Curriculum Vitae

• Research Professor

  Department of Physics, West Virginia University (August 2008 - present). Beginning in August 2008, half my time (Spring and Fall) is being spent at WVU, working with the WVU Pulsar Group

• Senior Research Associate

  Arecibo Observatory, Cornell University (June 2007 - present). Since 2001, my main duties have been: a) to provide scientific support of telescope operations and b) to conduct independent scientific research, which in my case is focused on the study of pulsars.

• Research Associate

  Arecibo Observatory, Cornell University (May 2003 - May 2007).

• Post-Doctoral Research Associate

  Arecibo Observatory, Cornell University (May 2001 - April 2003).

• Post-Doctoral Research Associate

  Jodrell Bank Observatory, University of Manchester, U.K. (October 2000 - March 2001).

• Ph.D. in Radioastronomy

  Jodrell Bank Observatory, The University of Manchester, U.K. (1997-2000).
• Thesis title: ``The Pulsars in 47 Tucanae''. Supervisor: Andrew G. Lyne. Download Ph.D. Thesis in .pdf.gz and .ps.gz formats.

• Mestrado em Física

  (M.Sc. degree in Physics) Instituto Superior Técnico, Lisboa, Portugal (1995-1997).

• Licenciatura em Engenharia Física Tecnológica

  (Graduation in Engineering, with specialization in Physics and Technology),
  Instituto Superior Técnico, Lisboa, Portugal (1989-1994).
• The last year was spent at Université Paris 7, Paris, France; working at the Observatoire de Paris a Meudon. The dissertation work was on a search for extra-solar planets using the photometric method.

1. Binary pulsars give us the only strong-field tests of gravitational theories. Russel Hulse and Joe Taylor earned the Nobel Prize in Physics in 1993 for their discovery of the first binary pulsar, PSR B1916+13, made using the Arecibo radio telescope. Precise tracking of the motion of this object led to a precise test of general relativity which confirmed the existence of gravitational waves (GWs), a fundamental prediction of general relativity.


2. Pulsars might also be important for the direct detection of GWs:
   • Fast-spinning pulsars could provide the terrestrial GW detectors like LIGO and VIRGO with a detectable source, particularly if the very fast rotators lose energy via the GW-induced Rossby instability.
   • Precise timing of bright millisecond pulsars can be used to search for GWs with frequencies of the order of nHz. This regime is complementary to the terrestrial detectors (F ~ 10^2 Hz) and space interferometers like LISA (~1 mHz < F < ~ 0.1 Hz).


3. Given the large mass (~ 1.4 solar masses) and small radius (10-16 km) of neutron stars, matter at their centers is one or two orders of magnitude denser than atomic nuclei. The composition and behavior of matter in these conditions is not known. Studies of recycled pulsars can constrain its bulk properties (more particularly, the relation between pressure and density, or the equation of state, ``EOS''):
   • The "soft" EOS models predict lower pressures for a given density. More compressible matter translates into smaller, more compact stars with very high gravitational fields, these have upper mass limits of about 1.6 solar masses (above that limit, the star implodes and forms a black hole). Finding a more massive star, we can exclude such "soft" EOS models.
   • Because of their different micro-physical assumptions, "stiff" EOS models predict that matter is highly incompressible. That would produce very large stars that can not withstand large spin frequencies without breaking apart. Finding fast-spinning pulsars could therefore disprove such EOS models.

Pulsars in globular clusters

• A prime example of an exotic binary is NGC 1851A. At the time of discovery, this was the binary pulsar with the most eccentric orbit known. I found it using the GMRT, in collaboration with Yashwant Gupta and Scott Ransom. It probably formed as a "normal" millisecond pulsar - light white dwarf binary, which was later disrupted by the intrusion of the present massive companion. This is the exact mechanism for the creation of MSP - black hole or MSP-MSP systems!


• I am part of the collaboration led by Scott M. Ransom, and including Jason Hessels, Ingrid Stairs, Fernando Camilo, Victoria Kaspi and David Kaplan that, in January 2005, announced the discovery of 21 new MSPs in the globular cluster Terzan 5 using the S-band receiver of the Green Bank Telescope. The timing of two of the new binary systems in Terzan 5, I and J, indicates that one of them has a mass > 1.68 M_sun at 95 % confidence.


