Final Report Summary - PULSARS WITH LOFAR (Radio pulsars with LOFAR: a study of extreme physics laboratories)
Due to their super-nuclear density, radio pulsars enable a wide range of fundamental physics science goals. This project focused on two of those: 1) the supernova mechanism, and 2) the nature of extreme gravity. For each of these two, objectives were defined, as detailed below; almost all these were met, resulting in nine refereed journal papers, four conference papers and one upcoming PhD thesis (2013, for Thijs Coenen, the graduate students this project partially funded).
- Background
Once the cores of massive stars grow to 1.4 solar masses, these cores cave in under their self-gravity. In less than a second the entire stellar core shrinks to a 10-km neutron star and the collapse is abruptly stopped by neutron repulsion. The outer layers of the star explode in a supernova. The newly formed neutron star can be seen as a pulsar.
- Project objectives
The project is part of a research programme with two main objectives. The first is to find all nearby radio pulsars and measure their velocities. From such a unique complete census we can determine the supernova progenitor mass range that forms neutron star, while the neutron-star velocity distribution can constrain the collapse asymmetry and energetics.
The second is to find several extremely stable individual radio-pulsar systems that can be used in either a pulsar timing array to directly detect gravitational waves or in the first ever strong-field test of gravity.
In this MC-IRG project, the objective was to find new PULSARS WITH LOFAR, to then characterise the newly-found systems for overall goal oen, and monitor which systems are suitable for overall goal two.
- Description of the work carried out
Objective one: 'Find and characterise new PULSARS WITH LOFAR' To reach our objective, Van Leeuwen, Coenen and collaborators commissioned pulsar observing for LOFAR, a revolutionary new radio telescope that was completed by the host institute in 2010. This low frequency array (LOFAR) has more collecting area by itself than all other radio telescopes in the world combined and it operates in a frequency range (130-220 MHz) in which radio pulsars shine at their brightest. Partly due to this ambitious nature, the LOFAR opening was two years delayed, until 2010. For this MC-IRG project that delay was beyond our control; and we were able to meet our project science goals by using data from different telescopes, as will be shown below. Still, the commissioning of this first-ever software telescope produces much fundamental science opportunities for objective one, resulting in three refereed papers (van Leeuwen & Stappers 2010, Stappers et al. 2010, Wise et al. 2012) and numerous invited talks and colloquiums for researcher Van Leeuwen.
After this commissioning, we undertook the first LOFAR pulsar survey, the LOFAR pilot pulsar survey (LPPS). LPPS was a shallow 140-MHz survey covering roughly half of the northern celestial hemisphere. This survey re-detected several tens of pulsars, validating our software and approach. More importantly, it already discovered one new pulsar, the first-ever with LOFAR, or any software telescope. [Coenen et al., 2012, ERPM; Coenen 2013, PhD thesis, U. Amsterdam]
For the second LOFAR pulsar survey, the LOFAR tied array survey (LOTAS), data was taken end 2011 and beginning 2012, and is currently being analysed. This survey uses 19 tied-array beams, the highest number of simultaneous beams for any pulsar survey so far, and offers close to an order of magnitude increase in sensitivity compared with LPPS. [Coenen et al., 2012, IAUS291; Coenen 2013, PhD thesis, U. Amsterdam]
Analyzing these surveys is extremely computationally intensive. We provide this computing power by combining four supercomputers (Dwingeloo, Groningen, Amsterdam, Manchester) via a private fiber network. For this novel data-reduction setup Van Leeuwen received the 2009 enlighten your research prize, awarded personally by the Dutch minister of Science.
The two-year LOFAR startup delay meant that the longer-term part of the characterisation (mainly, measuring velocities) could be finished before the end date of the proposal. The host institute, however, will continue to fund that research. Van Leeuwen was further able to mitigate the two-year LOFAR delay by increasing involvement in US surveys, thus further strengthening Europe-North America ties that we outlined for this proposal. In the green bank telescope and pulsar spigot at 350MHz (GBT350), green bank north celestial cap (GBNCC), and pulsar alpha (PALFA) surveys, the LOFAR pulsar search software was used; these found about 100 new pulsars and resulted in three new papers. [Boyles et al. 2011, AIP1357; Knispel et al. 2010, Science; van Leeuwen et al., ApJ, 2012]
On the theoretical front, the researcher and collaborators were able to explain for the first time the formation of the enigmatic eccentric-orbit millisecond pulsars, based on supernova and binary-evolution models. [Portegies Zwart et al., ApJ 2011]
Objective two: 'monitor which systems are suitable as high-sensitivity physics laboratories' --- Using the green bank telescope (GBT) in the United States and part of the same software we use for LOFAR, we monitored a unique binary star system. This new system turned out to be the best-visible laboratory for accretion and spin-up physics. It is a 'missing link' in the process that creates the fastest-spinning stars in our Universe: millisecond pulsars. The resulting paper appeared in Science (2009, 324, p1411).
