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Recent Science from NICER on the ISS

Rapid spin revealed

One of the most remarkable properties of neutron stars is how fast some of them spin: well over 700 rotations each second (42,000 RPM!). Measuring an object's rotation rate from a distance requires that it emits light non-uniformly, in a surface hot-spot or some form of directed "beam" that is swept around by the spinning star, appearing to pulse or flash regularly. Many neutron stars, especially those that accrete matter from a companion star in a binary system, are not known to exhibit detectable pulsations, and yet those that do make up the fastest rotators known. Investigating the high-spin population - understanding how they got there and what extreme physics may play out on their surfaces - means finding circumstances in which pulses, some of them short-lived, might arise. In this category are so-called burst oscillations, weak and fleeting pulsations that appear only during bursts of luminous X-ray emission associated with thermonuclear explosions on a neutron star's surface.

In a peer-reviewed paper recently published in The Astrophysical Journal, L Giridharan (CHRIST Univ., India) and collaborators describe the discovery of pulsations during one of two X-ray bursts detected from the neutron star binary 4U 1705-44 by NICER on 18 September, 2020. Lasting just about one second, the X-ray brightness near the peak of the first burst is seen to oscillate approximately 702 times, likely the result of ignition of light-element nuclear fuels (hydrogen burning to helium nuclei) in a well-localized region on the star. As the flame-front spreads and heating of the surface becomes more uniform, the detectable pulsations fade, once again hiding the underlying surface rotation from view. Approximately one hour later, a second burst is detected, with lower peak brightness and shorter duration; no oscillations are detectable in the second burst. Such "short waiting time" recurrent bursts are known to be especially common in the fastest-rotating neutron stars; it is hypothesized that centrifugal and shear forces in rapid rotators prevent all of the available fuel from being consumed in the initial explosion, leaving some residual matter to be ignited in a subsequent burst a short time later. 4U 1705 appears to fit this pattern, with additional aspects of the burst profiles suggesting that, in this case, ignition occurred at relatively high latitude - closer to the rotation poles than the equator - on the star. This new addition to the collection of known high-spin accreting neutron stars extends our ability to test models of accretion physics, nuclear burning, and the endpoints of stellar evolution in compact binary systems.


A rapid, short-lived oscillation in X-ray brightness during the peak of a burst from the neutron-star binary 4U 1705-44 is uncovered through two types of analysis of NICER data: a) a fast Fourier transform (FFT) and b) a folding method and statistic (Z_1^2) that capture the presence of sinusoidal variations. Narrow spikes in both panels indicate modulation at 702 cycles per second (Hz). Panel c) shows X-ray intensity over the duration of the burst, and contours of Z_1^2 > 15 that arise between approximately 4 and 5 seconds into the 20-sec-long burst. For this interval, panel d) shows two cycles of the rapid oscillation in brightness together with a fitted sine-curve model.

A rapid, short-lived oscillation in X-ray brightness during the peak of a burst from the neutron-star binary 4U 1705-44 is uncovered through two types of analysis of NICER data: a) a fast Fourier transform (FFT) and b) a "folding" method and statistic (Z_1^2) that capture the presence of sinusoidal variations. Narrow spikes in both panels indicate modulation at 702 cycles per second (Hz). Panel c) shows X-ray intensity over the duration of the burst, and contours of Z_1^2 > 15 that arise between approximately 4 and 5 seconds into the 20-sec-long burst. For this interval, panel d) shows two cycles of the rapid oscillation in brightness together with a fitted sine-curve model.



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