Robert F. Archibald

Postdoctoral Fellow at University of Toronto.

My research is centred around young neutron stars, especially those with high magnetic fields. Using X-ray telescopes like Swift and NuSTAR, I try to figure out what makes these extreme objects tick.


A full list of publications can be found in this ADS library.

Selected Papers

A Magnetar-like Outburst from a High-B Radio Pulsar

R. F. Archibald, V.M. Kaspi, S.P. Tendulkar, and P. Scholz

Radio pulsars are believed to have their emission powered by the loss of rotational kinetic energy. By contrast, magnetars show intense X-ray and gamma-ray radiation whose luminosity greatly exceeds that due to spin-down and is believed to be powered by intense internal magnetic fields. A basic prediction of this picture is that radio pulsars of high magnetic field should show magnetar-like emission. Here we report on a magnetar-like X-ray outburst from the radio pulsar PSR J1119-6127, heralded by two short bright X-ray bursts on 2016 July 27 and 28 (Kennea et al. 2016; Younes et al. 2016). Using Target-of-Opportunity data from the Swift X-ray Telescope and NuSTAR, we show that this pulsar's flux has brightened by a factor of > 160 in the 0.5-10 keV band, and its previously soft X-ray spectrum has undergone a strong hardening, with strong pulsations appearing for the first time above 2.5 keV, with phase-averaged emission detectable up to 25 keV. By comparing Swift-XRT and NuSTAR timing data with a pre-outburst ephemeris derived from Fermi Large Area Telescope data, we find that the source has contemporaneously undergone a large spin-up glitch of amplitude df/f = 5.74(8) E-6. The collection of phenomena observed thus far in this outburst strongly mirrors those in most magnetar outbursts and provides an unambiguous connection between the radio pulsar and magnetar populations.

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A High Braking Index for a Pulsar

R. F. Archibald, E.V. Gotthelf, R.D. Ferdman, V.M. Kaspi, S. Guillot, F.A. Harrison, E.F. Keane, M.J. Pivovaroff, D. Stern, S.P. Tendulkar, and J.A. Tomsick. The Astrophysical Journal Letters, Volume 819, Number 1, article id. L16, 2016.

We present a phase-coherent timing solution for PSR J1640–4631, a young 206 ms pulsar using X-ray timing observations taken with NuSTAR . Over this timing campaign, we have measured the braking index of PSR J1640–4631 to be n = 3.15 ± 0.03. Using a series of simulations, we argue that this unusually high braking index is not due to timing noise, but is intrinsic to the pulsar's spin-down. We cannot, however, rule out contamination due to an unseen glitch recovery, although the recovery timescale would have to be longer than most yet observed. If this braking index is eventually proven to be stable, it demonstrates that pulsar braking indices greater than three are allowed in nature; hence, other physical mechanisms such as mass or magnetic quadrupoles are important in pulsar spin-down. We also present a 3 σ upper limit on the pulsed flux at 1.4 GHz of 0.018 mJy.

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On the braking index of the unusual high-B rotation-powered pulsar PSR J1846-0258

R. F. Archibald, V. M. Kaspi, A. P. Beardmore, N. Gehrels, and J. A. Kennea. The Astrophysical Journal, Volume 810, Number 1, article id. 67, 2015.

PSR J1846-0258 is an object which straddles the boundary between magnetars and rotation powered pulsars. Though behaving for many years as a rotation-powered pulsar, in 2006, it exhibited distinctly magnetar-like behavior - emitting several short hard X-ray bursts, and a flux increase. Here we report on 7 years of post-outburst timing observations of PSR J1846-0258 using the Rossi X-ray Timing Explorer and the Swift X-ray Telescope. We measure the braking index over the post-magnetar outburst period to be n=2.19±0.03. This represents a change of Δn=−0.46±0.03 or a 14.5σ difference from the pre-outburst braking index of n=2.65±0.01, which itself was measured over a span of 6.5 yr. So large and long-lived a change to a pulsar braking index is unprecedented and poses a significant challenge to models of pulsar spin-down.

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Repeated, Delayed Torque Variations Following X-Ray Flux Enhancements in the Magnetar 1E 1048.1-5937

R. F. Archibald, V. M. Kaspi, C.-Y. Ng, P. Scholz, A.P. Beardmore, N. Gehrels, and J. A. Kennea. The Astrophysical Journal, Volume 800, Issue 1, article id. 33, 2015.

We report on two years of flux and spin evolution monitoring of 1E 1048.1-5937, a 6.5 s X-ray pulsar identified as a magnetar. Using Swift X-Ray Telescope data, we observed an X-ray outburst consisting of an increase in the persistent 1-10 keV flux by a factor of 6.3 ± 0.2, beginning on 2011 December 31 (MJD 55926). Following a delay of ~100 days, the magnetar entered a period of large torque variability, with \dot{ν } reaching a factor of 4.55 ± 0.05 times the nominal value, before decaying in an oscillatory manner over a timescale of months. We show by comparing to previous outbursts from the source that this pattern of behavior may repeat itself with a quasi-period of ~1800 days. We compare this phenomenology to periodic torque variations in radio pulsars, finding some similarities that suggest a magnetospheric origin for the behavior of 1E 1048.1-5937.

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An anti-glitch in a magnetar

R. F. Archibald, V. M. Kaspi, C. -Y. Ng, K. N. Gourgouliatos, D. Tsang, P. Scholz, A. P. Beardmore, N. Gehrels, and J. A. Kennea. Nature 497, 591–593, 2013.

Magnetars are neutron stars with X-ray and soft γ-ray outbursts thought to be powered by intense internal magnetic fields. Like conventional neutron stars in the form of radio pulsars, magnetars exhibit `glitches' during which angular momentum is believed to be transferred between the solid outer crust and the superfluid component of the inner crust. The several hundred observed glitches in radio pulsars and magnetars have involved a sudden spin-up (increase in the angular velocity) of the star, presumably because the interior superfluid was rotating faster than the crust. Here we report X-ray timing observations of the magnetar 1E 2259+586 (ref. 8), which exhibited a clear `anti-glitch'--a sudden spin-down. We show that this event, like some previous magnetar spin-up glitches, was accompanied by multiple X-ray radiative changes and a significant spin-down rate change. Such behaviour is not predicted by models of neutron star spin-down and, if of internal origin, is suggestive of differential rotation in the magnetar, supporting the need for a rethinking of glitch theory for all neutron stars.

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I work with the AstroMcGill outreach group. We put on a series on monthly public talks and visit elementary schools to teach kids about science & space. We also host a podcast series with visiting speakers.


  • NASA article about the 'Missing Link' Neutron Star PSR J1119-6127.
  • AAS NOVA article about PSR J1846-0258's change in braking index.
  • CBC article about the anti-glitch in 1E 2259+586.
  • coverage on the anti-glitch in 1E 2259+586.

Contact Me

Delivery and mailing address:
Department of Physics-Rutherford Physics Building, McGill University
3600 University St.,
Montreal, QC
H3A 2T8

Office address:
McGill Space Institute, Room 010
3550 University St.,
Montreal, QC
H3A 2A7