Thijs Coenen, the graduate student partially funded from this grant, and researcher Van Leeuwen next used data from GBT to further test the LOFAR census software, with the specific goal of finding stable systems for objective two. These results were accepted in astronomy & astrophysics, the top European astronomical journal. This work is in collaboration with Prof Dr Ingrid Stairs, and uses a US telescope; again exemplifying this proposal's collaboration between Europe and North America. [Coenen, van Leeuwen, Stairs, A&A 2011]
Together with US researcher Dr Andrey Timokhin, Van Leeuwen next made the highest-sensitivity estimates of the plasma physics around the pulsar magnetic poles to date. Results were published in ApJ, the world's highest-ranking astronomy journal. [van Leeuwen & Timokhin 2012]
For LOFAR, we designed and implemented a 'coherent dedispersion' software machine, running on top of the LOFAR supercomputer. This made possible a significant step in time sensitivity.
- Main results
Below we compare the originally proposed results and deliverables with those achieved:
[DONE] Integrated pulsar-survey computing cluster --- The LOFAR supercomputer was seamlessly connected with a cluster of thee further supercomputers via a private fiber network.
[DONE] Public archive of LOFAR survey data --- The LOFAR survey data is available for download through the Grid.
[DONE] Multiple research papers on nearby pulsars, supernova mass ranges and asymmetries --- 6 papers were written and published
[DONE] Dispersion-removal pulsar machine --- The LOFAR BG/P supercomputer was extended with a 'Coherent dedispersion' software machine.
[MOSTLY DONE] Multiple research papers on gravitational-wave background, and tests of relativity --- No papers yet on gravity-wave background; two papers were published on test of relativity
- Conclusions
The project deliverables were produced and the objectives were met. A revolutionary new telescope was commissioned, new pulsars were discovered, nine refereed papers were written, several EU-US collaborations were continues while two collaborations were formed.
- Potential Impact and Use
The LOFAR Pulsar observing modes commissioned under this project are now open for public use. A call for proposals went out in June 2012. The LOFAR survey data is available for download and processing on the Grid. The target groups for these are the scientific, astronomical community. The scientific findings of the project were presented to the wider general audience in a series of lectures, newspaper and magazine articles, and radio and television appearances by researcher Van Leeuwen. The target groups there the general audience in The Netherlands, with special focus on high-school age children.
- Background
Once the cores of massive stars grow to 1.4 solar masses, these cores cave in under their self-gravity. In less than a second the entire stellar core shrinks to a 10-km neutron star and the collapse is abruptly stopped by neutron repulsion. The outer layers of the star explode in a supernova. The newly formed neutron star can be seen as a pulsar.
- Project objectives
The project is part of a research programme with two main objectives. The first is to find all nearby radio pulsars and measure their velocities. From such a unique complete census we can determine the supernova progenitor mass range that forms neutron star, while the neutron-star velocity distribution can constrain the collapse asymmetry and energetics.
The second is to find several extremely stable individual radio-pulsar systems that can be used in either a pulsar timing array to directly detect gravitational waves or in the first ever strong-field test of gravity.
In this MC-IRG project, the objective was to find new PULSARS WITH LOFAR, to then characterise the newly-found systems for overall goal oen, and monitor which systems are suitable for overall goal two.
- Description of the work carried out
Objective one: 'Find and characterise new PULSARS WITH LOFAR' To reach our objective, Van Leeuwen, Coenen and collaborators commissioned pulsar observing for LOFAR, a revolutionary new radio telescope that was completed by the host institute in 2010. This low frequency array (LOFAR) has more collecting area by itself than all other radio telescopes in the world combined and it operates in a frequency range (130-220 MHz) in which radio pulsars shine at their brightest. Partly due to this ambitious nature, the LOFAR opening was two years delayed, until 2010. For this MC-IRG project that delay was beyond our control; and we were able to meet our project science goals by using data from different telescopes, as will be shown below. Still, the commissioning of this first-ever software telescope produces much fundamental science opportunities for objective one, resulting in three refereed papers (van Leeuwen & Stappers 2010, Stappers et al. 2010, Wise et al. 2012) and numerous invited talks and colloquiums for researcher Van Leeuwen.
