Jets are collimated outflows of matter and energy produced by accreting astrophyical objects. Such jets are found on many different scales in the universe, ranging from young stars to supermassive black holes in the centers of galaxies. Black holes and neutron stars that accrete gas from a companion star in an X-ray binary are prominent jet producers too. In these systems, the collimated jets are most prominently detected at radio wavelengths.
Starting in the 1970s, the radio jets of X-ray binaries have been studied in great detail. One key characteristic is that there is a very strong correlation between the radio brightness and the X-ray luminosity, which suggests a strong coupling between the inflow of matter (traced by the X-rays) and the outflow (traced by the radio emission). Early studies suggested, however, that the coupling between the X-rays and the radio emission, parametrized by the coupling index beta, is different for black holes and neutron stars. A plausible explanation for this difference could be that neutron stars have a solid surface; whereas gas that reaches a black hole can be carried across the event horizon without emitting any radiation, all energy contained by the gas will be converted into X-ray radiation when it hits the surface of a neutron star. This could translate into a different X-ray/radio correlation.
Collecting the largest sample of radio/X-ray points of X-ray binaries to date, we set out to perform a rigorous statistical analysis to investigate if the jets of neutrons stars are fundamentally different from those of black holes. Our analysis contained a total of 35 individual black holes, and 41 neutron stars and let to several important conclusions. Several common conjectures about neutron star jets were disproved by our analysis, while others were strengthened, leading to the following facts and myths:
Our rigorous analysis reinforces previous conjectures that the radio emission of neutron stars is fainter, by a factor ~20, than that of black holes accreting at similar X-ray luminosity. Correcting for different factors that might influence the comparison (e.g. their difference in mass, different bolometric correction factors and the extra X-ray emission of neutron stars coming from their surface) does not lift this difference. Therefore, we are left to conclude that, in general, neutron stars produce less bright radio emission than black holes accreting at similar rates.
1) For decades, the number of neutron stars observed in the radio band was much more modest than that of black holes, partly driven by the fact (see above) that neutron stars were considerably fainter in the radio band, hence more difficult to observe. However, exploiting the current generation of upgraded radio facilities, much more neutron stars have been observed in the radio band. In fact, our study included 41 different neutron stars, compared to 35 different black holes. Neutron stars are thus no longer underrepresented in radio studies.
2) It is commonly said that neutron stars display a larger scatter in the radio/X-ray plane, i.e. display more chaotic behavior. However, in our study we found that the statistical scatter in the neutron star sample is similar to that in the black hole sample.
3) It is often assumed that neutron stars, in general, show a different (namely steeper) correlation between their radio and X-ray luminosity. However, this conjecture is largely based on a detailed study of one individual neutron star. Considering the sample as a whole, we obtained a coupling index for the neutron stars that was consistent with being the same as that of the black hole sample. It thus appears that neutron stars do not show a different radio/X-ray coupling than black holes.
Apart from comparing the neutron star and black hole samples, we also investigated if sub-samples among the neutron stars may behave differently. Interestingly, we found that the sub-population of transitional millisecond radio pulsars, statistically behaves differently from the other neutron stars. This suggest that their jet properties are fundamentally different.
Gallo, Degenaar & van den Eijnden 2018, MNRAS Letters 478, L132: Hard state neutron star and black hole X-ray binaries in the radio:X-ray luminosity plane
Paper link: ADS