A NICER X-ray view on the magnetar-like 2020 outburst of PSR J1846-0258

ATel #13985; L. Kuiper (SRON-The Netherlands), A. K. Harding (NASA GSFC-USA), T. Enoto (RIKEN-Japan), W. C.G. Ho (Haverford College-USA), K. Gendreau (NASA GSFC-USA), Z. Arzoumanian (NASA GSFC-USA)
on 1 Sep 2020; 09:43 UT
Credential Certification: Lucien Kuiper (L.M.Kuiper@sron.nl)

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We report on the results of temporal and timing analyses of NICER AO2 GO monitoring (PI: A. Harding) and ToO observations of rotation-powered pulsar PSR J1846-0258, that showed a magnetar-like (<100 ms) burst on Aug. 1, 2020 20:11:47 UT (MJD 59062.8423169 MJD TT) as detected by the Swift BAT (ATel #13913) and Fermi GBM, indicating a new active episode after a quiescent period of about 14 years (the last outburst occurred in May/June 2006).

Analysing our AO2 monitoring observations, that started on March 26, 2020, we noticed that during the observation performed on June 26, 2020 (MJD 59026) the pulsed count rate in the 2.5-10 keV band had already increased by a factor of ~4 with respect to its normal (pre-outburst) quiescent level. The July 25, 2020 (MJD 59055) observation, performed about a week before the magnetar-like burst, showed even a 10 times higher 2.5-10 keV pulsed count rate than the pre-outburst value. Apparently, the source had already gone into outburst somewhere between May 27, 2020 (MJD 58996; last AO2 GO observation of the quiescent period) and June 26, 2020. The NICER ToO observations started at Aug. 2, 2020 (MJD 59063), one day after the magnetar-like burst, and are still ongoing.

The 2.5-10 keV pulsed flux evolution, using all available AO2 GO and ToO observations, since the start of the AO2 monitoring at MJD 58934 up to and including Aug. 24, 2020 (MJD 59085) is shown in Fig. 1. It displays a bizarre behavior with even a flux spike during a 3.1 ks ToO obs. performed on Aug. 21, 2020 on top of a nearly constant high level that set in since the start of the ToO observations.

From the densely sampled ToO observations and two enclosing AO2 GO observations we were able to phase-connect all timing data from July 25, 2020 till Aug. 24, 2020. The spin-frequency could be accurately modelled in terms of three parameters: Nu = 3.037919719(23) Hz, dNu/dt = -6.7256(25)E-11 Hz/s and d²Nu/dt²=-2.659(45)E-18 Hz/s^2 evaluated at Epoch 59064 (TDB) for validity interval MJD 59055-59086 (adopting the DE200 JPL-Solar System ephemeris; errors are at 1-sigma confidence), and thus shows accelerated spin-down for the initial part of the outburst.

Combining this model with the pre-outburst frequency model and incoherent measurements from the outburst period (see Fig. 2; dotted line represents pre-outburst spin extrapolation model) shows that at the start of the 2020 outburst period a strong timing glitch occurred of size Dnu/nu = 3E-6, comparable with the 2006 outburst value.

Next, we used the phase-coherent outburst model in a folding procedure to obtain pulse-profiles in different energy bands for the initial part of outburst period. We compared these outburst profiles with the pre-outburst quiescent profiles, and noticed clear differences between the two e.g. there is a 'shoulder' before pulse maximum in the 2.5-10 keV outburst profile (see Fig. 3). Also, contrary to the quiescent state pulsed emission is now detected at softer X-rays down to ~ 1 keV.

It is clear that future phase-resolved spectral analyses for different stages of the outburst will shed light on the evolution and nature of the soft-component. More NICER observations are scheduled coming week(s) covering later stages of the outburst. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.