NICER / ISS Science Nugget
for February 13, 2020

New paper shows profile shifts in PSR J0218+4232

For over two years, NICER has been regularly observing three rotation-powered millisecond pulsars (MSPs): PSRs B1937+21, B1821-24, and J0218+4232. NICER receives a pulse, across the electromagnetic spectrum, from these fast-spinning neutron stars for each rotation, but the mechanisms responsible for pulsar emissions remain poorly understood.

NICER's observations enable detailed analyses of the X-ray emission properties of these MSPs thanks to the payload's high timing accuracy and large collecting area. This means that, rather than constructing just one pulse profile averaged over a broad band of X-ray energies, we can construct profiles (or "lightcurves") within multiple energy ranges (see Figure). We model these energy-resolved lightcurves to observe the energy dependence of the pulse morphology.

For one of our pulsars, PSR J0218+4232, we find that the separation between its two peaks decreases with increasing photon energy. For the others, we find the separation is remarkably stable across the NICER X-ray energy band. The measurements fit well with an emerging model for magnetospheric pulsar emission, known as the current sheet model.

NICER X-ray pulse profiles for PSR J0218+4232 in different energy bands

Figure: X-ray brightness variations (or "pulse profiles", purple points) are seen as the "lighthouse" beams of PSR J0218+4232 swivel into and out of our line of sight at a rate of 430.5 revolutions per second. Three ranges of X-ray photon energy within the overall NICER band are shown in the three panels. The two brightness peaks (labeled P1 and P2) vary slightly in strength and relative phase as a function of energy. Model fits to the peaks are shown by the blue traces, and the dotted horizontal lines indicate the estimated background level.

These results, in a paper led by Haverford College undergraduate Dominic Rowan, have been accepted for publication in The Astrophysical Journal.

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