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Abstract
Abstract We report the detection of X-ray pulsations from the rotation-powered millisecond-period pulsars PSR J0740+6620 and PSR J1614−2230, two of the most massive neutron stars known, using observations with the Neutron Star Interior Composition Explorer (NICER). We also analyze X-ray Multi-Mirror Mission (XMM-Newton) data for both pulsars to obtain their time-averaged fluxes and study their respective X-ray fields. PSR J0740+6620 exhibits a broad double-peaked profile with a separation of ∼0.4 in phase. PSR J1614−2230, on the other hand, has a broad single-peak profile. We show the NICER detections of X-ray pulsations for both pulsars and also discuss the phase relationship to their radio pulsations. The XMM-Newton X-ray spectra of both pulsars shows they are thermally dominated but in the case of PSR J1614−2230 a weak nonthermal high energy tail appears to be present in the spectrum. The thermally dominated spectra along with broad modulations for both pulsars are indicative of thermal radiation from one or more small regions of the stellar surface. For PSR J0740+6620, this paper documents the data reduction performed to obtain the pulsation detection and prepare for pulse light curve modeling analysis.
Highlights
Rotation-powered millisecond pulsars (MSPs) are usually old neutron stars (NSs) with spin periods in the ∼1–30 ms range
We find evidence of weak nonthermal emission in the spectrum of PSR J1614−2230, and the fact that the detection of pulsations extends up to 2.29 keV in the Neutron Star Interior Composition Explorer (NICER) data opens the possibility that, with the accumulation of more data, pulsations may extend to higher energies
For PSR J0740+6620 the NICER observations have shown X-ray pulsations with two broad thermal X-ray peaks separated in phase by about ∼0.4
Summary
Rotation-powered millisecond pulsars (MSPs) are usually old neutron stars (NSs) with spin periods in the ∼1–30 ms range. This paper will focus on the X-ray emission from two pulsars found to be massive NSs. Radio pulse timing observations of MSPs have resulted in precisely measured NS masses near or above 2 Me for a number of objects to date. Obtaining NS radius measurements using this technique is one of the primary scientific goals of the Neutron star Interior Composition Explorer (NICER) mission The first such constraints were presented in Miller et al (2019) and Riley et al (2019) based on a deep NICER exposure of the nearby isolated MSP PSR J0030+0451 (Bogdanov et al 2019). We describe NICER observations of the two nearby binary MSP systems hosting massive NSs, PSR J0740+6620 and PSR J1614−2230, and report the detection of thermal X-ray pulsations from both.
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