Accretion is an important physical process in which an astronomical body gravitationally attracts material from its surroundings. This leads to growth and to the release of gravitational energy. We encounter accretion throughout the universe, on many different scales and in widely varying environments. For instance, stars and planets are formed through accretion, and the accretion behavior of a super-massive black hole determines how its host galaxy evolves over time.
Regardless of the nature of the object that is accreting (e.g. star, black hole), or the environment in which accretion occurs, it seems inevitable that part of the attracted material is spit back into space. This occurs to powerful collimated streams called jets. Despite being ubiquitous, exactly how jets are formed and being powered remains a mystery. To understand this, it is key to study jets in different types of accreting systems. This is one of the prime pursuits of modern astrophysics.
Strikingly, the only accreting systems for which jets had never been detected were neutron stars with strong magnetic fields (over a trillion times – a one with twelve zero’s that is – more powerful than the Earth’s magnetic field). This led to the long-standing paradigm that the presence of very strong magnetic field prevent jets from being formed.
Jets from accreting black holes and neutron stars with low magnetic fields (only a billion times the strength of the Earth’s magnetic field, i.e. a one with only nine zero’s) are most commonly detected at radio wavelengths. Despite various searches, radio emission had never been detected from accreting neutron stars with strong magnetic fields. Luckily, the Very Large Array (VLA) radio facility in New Mexico underwent major technical upgrades in recent years that greatly improved its sensitivity. We therefore decided to revisit if strong magnetic field neutron stars truly do not produce radio jets.
Somewhat to our surprise, Jakob made the startling discovery that the two strong-magnetic field neutron stars that we observed with the VLA, the well-studied sources GX 1+4 and Her X-1, were both detected in the radio band (at 8 GHz). While for GX 1+4 the radio properties allow for a different origin than a jet (e.g. shocks in the magnetosphere of the neutron star), the radio properties of Her X-1 more strongly suggest a jet origin. If confirmed, it would show that high-magnetic field neutron stars can launch jets after all. These findings would have important implications for understanding jet formation in general.
van den Eijnden, Degenaar, Russell, Miller-Jones, Wijnands, Miller, King, Rupen 2018, MNRAS Letters 473, L141: Radio emission from the X-ray pulsar Her X-1: a jet launched by a strong magnetic field neutron star?
Paper link Her X-1: ADS
van den Eijnden, Degenaar, Russell, Miller-Jones, Wijnands, Miller, King, Rupen 2018, MNRAS Letters 474, L91: Discovery of radio emission from the symbiotic X-ray binary system GX 1+4
Paper link GX 1+4: ADS