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NICER / ISS Science Nugget
for April 27, 2023



Confirming an outbursting AMXP

During its continuing imaging survey of the inner regions of our Galaxy, the European INTEGRAL observatory detected, on April 13 through 15, high-energy X-ray emission from the direction of the known neutron-star binary system IGR J17498-2921. Discovered and last seen in outburst in 2011, IGR J17498 is a member of the rare class of accreting X-ray pulsars with millisecond spin periods (AMXPs). To unambiguously identify the INTEGRAL transient, NICER acquired a series of snapshot observations of IGR J17498 and found that, indeed, a fraction of the measured X-ray emission brightens and dims 401 times each second. News of NICER's confirmation of the second known outburst of IGR J17498 was published by A. Sanna (Univ. of Cagliari, Italy) and collaborators in Astronomer's Telegram #15998 on April 20, 2023.

Since AMXP outbursts typically lasting 2-3 weeks, NICER continues to monitor IGR J17498. In addition to enabling high-precision timing of the millisecond pulsations, which provides measurements of the neutron star's binary orbit with exquisite precision, follow-up observations have captured an unusual Type I X-ray burst, a flash that is attributed to a thermonuclear explosion on the surface of a neutron star accreting light-element (primarily hydrogen) "fuel" from a companion star - the matter is compressed to extreme temperatures and densities until unstable nuclear fusion occurs. Time-resolved spectroscopy of the X-ray burst from IGR J17498, to better understand the chemical composition of the accreted fuel, the surface conditions, and the impact of the explosion on the accretion flow, is underway.


Pulse profile of the transient accreting millisecond X-ray pulsar IGR J17498-2921, from a 1,800-second NICER exposure on April 20. Approximately 5% of the measured photon count-rate is modulated with the neutron star's 2.5 ms spin period, here divided into 10 bins; for clarity, the same data values are plotted twice in pulse phase. The rapid rise and gradual decay of photon count-rate over approximately 10 seconds, in an observation on April 23, is characteristic of a Type I X-ray burst, but this event from IGR J17498 shows unusual features in its shape: a complex peak and a plateau (or shoulder) during its decay.

Figure: Left: Pulse profile of the transient accreting millisecond X-ray pulsar IGR J17498-2921, from a 1,800-second NICER exposure on April 20. Approximately 5% of the measured photon count-rate is modulated with the neutron star's 2.5 ms spin period, here divided into 10 bins; for clarity, the same data values are plotted twice in pulse phase. Right: The rapid rise and gradual decay of photon count-rate over approximately 10 seconds, in an observation on April 23, is characteristic of a Type I X-ray burst, but this event from IGR J17498 shows unusual features in its shape: a complex peak and a plateau or "shoulder" during its decay.



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