NICER / ISS Science Nugget for April 5, 2018

NICER sees an accreting millisecond pulsar

In a rapid response to a target-of-opportunity trigger, NICER executed a set of pointed observations of the X-ray transient Swift J1756.9-2508 on April 4. Within hours, the NICER team confirmed this object's new outburst through the detection of coherent brightness oscillations at a period of 5.5 milliseconds (182 Hz; see Figure). This initial detection, published on April 5 as Astronomer's Telegram #11502, opens up an exciting opportunity for NICER.

A known accreting millisecond pulsar, Swift J1756 has been seen in outburst only twice before: for the first time in 2007, and again in 2009. With NICER we now add the 2018 outburst, enabling high-precision timing of this pulsar over a span of 11 years. Such long baselines are crucial to establishing long-term changes in spin period, which are typically well under 1 microsecond per decade. Measured for only a handful of accreting systems, these spin changes are among the most informative observable parameters for a neutron star, with implications for their rotational ("clock") stability, magnetic field strength, and energy budget.

In addition to detecting spin evolution, measurement of the pulse profile – the brightness variation as the star rotates – offers a host of information. The profile encodes information about the neutron star's interior makeup and the dynamics of its interaction with the surrounding accretion flow, all of which is made possible through NICER's unique spectral-timing capability.

Pulsations detected in X-ray transient Swift J1756.9-2508
Figure: Power spectrum from a 9,500 sec NICER exposure of Swift J1756.9-2508, with a narrow peak rising high above the noise level. This spike is essentially a "delta function", much narrower than the width of individual spectral bins. Accounting for the pulsar's motion in its 55-min orbit and co-adding the X-ray data at the pulsation period reveals the pulse profile (inset), a coherent, double-peaked brightness oscillation that makes much of the exciting science from this object possible.

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