After this commissioning, we undertook the first LOFAR pulsar survey, the LOFAR pilot pulsar survey (LPPS). LPPS was a shallow 140-MHz survey covering roughly half of the northern celestial hemisphere. This survey re-detected several tens of pulsars, validating our software and approach. More importantly, it already discovered one new pulsar, the first-ever with LOFAR, or any software telescope. [Coenen et al., 2012, ERPM; Coenen 2013, PhD thesis, U. Amsterdam]
For the second LOFAR pulsar survey, the LOFAR tied array survey (LOTAS), data was taken end 2011 and beginning 2012, and is currently being analysed. This survey uses 19 tied-array beams, the highest number of simultaneous beams for any pulsar survey so far, and offers close to an order of magnitude increase in sensitivity compared with LPPS. [Coenen et al., 2012, IAUS291; Coenen 2013, PhD thesis, U. Amsterdam]
Analyzing these surveys is extremely computationally intensive. We provide this computing power by combining four supercomputers (Dwingeloo, Groningen, Amsterdam, Manchester) via a private fiber network. For this novel data-reduction setup Van Leeuwen received the 2009 enlighten your research prize, awarded personally by the Dutch minister of Science.
The two-year LOFAR startup delay meant that the longer-term part of the characterisation (mainly, measuring velocities) could be finished before the end date of the proposal. The host institute, however, will continue to fund that research. Van Leeuwen was further able to mitigate the two-year LOFAR delay by increasing involvement in US surveys, thus further strengthening Europe-North America ties that we outlined for this proposal. In the green bank telescope and pulsar spigot at 350MHz (GBT350), green bank north celestial cap (GBNCC), and pulsar alpha (PALFA) surveys, the LOFAR pulsar search software was used; these found about 100 new pulsars and resulted in three new papers. [Boyles et al. 2011, AIP1357; Knispel et al. 2010, Science; van Leeuwen et al., ApJ, 2012]
On the theoretical front, the researcher and collaborators were able to explain for the first time the formation of the enigmatic eccentric-orbit millisecond pulsars, based on supernova and binary-evolution models. [Portegies Zwart et al., ApJ 2011]
Objective two: 'monitor which systems are suitable as high-sensitivity physics laboratories' --- Using the green bank telescope (GBT) in the United States and part of the same software we use for LOFAR, we monitored a unique binary star system. This new system turned out to be the best-visible laboratory for accretion and spin-up physics. It is a 'missing link' in the process that creates the fastest-spinning stars in our Universe: millisecond pulsars. The resulting paper appeared in Science (2009, 324, p1411).
Thijs Coenen, the graduate student partially funded from this grant, and researcher Van Leeuwen next used data from GBT to further test the LOFAR census software, with the specific goal of finding stable systems for objective two. These results were accepted in astronomy & astrophysics, the top European astronomical journal. This work is in collaboration with Prof Dr Ingrid Stairs, and uses a US telescope; again exemplifying this proposal's collaboration between Europe and North America. [Coenen, van Leeuwen, Stairs, A&A 2011]
Together with US researcher Dr Andrey Timokhin, Van Leeuwen next made the highest-sensitivity estimates of the plasma physics around the pulsar magnetic poles to date. Results were published in ApJ, the world's highest-ranking astronomy journal. [van Leeuwen & Timokhin 2012]
For LOFAR, we designed and implemented a 'coherent dedispersion' software machine, running on top of the LOFAR supercomputer. This made possible a significant step in time sensitivity.
- Main results
Below we compare the originally proposed results and deliverables with those achieved:
[DONE] Integrated pulsar-survey computing cluster --- The LOFAR supercomputer was seamlessly connected with a cluster of thee further supercomputers via a private fiber network.
[DONE] Public archive of LOFAR survey data --- The LOFAR survey data is available for download through the Grid.
[DONE] Multiple research papers on nearby pulsars, supernova mass ranges and asymmetries --- 6 papers were written and published
[DONE] Dispersion-removal pulsar machine --- The LOFAR BG/P supercomputer was extended with a 'Coherent dedispersion' software machine.
[MOSTLY DONE] Multiple research papers on gravitational-wave background, and tests of relativity --- No papers yet on gravity-wave background; two papers were published on test of relativity
- Conclusions
The project deliverables were produced and the objectives were met. A revolutionary new telescope was commissioned, new pulsars were discovered, nine refereed papers were written, several EU-US collaborations were continues while two collaborations were formed.
- Potential Impact and Use
The LOFAR Pulsar observing modes commissioned under this project are now open for public use. A call for proposals went out in June 2012. The LOFAR survey data is available for download and processing on the Grid. The target groups for these are the scientific, astronomical community. The scientific findings of the project were presented to the wider general audience in a series of lectures, newspaper and magazine articles, and radio and television appearances by researcher Van Leeuwen. The target groups there the general audience in The Netherlands, with special focus on high-school age children.