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NICER / ISS Science Nugget
for February 6, 2025



Piecing together a pulsar puzzle

Seen only in X-rays that pulse 17 times each second, the neutron star known as Calvera (also PSR J1412+7922) presented a puzzle. Its location on the sky, well off the plane of the Milky Way, combined with estimates of its age and distance from the Earth derived from its X-ray emissions and spin, suggested an exceptionally high velocity - several thousands of kilometers per second - if, like other neutron stars, it was born in the disk of our Galaxy. Two recent results, bringing together X-ray and radio measurements in a peer-reviewed paper published in The Astrophysical Journal by M. Rigoselli (National Inst. for Astrophysics, Italy) and collaborators, appear to now lay to rest questions about the origin of Calvera.

Alongside NICER's pulse timing measurements, Rigoselli et al. report high-precision measurements of Calvera's position on the sky, spanning 10 years, using NASA's Chandra X-ray telescope. They measure a change - Calvera's "proper motion" - amounting to 7.85 seconds of arc (about one-sixth the apparent angular diameter of Jupiter as seen from the Earth) per century. The direction of motion is found to be toward the Galactic plane, not away from it, suggesting that the pulsar was born out of the plane, likely close to where it is now. This conclusion is reinforced by the 2022 discovery of a faint ring of radio emission surrounding Calvera: the likely remnant of the supernova explosion in which the neutron star was created. Indeed, the pulsar appears to be moving away from the center of the ring, with a projected speed (hundreds of km/s) similar to those of many other pulsars. A self-consistent picture emerges, in which the pulsar was born a few thousand years ago, with a spin rate close to its present value, from a relatively high-speed progenitor star that wandered far above the Galactic plane before exploding. The radio remnant of that explosion is large because it is relatively nearby, and faint because it is expanding into low-density gas far from the Galactic disk. The new proper motion measurement helps further refine NICER's continuing timing measurements, which are used in searches for gravitational-wave emission from isolated neutron stars using facilities such as the U.S. Laser Interferometer Gravitational-wave Observatory (LIGO).


Six years of X-ray pulse timing with NICER of the neutron star known as Calvera. Blue points represent pulse arrival-time measurements with exposures of at least 5,000 seconds; the data span, shown in Modified Julian Days, is 15 Sept 2017 to 30 Oct 2023. The difference (residual) between measured and predicted X-ray pulse arrival times exhibits only small departures from zero, implying a good predictive model of the neutron star's spin behavior. (Credit: Rigoselli et al. 2025) Radio-band (120-168 MHz) image of a 1-degree-square patch of sky around Calvera (its position indicated by the open red circle), obtained by the European Low-Frequency Array telescope. The remnant of a supernova explosion appears as a faint ring of emission (bounded by dotted red circles), the geometric center of which is shown by a red cross. The arrow indicates the recently measured direction of Calvera's motion on the sky, and its length corresponds to the distance the neutron star would travel in 4,000 years. (Credit: Rigoselli et al. 2025)

Left: Six years of X-ray pulse timing with NICER of the neutron star known as Calvera. Blue points represent pulse arrival-time measurements with exposures of at least 5,000 seconds; the data span, shown in Modified Julian Days, is 15 Sept 2017 to 30 Oct 2023. The difference (residual) between measured and predicted X-ray pulse arrival times exhibits only small departures from zero, implying a good predictive model of the neutron star's spin behavior. (Credit: Rigoselli et al. 2025) Right: Radio-band (120-168 MHz) image of a 1-degree-square patch of sky around Calvera (its position indicated by the open red circle), obtained by the European Low-Frequency Array telescope. The remnant of a supernova explosion appears as a faint ring of emission (bounded by dotted red circles), the geometric center of which is shown by a red cross. The arrow indicates the recently measured direction of Calvera's motion on the sky, and its length corresponds to the distance the neutron star would travel in 4,000 years. (Credit: Rigoselli et al. 2025)



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