• Nine new pulsars have since been found in Terzan 5. One of them, Terzan 5 ad, is an eclipsing binary system, and it is now the fastest spinning pulsar known. Although it is still not fast enough to constrain the equation of state seriously, other faster pulsars might still be found in globular clusters. Also, Terzan 5 ad already constrains models of gravitational wave emission. I have obtained its timing solution, which might enable its detection by LIGO in the near future.


• More recently, we have started using the S-band receiver of the Green Bank Telescope in a systematic survey of other globular clusters. To date, we have discovered 29 new MSPs in 7 globular clusters. One of the most interesting new discoveries is NGC 6440B. This is a 16.7-ms pulsar in a 20.5-day orbit with a ~0.1 solar mass companion. The high eccentricity of this system (e = 0.57) has allowed a measurement of the precession of periastron, and from this an estimate of the total mass of the binary system and of the mass of the pulsar: (2.74 +/- 0.21) solar masses. This is by far the highest neutron star mass estimate to date.
  If confirmed by further timing, this mass measurement represents the most stringent constraint of the EOS. This mass estimate is so high that the products of the coalescence of double neutron star systems might be stable neutron stars themselves! It would also imply that at high densities the matter at the center of neutron stars is so incompressible that it propagates sound waves near the speed of light.


• After timing M5B for 19 years with the Arecibo radio telescope, we measured the advance of periastron for this binary system. The total binary mass is 2.29 +/- 0.17 M_sun. The estimate for the pulsar mass (2.08 ^+0.18_-0.19 M_sun) is the second largest ever, after that of NGC 6440B. There is a 95% probability that the mass of the pulsar is larger than 1.72 solar masses. This excludes most "soft" EOS models. The non-detection of the companion in deep HST images implies that the precession of periastron is relativistic.

Pulsars in the Galactic disk

• I helped with the Parkes High-Latitude Survey. This found the first double pulsar, PSR J0737-3039 A and B. Timing this pulsar for only three years has already yielded the most precise strong-field tests of general relativity ever!


• I am heavily involved in the ALFA pulsar survey (see also the Arecibo website of this survey). This survey has already produced very interesting results:
  • It has discovered the youngest binary pulsar, PSR J1906+0746; which is also the second most relativistic system known. It belongs to a new population of young, binary systems that could include even more eccentric and compacts orbits! This discovery shows that the ALFA pulsar survey is specially sensitive to pulsars in compact orbits.
  • This survey also has unprecedented sensitivity to fast-spinning pulsars at high DMs, as recently demonstrated by the discovery of PSR J1903+0327, a 2.15-ms pulsar at a DM of 297 cm^-3pc (the highest for a known millisecond pulsar). This indicates that the ALFA pulsar survey can detect millisecond pulsars in a much larger volume of our Galaxy than any previous survey. Apart from its high DM and spin frequency, this object has extraordinary characteristics: it is the first MSP in the Galaxy found in an eccentric (e = 0.44) orbit, its companion is massive (Mc ~ 1 solar mass) and it might be a main-sequence star! Presently, no stellar evolution theory can explain the formation of such a bizarre system! Furthermore, the orbital characteristics allow the measurement of relativistic effects that might give us a very precise estimate of the mass of this pulsar. The preliminary value is 1.74 +/- 0.04 solar masses, if confirmed with further timing (which will allow the measurement of extra relativistic parameters) this would very firmly exclude several equations of state for dense matter.


• Timing the PSR J1738+0333 binary system at Arecibo, I have recently derived stringent constraints on emission of dipolar gravitational waves. This is very important for constraining alternative theories of gravitation! The precision of the measurement of the orbital decay of PSR J1738+0333 will continue to improve fast in the near future, and this pulsar might soon surpass PSR J1141-6545 as the best gravitational wave laboratory known. Furthermore, the timing precision of this object and of PSR J1741+1351 is such that they will be used in the array of MSPs that will be used to search for gravitational waves (see below).


• I am a founding member of the recently formed North American Nano-Hertz Observatory of GRAVitational waves (NANOGrav). Over the next few years, this Consortium will coordinate the efforts of North American pulsar astronomers towards detecting very low frequency gravitational waves via improved timing of bright millisecond pulsars, in close association with sister organizations in Australia and Europe.