Two New Outbursts and Transient Hard X-Rays from 1E 1048.1-5937

Abstract

Since its discovery, 1E 1048.1−5937 has been one of the most active magnetars, both in terms of radiative outbursts, and changes to its spin properties. Here we report on a continuing monitoring campaign with the Neil Gehrels Swift Observatory X-ray Telescope in which we observe two new outbursts from this source. The first outburst occurred in 2016 July, and the second in 2017 December, reaching peak 0.5-10 keV absorbed fluxes of 3.2 +0.2 −0.3 × 10 −11 erg s −1 cm −2 and 2.2 +0.2 −0.2 × 10 −11 erg s −1 cm −2 , respectively, factors of ∼5 and ∼ 4 above the quiescent flux. Both new outbursts were accompanied by spin-up glitches with amplitudes of ∆ν = 4.47(6) × 10 −7 Hz and ∆ν = 4.32(5) × 10 −7 Hz, respectively. Following the 2016 July outburst, we observe, as for past outbursts, a period of delayed torque fluctuations, which reach a peak spin-down of 1.73 ± 0.01 times the quiescent rate, and which dominates the spin evolution compared to the spin-up glitches. We also report an observation near the peak of the first of these outbursts with NuSTAR in which hard X-ray emission is detected from the source. This emission is well characterized by an absorbed blackbody plus a broken power law, with a powerlaw index above 13.4 ± 0.6 keV of 0.5 +0.3 −0.2 , similar to those observed in both persistent and transient magnetars. The hard X-ray results are broadly consistent with models of electron/positron cooling in twisted magnetic field bundles in the outer magnetosphere. However the repeated outbursts and associated torque fluctuations in this source remain puzzling.

NICER Observation of the Temporal and Spectral Evolution of Swift J1818.0−1607: A Missing Link between Magnetars and Rotation-powered Pulsars

“…A hard power-law spectral component above ∼10 keV has been reported from some of the persistently bright magnetars and from the early phases of transient outbursts, where the emission is thought to be radiated from the magnetosphere (see, e.g., Younes et al 2017b;Archibald et al 2020). Using the reported correlation between the soft and hard X-ray luminosity of known magnetars (Equation (4) of Enoto et al 2017) with the unabsorbed X-ray flux of SwiftJ1818, ∼3× 10 −11 ergscm −2 observed in the soft band, we would expect the hard power-law flux at ∼2×10 −11 ergscm −2 in the 15-60 keV band with a flat photon index of ∼0.8 (Equation 7of Enoto et al 2017).…”

“…The torque may be larger than the nominal value by 1 order of magnitude at this early stage in the outburst. In several other magnetars, the torque decreases to the quiescent level on a timescale of a few months to 10 years (Camilo et al 2016;Younes et al 2017a;Archibald et al 2020). Future longterm monitoring of the timing behavior of SwiftJ1818 is necessary.…”

A Month of Monitoring the New Magnetar Swift J1555.2−5402 during an X-Ray Outburst

“…The suggestion is supported by the observed strong timing noise, which requires a model with high-order polynomials (Section 3.1), similar to that of the young magnetar Swift J1818.0−1607 (Hu et al 2020). We caution that the derived pulsar parameters during the outburst may deviate from those in the quiescent state (e.g., Younes et al 2017a;Archibald et al 2020). Frequency derivatives are known to fluctuate during magnetar outbursts, with variations of a factor of 1-50 (see, e.g., Dib et al 2012;Dib & Kaspi 2014;Levin et al 2019).…”

The X-ray evolution and geometry of the 2018 outburst of XTE J1810-197

“…Moreover, we found evidence of spin-down variations during our monitoring campaign: the period derivative measured during the first two months of the outburst decay ( 𝑃 ∼7.2×10 −12 s s −1 ) is a factor of ∼2.5 higher than the value in quiescence ( 𝑃 ∼2.8×10 −12 s s −1 ) and a factor of ∼2 smaller than the value derived for the rest of its first year evolution ( 𝑃 ∼1.5×10 −11 s s −1 ). This behaviour has already been witnessed in XTE J1810 after the 2003 outburst and appears to be a common feature following magnetar outbursts (e.g., 1E 1048.1-5937; Archibald et al 2020). Spin-down rate variations associated with outbursts are commonly explained in terms of the untwisting bundle scenario, according to which outbursts are most likely driven by magnetic stresses resulting in twistings of the external magnetic field with the formation of current-carrying localized bundles (Beloborodov 2009).…”