Diego F. Torres

Institut de Ciències de l'Espai (CSIC-ICE) & Institut d’Estudis Espacials de Catalunya (IEEC)

Using the largest radio telescope on Earth, the Five-hundred-meter Aperture Spherical radio Telescope (FAST, in China), an international team including ICREA Prof. Diego Torres has discovered radio pulsations from the system LS I 61 303. This is the first evidence for pulsations from this source at any frequency, and proves the existence of a rotating neutron star in this system, something that was debated for decades.

LS I 61 303 is a rare system, one of the few known gamma-ray binaries. These systems emit most of their luminosity in photons with very high energies. They are stellar systems formed by a massive star and a compact object, that can be either a black hole or a neutron star. To know for sure what the compact object is has implications for understanding the multifrequency emission and the evolutionary path of gamma-ray binaries, and how to relate them to other classes. In this case, LS I 61 303 has also shown super-orbital variability and magnetar-like flares.

Torres’ group have carried out many deep searches for pulsations in X-rays, hard X-rays, and GeV gamma-rays before actually finding them in radio frequencies. It was a difficult task: not only they were trying to detect a pulsar that is not particularly bright, but, similarly to other pulsars, one for which pulsations are not permanent.

Torres is particularly concerned with the period actually found also from a theoretical perspective. The period of the pulsar, 0.26 s, is right at the predicted range of a multi-frequency model he did a decade ago (Torres et al. 2012, and A. Papitto,Torres, & Rea 2012, both published in The Astrophysical Journal). In this model, the system transitions between two states along the orbit, according to the mass pressure in the vicinity, explaining long-term multi-frequency recurrencies as well. The current period measurement brings increased interest to this possibility.