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 March 2016, we have already discovered a total
of 163 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 20 as of August 2015).
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. Another unusual system
is PSR J1950+2414,
a MSP-WD system with a low-mass companion, an orbital period of 22 days
and orbital eccentricity of 0.08. The origin of this system is still mysterious,
but there are a few others that are very similar.
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.
Apart from pulsars, one of the types of objects targeted by the PALFA survey
is fast radio bursts (FRBs).
The first FRB to be discovered with a telescope other than
the Parkes radio telescope was found in the PALFA survey.
However, the unparallelled sensitivity of the Arecibo telescope has
allowed us to see what no other telescope has been able to see before:
FRBs repeat!
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 several million signal
candidates so far.
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 2015), it has discovered 30
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.
Membership and Affiliations