Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17498-2921

TL;DR

This paper investigates the spectral and timing characteristics of the accreting millisecond X-ray pulsar IGR J17498-2921, revealing stable spectral properties, pulsation behavior, and insights into burst composition and ignition processes.

Contribution

It provides detailed spectral modeling, timing analysis, and burst composition insights, advancing understanding of accreting millisecond pulsars and their X-ray emission mechanisms.

Findings

Spectral fits indicate thermal Comptonization with kT_e ~ 50 keV.

Pulsations detected at 2.5 ms up to 65 keV with constant pulsed fraction.

Burst analysis suggests hydrogen-poor material and challenges helium-ignition models.

Abstract

We analyze the spectral and timing properties of IGR J17498-2921 and the characteristics of X-ray bursts to constrain the physical processes responsible for the X-ray production in this class of sources. The broad-band average spectrum is well-described by thermal Comptonization with an electron temperature of kT_e ~ 50 keV, soft seed photons of kT_bb ~ 1 keV, and Thomson optical depth \taut ~ 1 in a slab geometry. The slab area corresponds to a black body radius of R_bb ~9 km. During the outburst, the spectrum stays remarkably stable with plasma and soft seed photon temperatures and scattering optical depth that are constant within the errors. This behavior has been interpreted as indicating that the X-ray emission originates above the neutron star (NS) surface in a hot slab (either the heated NS surface or the accretion shock). The INTEGRAL, RXTE, and Swift data reveal the X-ray pulsation at a period of 2.5 milliseconds up to ~65 keV. The pulsed fraction is consistent with being constant, i.e. energy independent and has a typical value of 6-7%. The nearly sinusoidal pulses show soft lags that seem to saturate near 10 keV at a rather small value of ~ -60\mu s with those observed in other accreting pulsars. The short burst profiles indicate that there is a hydrogen-poor material at ignition, which suggests either that the accreted material is hydrogen-deficient, or that the CNO metallicity is up to a factor of about two times solar. However, the variation in the burst recurrence time as a function of \dot{m} (inferred from the X-ray flux) is much smaller than predicted by helium-ignition models.