The Quiescent Neutron Star and Hierarchical Triple: 4U 2129+47
Abstract
After a period of active accretion, neutron stars can enter a phase where their X-ray emission is dominated by thermal emission from their surface. The rate of cooling of this emission can yield insight into neutron star structure. Furthermore, emission models may help determine the neutron star radius. A number of questions arise when modeling such X-ray spectra as observed by Chandra or XMM-Newton. Is there ongoing, low level active accretion that is contributing to the observed soft X-ray emission? In a number of cases, a hard X-ray tail is also observed. What is the origin of this hard tail? The quiescent neutron star system 4U 2129+47 presents a unique opportunity to study these questions. This system is viewed nearly edge on, as evidenced by a periodic, total eclipse that lasts 1585 seconds out of the 5.24 hour orbit. As we are viewing this system edge on, both observed neutral column variations and an observed hard X-ray tail in year 2000 Chandra observations indicated ongoing active accretion. Subsequent XMM and Chandra observations over the next 15 years showed that both the neutral column variability and the hard X-ray tail vanished. Thus, these later observations may represent a true quiescent, cooling neutron star state. We assess the evidence for cooling in the 4U 2129+47 system. Furthermore, we use the timing of the X-ray eclipses to discuss evidence for a third body in the system, and derive likely orbital periods. Finally, we discuss how future X-ray missions, e.g., Athena and Lynx, could place more stringent limits on neutron star cooling and the presence of a hard tail (i.e., active accretion) in this system.
- Publication:
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AAS/High Energy Astrophysics Division #16
- Pub Date:
- August 2017
- Bibcode:
- 2017HEAD...1610820N