Hard X-Ray Excess from the Magnificent Seven Neutron Stars
Abstract
We report significant hard X-ray excesses in the energy range 2-8 keV for two nearby isolated neutron stars: RX J1856.6-3754 and RX J0420.0-5022. These neutron stars have previously been observed in soft X-rays to have nearly thermal spectra at temperatures ∼100 eV, which are thought to arise from the warm neutron star surfaces. We find nontrivial hard X-ray spectra well above the thermal surface predictions with archival data from the XMM-Newton and Chandra X-ray telescopes. We analyze possible systematic effects that could generate such spurious signals, such as nearby X-ray point sources and pileup of soft X-rays, but we find that the hard X-ray excesses are robust to these systematics to the extent that is possible to test. We also investigate possible sources of hard X-ray emission from the neutron stars and find no satisfactory explanation with known mechanisms, suggesting that a novel source of X-ray emission is at play. We do not find high-significance hard X-ray excesses from the other five Magnificent Seven isolated neutron stars.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2020
- DOI:
- 10.3847/1538-4357/abb4ea
- arXiv:
- arXiv:1910.02956
- Bibcode:
- 2020ApJ...904...42D
- Keywords:
-
- Neutron star cores;
- Neutron stars;
- X-ray point sources;
- X-ray stars;
- X-ray astronomy;
- 1107;
- 1108;
- 1270;
- 1823;
- 1810;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 42 pages, 34 figures. v2: updated version to be published in ApJ