Abstract
Abstract Magnetars, isolated neutron stars with magnetic-field strengths typically ≳1014 G, exhibit distinctive months-long outburst epochs during which strong evolution of soft X-ray pulse profiles, along with nonthermal magnetospheric emission components, is often observed. Using near-daily NICER observations of the magnetar SGR 1830-0645 during the first 37 days of a recent outburst decay, a pulse peak migration in phase is clearly observed, transforming the pulse shape from an initially triple-peaked to a single-peaked profile. Such peak merging has not been seen before for a magnetar. Our high-resolution phase-resolved spectroscopic analysis reveals no significant evolution of temperature despite the complex initial pulse shape, yet the inferred surface hot spots shrink during peak migration and outburst decay. We suggest two possible origins for this evolution. For internal heating of the surface, tectonic motion of the crust may be its underlying cause. The inferred speed of this crustal motion is ≲100 m day−1, constraining the density of the driving region to ρ ∼ 1010 g cm−3, at a depth of ∼200 m. Alternatively, the hot spots could be heated by particle bombardment from a twisted magnetosphere possessing flux tubes or ropes, somewhat resembling solar coronal loops, that untwist and dissipate on the 30–40 day timescale. The peak migration may then be due to a combination of field-line footpoint motion (necessarily driven by crustal motion) and evolving surface radiation beaming. This novel data set paints a vivid picture of the dynamics associated with magnetar outbursts, yet it also highlights the need for a more generic theoretical picture where magnetosphere and crust are considered in tandem.
Highlights
Using near-daily NICER observations of the magnetar SGR 1830-0645 during the first 37 days of a recent outburst decay, a pulse peak migration in phase is clearly observed, transforming the pulse shape from an initially triple-peaked to a single-peaked profile
These novel dataset paints a vivid picture of the dynamics associated with magnetar outbursts, yet it highlights the need for a more generic theoretical picture where magnetosphere and crust are considered in tandem
Daily high-throughput X-ray observations were extremely rare prior to the launch of NICER and while only a few (e.g., XMM-Newton) observations may have been able to spot the peak motion we report here, typically these are spread over the entire outburst decay period of months to years, insufficiently sampling such comparatively rapid evolution
Summary
Magnetar outburst epochs start with an increase of the quiescent X-ray flux by as many as three orders of magnitude, accompanied by drastic spectral changes to the neutron star’s surface thermal and magnetospheric emission, as well as strong temporal variability in the form of timing noise, glitch activity, and altered pulse shape (e.g., Woods et al 2004; Gavriil et al 2006; Rea et al 2009; Israel et al 2010; Esposito et al 2010; Gavriil et al 2011; Scholz et al 2014; Hu et al 2020). The outbursts last anywhere from months to years during which the source properties typically return back to their initial state (Coti Zelati et al 2018, and see Younes et al 2017a; Coti Zelati et al 2020). Given these variability patterns, outburst epochs are distinctly revealing of a magnetar’s highly dynamic magnetosphere and its interplay with the surface thermal emission, both of which are governed by the decay of the super-critical internal and external B-fields (e.g., Thompson & Duncan 1995, 1996; Vigano et al 2013). In the several months following the source discovery, its soft X-ray flux decreased by a factor 6 due to the shrinkage of the total emitting area (Y21)
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