ALFA Pulsar Studies

Most of the advances in pulsar astronomy were due to the discovery of new objects. A major increase in search sensitivity has already started a new era of discovery at the Arecibo Observatory.

This increase in search sensitivity is due first and foremost by the ALFA receiver and the pulsar surveys it makes possible, which are now being carried by the Pulsar Consortium. Preliminary estimates (see below) indicate that the Arecibo Galactic plane survey using ALFA could find many hundreds of new pulsars. As of Feb. 27 2013, we have already discovered a total of 116 new pulsars.

From the superior spectral and time resolution of the survey, it was expected at the start that it should be able to find pulsars deep in the Galaxy that were undetectable to previous surveys because of dispersive smearing. This expectation is now being comfirmed, with the discovery of many distant millisecond pulsars (the number of discoveries with spin periods smaller than 20 ms is 15 as of Feb. 27 2013). A fine example of such systems is PSR J1903+0327, the first eccentric binary millisecond pulsar in the Galactic plane. This system is also unusual for having a main-sequence companion. It is the first of a new class of systems that is thought to have started their evolution as triple systems.

Given the very high sensitivity of the Arecibo telescope, we can combine very high sensitivity with very short (5 minute) pointings. This allows us to retain sensitivity to extremely compact binary systems, which are likely to represent the best laboratories for the study of gravitational physics. Great examples of this are the discovery of the second most relativistic binary pulsar known, PSR J1906+0746 - which is also, by far, the youngest pulsar known to be in a binary system. The discovery observation was so short that the high acceleration of the pulsar was not noticeable; this implies that it was detected with the same sensitivity as a normal isolated pulsar would be.

Left: Galactic location of the new P-ALFA pulsar survey discoveries. The center of the Galaxy is indicated with a cross, the position of the Solar System is indicated by the black dot at the center of the black circle. Right: Positions of the pulsars as a function of Galactic longitude and DM. The black curve indicates the maximum Galactic column density according to the Cordes and Lazio (2001) model of the electron distribution of the Galaxy. In both plots, red dots indicate positions of known pulsars with Galactic latitude smaller than 5 degrees, blue indicates new discoveries by the Arecibo P-ALFA survey. There seems to be a deficiency in the number of pulsars at Galactic latitudes of 60 degrees.

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 now 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 will lead to greatly increased search sensitivity, provided 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). Each pointing lasts about 268 seconds in the Inner Galaxy and 134 seconds in the Anti-center.

Data processing and storage

Many of the detections to date have been made with a quick reduction package that allows us to find pulsars 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. Re-processing these data with full resolution is, computationally, a very challenging task but is essential for detecting many fast (both young and recycled) pulsars so far hidden by Galactic plasma.

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. The full-resolution raw data is processed independently by three software pipelines.

The Cornell University pipeline does a standard periodicity search and single-pulse search without doing an acceleration search. It has been run on all WAPP data archived at the Cornell University Center for Advanced Computing and has provided 2.5 million signal candidates. Winnowing of this vast set of candidates is currently under way.

The second pipeline is based on PRESTO, a large suite of pulsar search and analysis software developed by Scott Ransom. 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. This approach thus significantly boosts sensitivity to binary pulsars. The PRESTO pipeline is run on dedicated clusters at several institutions that participate in the ALFA survey, producing ~3 million signal candidates so far.

Since March 2009, part of the Einstein@Home computing power is used to analyze WAPP 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, it has discovered 23 previously unknown pulsars .

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.