NICER follow-up observations of the magnetar 1E 1048.1-5937

ATel #11327; T. Guver (Istanbul Univ.), T. Enoto (Kyoto Univ.), Z. Arzoumanian (NASA/GSFC), A. Harding (NASA/GSFC), K. Gendreau (NASA/GSFC), and the NICER Magnetars & Magnetospheres Working Group
on 21 Feb 2018; 05:25 UT
Credential Certification: Tolga Guver (tolga.guver@istanbul.edu.tr)

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Monitoring with the Gehrels Swift observatory's X-Ray Telescope detected an X-ray flux increase from the magnetar 1E 1048.1-5937 in January 2018 (Archibald et al., ATEL #11241). The Neutron star Interior Composition Explorer (NICER; Gendreau et al. 2012, SPIE Conference Series, Vol. 8443) performed two follow-up X-ray observations on January 30 (ObsID 1020240126) and February 6 (ObsID 1020240127), 2018, with exposures of 728 and 2520 seconds, respectively.

The recently acquired 0.7-8.0 keV NICER spectra were fit jointly with earlier data, including an 8.9 ks NICER dataset from late July and early August 2017, as well as an XMM-Newton EPIC-pn 0.3-8.0 keV observation from December 28, 2000 (ObsID 0112780401; Tiengo et al. 2002, A&A 383, 182). The spectra are successfully modeled (chi2/d.o.f. = 1.052 at 1079 d.o.f.) by a blackbody plus a power-law component, with independent parameters at each epoch, attenuated by interstellar absorption ('phabs' in XSPEC, Anders & Grevesse 1989 abundance), with the absorbing column-density linked across all datasets. We confirmed the increase in flux, to (2.2-2.3)E-11 erg/s/cm2 (absorbed) in the 0.5-10 keV band, during the last two NICER observations. Preliminary best-fit blackbody temperatures are 0.64-0.69 keV, with column density NH = (0.85+/-0.06)E22 cm-2 (1-sigma statistical uncertainty) and power-law photon index 2.2-3.9. In each observation, typical statistical uncertainties of the spectral parameters are at the level of a few percent.

The NICER observations of July-August 2017 show that, compared to its quiescent level in 2000, the source was already in a higher flux state, by almost a factor of three. In 2018, the flux increased further so that the source was approximately four times brighter than in quiescence. In contrast to conclusions drawn from past brightening episodes, where the dominant spectral change was found (Tam et al. 2008, ApJ 677, 573) to be in the power-law component, the derived 2018 spectral parameters suggest that this activation is mainly due to an increase in the blackbody emitting radius, which changed from ~1.6 km to ~3.4 km assuming a distance of 9 kpc (Durant & van Kerkwijk 2006, ApJ, 650, 1070). In the 0.7-8.0 keV range, we also computed the pulsed fraction [defined as PF = (max - min)/(max + min) of the pulse profile folded with period 6.4629755 s] for the last NICER observation on February 6, 2018, and found PF ~48%, significantly lower than the 81% measured in the December 2000 XMM-Newton observation. A similar anticorrelation between flux and pulsed fraction has been reported several times for this source (Tiengo et al. 2005, A&A 437, 997; Gavriil et al. 2006, ApJ 641, 418; Tam et al. 2008, ApJ 677, 573; Dib, Kaspi, & Gavriil 2009, ApJ 702, 614).

These results are in agreement with the findings of Guver et al. (2015; ApJ 801, 48), where the authors modeled pulse profiles for this source using neutron star atmosphere models and inferred that the size of the hot spot changed from 80 degrees to 30 degrees from an outburst state in 2007 to quiescence in 2011. Such a change in the spot size was also suggested by Tam et al. (2008, ApJ 677, 573) as a reason for the observed anticorrelation between flux and pulsed fraction.

This work was supported by NASA through the NICER mission and the Astrophysics Explorers Program. It made use of data and software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), a service of the Astrophysics Science Division at NASA/GSFC and the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory.