NICER / ISS Science Nugget for December 14, 2018

NICER demonstrates readiness to provide long-term timing for unusual accreting pulsar

The Galactic source FIRST J102347.6+003841 (also known as PSR J1023+0038 or AY Sextantis) contains a 1.69 ms pulsar in a 4.75 hr binary orbit around a bloated ~0.2 solar-mass main-sequence-like companion star. This neutron-star system is notable in that it was the first to exhibit compelling evidence for the transformation process between an accretion disk-dominated low-mass X-ray binary (LMXB) state and a disk-free radio millisecond pulsar state, thereby providing observational confirmation of the long-suspected evolutionary connection between these two classes of objects.

FIRST J1023 unexpectedly underwent a transformation back into a low-luminosity accreting state in June of 2013, in which it has remained since. Observations obtained shortly thereafter with ESA's XMM-Newton revealed coherent X-ray pulsations at the 1.69 ms pulsar spin period. This discovery came as a surprise because the implied accretion rate is orders of magnitude below what is seen in the commonly observed transient accreting millisecond X-ray pulsars (AMXPs), so theory predicts that the accretion flow should be unable to overcome the "centrifugal barrier" imposed by the pulsar's strong magnetic field and rapid spin. Therefore, this system falls in an unusual and poorly explored accretion regime and may provide new insight into the physics of accretion onto magnetized objects.

A long-term X-ray timing study with XMM-Newton revealed that in its accreting state PSR J1023+0038 is spinning down 25% faster compared to its previous disk-free radio pulsar state. The implication of this finding is that accretion does not dramatically alter the pulsar spin-down rate in this state, and that the pulsar wind is still active at a similar level as in the disk-free rotation-powered state. Since NICER was specifically designed for precision timing of millisecond pulsars (with two orders of magnitude better absolute timing capability compared to XMM-Newton), in November 2018 we carried out observations with NICER of PSR J1023+0038 to evaluate the possibility of using NICER alone for the long-term X-ray timing campaign.

Output of a pulsation search analysis for the known pulsar FIRST J102347.6+003841
Figure: Output of a pulsation search analysis for the known millisecond X-ray and radio pulsar FIRST J102347.6+003841. The mosaic of plots includes at upper left the X-ray brightness as a function of the star's 1.69 millisecond rotation period (two full rotations shown for clarity), in a 16,000 sec exposure integrated over nine ISS orbits. The plot immediately below, at lower left, shows the time dependence of the signal, with its statistical significance increasing with time; the disappearance of the dark bands around 30,000 sec is a known "nulling" phenomenon, in which the pulsar switches off intermittently and unpredictably. The plot at lower right shows the significance (colors) of the pulsation detection across the search grid in pulse period (or frequency) and any evolution of that frequency with time (the time-derivative 'F-dot', on the vertical axis).

As expected, due to NICER's ~2x higher collecting area relative to the EPIC-pn (the fast timing capable X-ray instrument onboard XMM-Newton), the pulsed X-ray signal is easily detectable in significantly shorter exposure times. As an example, the accompanying plot shows the result of a blind periodicity search (i.e., no prior knowledge of the spin period is assumed) on the 16 ks NICER observation from November 11 after correcting for the binary orbital motion of the pulsar; the double-peaked X-ray pulses from PSR J1023+0038 are detected with a very high statistical significance (with a false positive probability of 10-102). This demonstrates that NICER can provide improved measurements of the long term spin behavior of this unusual system.

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