NICER, NuSTAR, and Swift-XRT observations of the Magnetar 1E 1841-045
ATel #16802; G. Younes (NASA/CRESST II/UMBC), C.-P. Hu (NCUE), T. Enoto (Kyoto University/RIKEN), K. C. Gendreau, Z. Arzoumanian (NASA GSFC), J. Hare (NASA GSFC/CRESST II/CUA), M. Ng (MIT), Z. Wadiasingh (UMD / NASA/GSFC / CRESST II), on behalf of a larger collaboration
on 4 Sep 2024; 16:49 UT
Credential Certification: George Younes (gyounes@email.gwu.edu)
Subjects: X-ray, Neutron Star, Soft Gamma-ray Repeater, Transient, Pulsar, Magnetar
Referred to by ATel #: 16927
Since the reported bursting activity of the magnetar 1E 1841-045 with Swift-BAT (ATEL #16784), Fermi-GBM (ATEL #16786), and NICER (ATEL #16789), we have acquired more NICER observations of the target, covering the time period 2024 August 20 at 23:28 to 2024 August 31 at 10:58 UT, for a total exposure of 27.6 ks. Additionally, we acquired a NuSTAR Directors Discretionary Time observation starting on 2024 August 29 at 04:21 UT for a total exposure of approximately 50 ks. Lastly, we analyze the Swift-XRT photon counting mode observation acquired beginning 2024-08-22 at 03:52 UT for a total of 1.8 ks.
The NICER monitoring shows that the source bursting activity has strongly diminished since August 20. We detect only two faint bursts in addition to the ones reported in ATEL #16789.
Preliminary phase-averaged spectral analysis, utilizing the NuSTAR and Swift-XRT imaging observations, shows that the spectra can be well modeled with an absorbed blackbody (BB) and double power-law (PL) model, in line with other magnetars (C-stat of 1887 for 1824 degrees-of-freedom). The photon indices Γ for the soft and hard PLs are 3.5±0.2 and 1.26±0.03, whereas the temperature of the BB is 0.41±0.02 keV. We find a 2-79 keV absorption corrected flux of 1.3 × 10-10 erg/s/cm2, and a hard PL flux in the 3-79 keV range of about 9.3 × 10-11 erg/s/cm2. The latter flux is a factor of 2 larger than the corresponding historical measurement established with XRT+NuSTAR (e.g., An, H. et al. 2015 ApJ, 807, 93A). We verified this assertion through our own analysis of the archival NuSTAR observation (ID 30001025002) and XRT observation (ID 00080220003), which indeed confirms the factor 2 elevated flux level in the most recent data.
The 10-day NICER baseline allowed us to establish an accurate timing solution for the post-outburst data. We find a pulsation frequency of 0.0846995(1) Hz and a frequency derivative of -2.9(4)× 10-13 Hz s-1 (reference time 60550 MJD). Comparing this solution to the long-term NICER and XRT monitoring of the source shows no evidence of a glitch so far, however, a longer baseline is required to establish the post-outburst timing behavior of the source.
Folding the post-outburst NICER and NuSTAR data utilizing the above timing solution shows a multi-peaked, complex pulse profile in the 2-8 keV range that significantly differs from the pre-outburst profile. Similarly, the NuSTAR 10-30 keV profile exhibits complex structure, differing from the profile in soft X-rays, as well as from the historical measurement (An, H. et al. 2015 ApJ, 807, 93A).
We note that, to our knowledge, this is the first time that radiative changes are reported for this magnetar following bursting activity (e.g., Dib, R. & Kaspi, V. M. 2014, ApJ, 784, 37). We encourage continued X-ray and multiwavelength monitoring to study the evolution of this outburst.
We graciously thank the NICER, NuSTAR, and Swift teams for approving and promptly executing our observational requests.
