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Register a new userNuSTAR Observations of the Magnetar 1E 2259+586
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Julia K. Vogel
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We report on new broad band spectral and temporal observations of the magnetar 1E 2259+586, which is located in the supernova remnant CTB 109. Our data were obtained simultaneously with the Nuclear Spectroscopic Telescope Array (NuSTAR) and Swift, and cover the energy range from 0.5-79 keV. We present pulse profiles in various energy bands and compare them to previous RXTE results. The NuSTAR data show pulsations above 20 keV for the first time and we report evidence that one of the pulses in the double-peaked pulse profile shifts position with energy. The pulsed fraction of the magnetar is shown to increase strongly with energy. Our spectral analysis reveals that the soft X-ray spectrum is well characterized by an absorbed double-blackbody or blackbody plus power-law model in agreement with previous reports. Our new hard X-ray data, however, suggests that an additional component, such as a power-law, is needed to describe the NuSTAR and Swift spectrum. We also fit the data with the recently developed coronal outflow model by Beloborodov for hard X-ray emission from magnetars. The outflow from a ring on the magnetar surface is statistically preferred over outflow from a polar cap.
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Magnetic field geometry is expected to play a fundamental role in magnetar activity. The discovery of a phase-variable absorption feature in the X-ray spectrum of SGR 0418+5729, interpreted as cyclotron resonant scattering, suggests the presence of very strong non-dipolar components in the magnetic fields of magnetars. We performed a deep XMM-Newton observation of pulsar 1E 2259+586, to search for spectral features due to intense local magnetic fields. In the phase-averaged X-ray spectrum, we found evidence for a broad absorption feature at very low energy (0.7 keV). If the feature is intrinsic to the source, it might be due to resonant scattering/absorption by protons close to star surface. The line energy implies a magnetic field of ~ 10^14 G, roughly similar to the spin-down measure, ~ 6x10^13 G. Examination of the X-ray phase-energy diagram shows evidence for a further absorption feature, the energy of which strongly depends on the rotational phase (E >~ 1 keV ). Unlike similar features detected in other magnetar sources, notably SGR 0418+5729, it is too shallow and limited to a small phase interval to be modeled with a narrow phase-variable cyclotron absorption line. A detailed phase-resolved spectral analysis reveals significant phase-dependent variability in the continuum, especially above 2 keV. We conclude that all the variability with phase in 1E 2259+586 can be attributed to changes in the continuum properties which appear consistent with the predictions of the Resonant Compton Scattering model.
We report on the timing and spectral properties of the soft X-ray emission from the magnetar 1E 2259+586 from January 2013, $sim 8$ months after the detection of an anti-glitch, until September 2019, using the Neil Gehrels Swift and NICER observatories. During this time span, we detect two timing discontinuities. The first, occurring around 5 years after the April 2012 anti-glitch, is a relatively large spin-up glitch with a fractional amplitude $Delta u/ u=1.24(2)times10^{-6}$. We find no evidence for flux enhancement or change in the spectral or pulse profile shape around the time of this glitch. This is consistent with the picture that a significant number of magnetar spin-up glitches are radiatively-quiet. Approximately 1.5 years later in April 2019, 1E 2259+586 exhibited an anti-glitch with spin-down of a fractional amplitude $Delta u/ u=-5.8(1)times10^{-7}$; similar to the fractional change detected in 2012. We do not, however, detect any change to the pulse-profile shape or increase in the rms pulsed flux of the source, nor do we see any possible bursts from its direction around the time of the anti-glitch; all of which occurred during the 2012 event. Hence, similar to spin-up glitches, anti-glitches can occur silently. This may suggest that these phenomena originate in the neutron star interior, and that their locale and triggering mechanism do not necessarily have to be connected to the magnetosphere. Lastly, our observations suggest that the occurrence rate of spin-up and spin-down glitches is about the same in 1E 2259+586, with the former having a larger net fractional change.
Ages of the magnetar 1E 2259+586 and the associated supernova remnant CTB~109 were studied. Analyzing the Suzaku data of CTB~109, its age was estimated to be $sim$14~kyr, which is much shorter than the measured characteristic age of 1E 2259+586, 230 kyr. This reconfirms the previously reported age discrepancy of this magnetar/remnant association, and suggests that the characteristic ages of magnetars are generally over-estimated as compared to their true ages. This discrepancy is thought to arise because the former are calculated without considering decay of the magnetic fields. This novel view is supported independently by much stronger Galactic-plane concentration of magnetars than other pulsars. The process of magnetic field decay in magnetars is mathematically modeled. It is implied that magnetars are much younger objects than previously considered, and can dominate new-born neutron stars.
Magnetars are an extreme, highly magnetized class of isolated neutron stars whose large X-ray luminosity is believed to be driven by their high magnetic field. In this work we study for the first time the possible very high energy gamma-ray emission above 100 GeV from magnetars, observing the sources 4U 0142+61 and 1E 2259+586. We observed the two sources with atmospheric Cherenkov telescopes in the very high energy range (E > 100 GeV). 4U 0142+61 was observed with the MAGIC I telescope in 2008 for ~25 h and 1E 2259+586 was observed with the MAGIC stereoscopic system in 2010 for ~14 h. The data were analyzed with the standard MAGIC analysis software. Neither magnetar was detected. Upper limits to the differential and integral flux above 200 GeV were computed using the Rolke algorithm. We obtain integral upper limits to the flux of 1.52*10^-12cm^-2 s^-1 and 2.7*10^-12cm^-2 s^-1 with a confidence level of 95% for 4U 0142+61 and 1E 2259+586, respectively. The resulting differential upper limits are presented together with X-ray data and upper limits in the GeV energy range.
We report on a 350-ks NuSTAR observation of the magnetar 1E 1841-045 taken in 2013 September. During the observation, NuSTAR detected six bursts of short duration, with $T_{90}<1$ s. An elevated level of emission tail is detected after the brightest burst, persisting for $sim$1 ks. The emission showed a power-law decay with a temporal index of 0.5 before returning to the persistent emission level. The long observation also provided detailed phase-resolved spectra of the persistent X-ray emission of the source. By comparing the persistent spectrum with that previously reported, we find that the source hard-band emission has been stable over approximately 10 years. The persistent hard X-ray emission is well fitted by a coronal outflow model, where $e^{+/-}$ pairs in the magnetosphere upscatter thermal X-rays. Our fit of phase-resolved spectra allowed us to estimate the angle between the rotational and magnetic dipole axes of the magnetar, $alpha_{mag}=0.25$, the twisted magnetic flux, $2.5times10^{26}rm G cm^2$, and the power released in the twisted magnetosphere, $L_j=6times10^{36}rm erg s^{-1}$. Assuming this model for the hard X-ray spectrum, the soft X-ray component is well fit by a two-blackbody model, with the hotter blackbody consistent with the footprint of the twisted magnetic field lines on the star. We also report on the 3-year Swift monitoring observations obtained since 2011 July. The soft X-ray spectrum remained stable during this period, and the timing behavior was noisy, with large timing residuals.
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