What gamma-ray pulsars are X-ray bright? And why?

Neutron stars are a common compact endpoint of the life of stars. They have an extreme density (stars of about 10 km in size, with the mass of our Sun), and harbour the strongest magnetic fields (from 108 to 1014 times that of our Sun). Magnetized and rotating neutron stars emit beams of radiation, which can only be seen when the observer and the beam stand aligned. Periodic recurrence of such alignment gives rise to pulsations, and to the name pulsar used for these objects.

Pulsars were discovered 50 years ago, but many of their main characteristics are still elusive. They emit at essentially all frequencies, and their energy distribution (that is, how much power they yield at each frequency band) is very varied. In fact, one of the most intriguing mysteries of pulsars relates to the origin of this spectral variety.

From the more than 2000 radio pulsars known, and the more than 200 gamma-ray pulsars known, we only know less than 20 X-ray pulsars. This lack of pulsators in X-rays hampers having a global population understanding, as well as the ability of doing individual pulsar studies.

What makes a pulsar shine in gamma-rays and/or X-ray energies? Why some do at only one and not the other frequency? Ultimately, how can we predict, based on observations on only a part of the emission spectra, what will the pulsar emit in other bands?

Diego F. Torres, director of the Institute of Space Sciences (ICE, CSIC) in Barcelona, ICREA Professor and also a member of the Institut d’Estudis Espacials de Catalunya (IEEC) has presented a theoretical model with which to handle these questions. His results are published today in an article in Nature Astronomy.

Despite the extreme precision of the observations, and the underlying complexity of the processes involved, just four physical parameters suffice in his model to fit the spectrum of all gamma and/or X-ray pulsars known.

When analyzing the fitting for all pulsars, grouping of these parameters and relevant correlations appear, explaining the different observational behaviors.

“This model answers at once what process is behind the emission spectra and how the spectral variety arises. It explains intricacies such as why we have detected flat spectra at low and high energies. And most importantly, it provides a predictive tool by which to identify new X-ray pulsars.”

In fact, testing of the model with archival data has proven that it correctly pinpoint already known pulsars, and has already lead to new detections.

It is expected that via the use of the model by Prof. Torres, not only we shall understand the physics of these objects better, but that the population of pulsars detected at X-ray energies will rise notably.

To read the original article:

“Order Parameters for the high-energy spectra of pulsars”

D. F. Torres

Nature Astronomy (2018), DOI: 10.1038 / s41550-018-0384-5 (http://dx.doi.org).