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.

Feebly-interacting particles: FIPs 2020 workshop report

“…Both cases were applied to axion like particles and resulted in strong bounds on the ALP-photon coupling, comparable to the bounds from SN 1987A. Another notable example is the observed excess of hard X-ray events from a group of NS known as the Magnificent Seven (M7) [140]. The origin of this anomaly is not understood but it was speculated in Ref.…”

Astroparticle physics with compact objects

“…Their relative proximity to the Earth, with distances in 100-600 pc range also benefits searches. Deep observations in 2-8 keV range by XMM-Newton and Chandra revealed existence of X-ray excess for two XDINS: RX J1856.6-3754 and RX J0420.0-5022 [103]. This excess could be explained by axion-photon conversion if g aγγ × g ann ∼ 4 × 10 −20 GeV −1 (only central value is quoted, systematic and statistical uncertainties could exceed an order of magnitude ) [97].…”

Astroparticle Physics with Compact Objects

“…Their relative proximity to the Earth, with distances in a 100-600 pc range, also benefits searches. Deep observations in a 2-8 keV range by XMM-Newton and Chandra revealed the existence of X-ray excess for two XDINS: RX J1856.6-3754 and RX J0420.0-5022 [103]. This excess could be explained by axion-photon conversion if g aγγ × g ann ∼ 4.0 × 10 −20 GeV −1 (only a central value is quoted, systematic and statistical uncertainties could exceed an order of magnitude ) [97].…”