The mystery of unbreakable radio jets

Black holes and neutron stars are notorious for sucking material from their surroundings towards themselves, a process that we call accretion. However, both types of objects also blast material back into space, for instance via highly collimated streams of gas and energy that we call jets. The material that is hurdling towards black holes and neutron stars, the accretion flow, is hot and emits heat radiation at X-ray wavelengths. The jets, on the other hand, emit radiation at radio wavelengths.

It is natural to assume that there is some kind of connection between how (much) material is flowing in and how (much) is pushed out in a radio jet. Indeed, a strong correlation between the X-ray and radio brightness is observed for both black holes and neutron stars, which points towards a strong connection. For black holes, it has also been observed that when material is flowing in extremely rapidly, it is no longer possible to push out a continuous jet. Rather, when matter is pushing in at high speed, it is spewed out in spurts while the continuous steady jet seen at low accretion rates disappears. The latter is observed as a sudden strong reduction in the radio brightness once the X-ray luminosity, hence the rate of matter inflow, climbs up to very high levels. Surprisingly, some neutron stars do not show a strong reduction of their radio brightness when we see them move up to high accretion rates. It therefore appears that, somehow, these neutron stars are able to sustain their continuous, steady radio jets. It is a long-standing puzzle why this is the case.

In a recent study, we investigated the coupled radio and X-ray behavior of the accreting neutron star 4U 1820-30. This is one of those few neutron stars that was thought to sustain its continuous radio jets because its radio brightness never becomes very low. What we found, however, is that the brightness at different radio frequencies does vary by a lot causing the radio energy spectrum to change strongly. In particular, we found that between X-ray “low and high modes” that differ a factor of about 10 in X-ray brightness, 4U 1820-30 is switching between sending out a steady continuous jet and ballistic ejections, represented by the two different radio spectra. Contrary to what was thought, the neutron star is thus not sustaining its steady jets, but behaving in the same way as black holes. These findings motivate similar studies of other neutron stars as well as a more detailed study of 4U 1820-30 itself to resolve the changes on shorter timescales (days or even hours) than we have done now (weeks).

Russell, Degenaar, van den Eijnden, Del Santo, Segreto, Altamirano, Beri, Diaz Trigo, Miller Jones 2020, MNRAS 508, L6: The evolving radio jet from the neutron star X-ray binary 4U 1820-30

Paper link: ADS

Radio observations of 4U1820 during its X-ray high and low modes. The figure shows that there is a clear difference in the radio spectrum between low modes (for instance the green, red and yellow data points+curves) and high modes (for instance the blue, purple and pink data points+curves). These large changes are highly surprising since the change in X-ray luminosity between the two modes is very small (a factor of 2 or so) and also the X-ray spectrum remains largely the same.