. Web based tools have been developed so that students could rank the pulsar candidates created by the PRESTO analysis.