1. ALFA Pulsar Surveys: Searching for Laboratories of Extreme Physics
Paulo César C. Freire
Cornell University / Arecibo Observatory
on behalf of the
THE ALFA PULSAR CONSORTIUM
207th AAS Meeting, Washington D.C., 4/27/2019

2. In This Talk… Pulsar surveys and the ALFA system
Finding fast pulsars with ALFA
Finding pulsars in tight binaries with ALFA
How to use the newly discovered laboratories to study extreme physics: one example
207th AAS Meeting, Washington D.C. 4/27/2019

3. Pulsar Surveys
From 1997 to 2002, the Parkes multi-beam pulsar survey used a set of 13 beams to search the Galaxy from –5º < b < 5º and 260º < l < 50º.
The use of L-band allows us to peer through the Galactic plasma, the large number of beams, large bandwidth and low system temperature more than compensate for the low flux at 1400 MHz.
207th AAS Meeting, Washington D.C. 4/27/2019

4. Pulsar Surveys
Using a set of thirteen 2 x 96 x 3 MHz filterbanks, and using 1-bit digitization, this survey found 728 pulsars so far!
Despite its tremendous success (doubling the number of known pulsars!), the 3 MHz channels make this survey almost unable to detect millisecond pulsars at DMs >~ 100 cm–3 pc. Most new objects are “boring” slow pulsars.
The beams are quite wide, so the lower gain can be compensated by large integration times (35 minutes). However, the discovery of compact binary systems in such long integrations requires the use of very complex and computer intensive techniques.
207th AAS Meeting, Washington D.C. 4/27/2019

5. The ALFA system
In 2002, the Arecibo Observatory purchased from the ATNF the Arecibo L-band Feed Array. This consists of a set of seven beams, adapted to the smaller focal plane of the Arecibo telescope.
This was delivered to Arecibo in April 1st 2004, and is now working
207th AAS Meeting, Washington D.C. 4/27/2019

6. The ALFA system
The gain of the central beam is 11 K/Jy, the system temperature is about 25 K at 1400 MHz
The gain of the outer beams is about 8 K/Jy
The average beam size is 204 x 232 arcseconds. The six outer beams sit on an ellipse of 329 x 384 arcseconds.
207th AAS Meeting, Washington D.C. 4/27/2019

7. The ALFA system
We can cover the sky almost completely with three interlaced pointings. Using this strategy, one needs 50 pointings per square degree.
207th AAS Meeting, Washington D.C. 4/27/2019

8. The ALFA system
The sparse mode does only one in three necessary pointings. Per unit time, it searches the same area for faint pulsars as a dense survey, but three times as much area for bright pulsars! It favors the early detections of bright pulsars, the best for timing purposes. Example: PSR J
207th AAS Meeting, Washington D.C. 4/27/2019

9. The ALFA system
The back-ends used to detect the spectra (for pulsar and continuum purposes) are the Wideband Arecibo Pulsar processors. Each of the eight boards can deal with bandwidths of up to 100 MHz. For the L-band searches, this is divided in 256 channels, samples every 64.
Eventually, these will be replaced (for the purpose of pulsar searches with ALFA) with the new PALFA spectrometers. These will be able to detect 300 MHz, with a total of 1024 channels, sampled every 64 μs.
207th AAS Meeting, Washington D.C. 4/27/2019

10. Finding Fast Pulsars
This will address a fundamental limitation of the PMB survey; we will be able to detect MSPs at DMs 16 times larger, and be able to detect sub-millisecond pulsars (if they exist).
207th AAS Meeting, Washington D.C. 4/27/2019

11. Finding Fast Pulsars
In the Arecibo sky, the Parkes Multi-Beam survey (PMB) detected 134 pulsars.
207th AAS Meeting, Washington D.C. 4/27/2019

12. Finding Fast Pulsars
Of these, 4 are millisecond pulsars (P< 20 ms) !
207th AAS Meeting, Washington D.C. 4/27/2019

13. Finding Fast Pulsars All of these have DMs lower than 40 cm-3 pc!
Clearly, dispersive smearing is what is preventing the detection of MSPs at higher DMs.
207th AAS Meeting, Washington D.C. 4/27/2019

14. Finding Fast Pulsars
The number of MSPs below a given DM is about half of the total number of pulsars for the lower DMs.
207th AAS Meeting, Washington D.C. 4/27/2019

15. Finding Fast Pulsars
If we increase the time resolution by a factor of 10, then in a survey with the same sensitivity as the PMB, covering just the part of the Arecibo sky seen by PMB, we should detect MSPs up to DMs ten times larger.
If the ratio relative to slow pulsars is similar, then we should detect ~ 50 MSPs.
207th AAS Meeting, Washington D.C. 4/27/2019

16. Finding Fast Pulsars
For pulsars with 10 times the period (up to 200 ms), we also find a linear trend. Therefore, the previous linear extrapolation is adequate for pulsars with 10 times smaller periods observed with 10 times the time resolution.
207th AAS Meeting, Washington D.C. 4/27/2019

17. Finding Fast Pulsars Is the sensitivity similar to PMB?
In the area surveyed by both surveys, the PALFA survey (in its extended phase alone – going on since the start of 2005, with integration times of 268 seconds) has detected all the 24 pulsars detected by the PMB.
The PALFA survey has detected 8 other pulsars in this area, not previously detected by the PMB.
Sensitivity is therefore better – but more was probably to be expected! However, 2/3 of the faint pulsars are still eluding us – we are doing the survey in sparse mode.
Therefore, for the part of the Arecibo sky surveyed by the PMB, we should expected to detect, _with present sensitivity_, at least ~65 MSPs. This is a conservative estimate – assumes that no further faint pulsars are found in the 2nd and 3rd passes needed for dense coverage.
207th AAS Meeting, Washington D.C. 4/27/2019

18. Finding Fast Pulsars
However, an even larger area of the Arecibo sky lies north of the upper limit of the PMB survey. Assuming, conservatively, that the number of MSPs in that larger area is half of the number found in the smaller area covered by the PMB (the pulsar density decreases as we move away from the center of the Galaxy), that would give us a total of about 100 MSPs.
The total could be larger than this!
First, more MSPs might be discovered in the PMB.
Second, a new back-end might (depending on RFI) enable us to use 300 MHz bandwidth, which will increase the sensitivity of the system.
Third, we will have better sensitivity to pulsars with short orbital periods.
207th AAS Meeting, Washington D.C. 4/27/2019

19. Finding Tight Binaries
Because the Arecibo dish is so much larger than Parkes, one can reach a better sensitivity with much smaller integration times for a similar sky coverage per unit time. This means that this survey will be much more sensitive to binary pulsars: no complex and CPU-intensive algorithms are needed to detect fast binary systems.
This, coupled with the much increased sensitivity to MSPs, should enable us to detect a much larger number of pulsars in tight binary systems than the PMB survey.
207th AAS Meeting, Washington D.C. 4/27/2019

20. Finding Tight Binaries
This is the discovery plot for PSR J With Arecibo, no curvature is seen. Pulsar is faint because it is so far from the beam (sparse survey).
207th AAS Meeting, Washington D.C. 4/27/2019

21. Finding tight binaries
In the PMB, pulsar was detected only after detection at Arecibo! Acceleration prevented the early detection of this system!
207th AAS Meeting, Washington D.C. 4/27/2019

22. Finding Fast Pulsars in Tight Binaries
This class of objects is the most exciting for relativistic studies! The PMB could not see these because of long integrations and poor time resolution. The PALFA survey has both of these problems sorted out.
To conclude, the discovery of a large number of interesting objects is to be expected from the PALFA survey.
Large computing resources will be needed to make it come true! Fortunately, Moore is likely to give us a hand.
207th AAS Meeting, Washington D.C. 4/27/2019

23. The pulsar consortium thanks…
…the people that made it possible:
Jeff Hagen, Bill Sisk and now Mikael Lerner for the magnificent work with the WAPP and CIMA gui
Avinash Deshpande, and the electronics department for all their work on the ALFA system and its characterization
Arun Venkataraman, for all of his efficient data transferring and archiving
NAIC and Bob Brown for pursuing ALFA, a great research instrument, and ATNF for building it.
207th AAS Meeting, Washington D.C. 4/27/2019

24. Using the new binaries: Two examples.
Although we expect to find (and have found already) exciting binary systems, even relatively “dull” systems can, with persistent timing, yield significant results.
PSR J , a circular MSP/WD system with an orbital period of 6.5 hours, has been timed at Arecibo for 10 years. Measurement of the relativistic orbital decay and Shapiro delay have lead to an estimate of its mass of 2.1+/- 0.2 solar masses (Nice et al. 2005, ApJ 634, 1242). If measured with greater precision, this value will have significant implications for the study of matter at very high densities.
Similar systems in very wide orbits (even more boring!) can be used to put fundamental constraints on violations of the strong equivalence principle, and constrain alternative theories of gravitation (Stairs et al. 2005, ApJ 632, 1060).
207th AAS Meeting, Washington D.C. 4/27/2019

25. Thank you for your time!
The National Astronomy and Ionosphere Center is operated by Cornell University, under a cooperative agreement with the National Science Foundation.
For any questions and suggestions, contact me at:
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207th AAS Meeting, Washington D.C. 4/27/2019