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Recent Swift X-ray Monitoring of the Magnetar XTE J1810-197

ATel #12689; M. Balakrishnan, S. Ali, J. Chen, N. Kebede, J. M. Miller, M. T. Reynolds, B. E. Tetarenko, D. Vozza (Univ. of Michigan)
on 25 Apr 2019; 14:05 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

Subjects: X-ray, Neutron Star, Transient, Magnetar

Referred to by ATel #: 16017

XTE J1810-197 is a transient magnetar that had its first X-ray outburst in 2003. The magnetar has a pulse period of 5.54s, and lies at a distance of about 4 kpc (Ibrahim et al. 2004). After spending almost a decade in a radio-quiet state, the magnetar turned back on in early December 2018 (Levin et al. 2019). SWIFT is continuing to follow the recent outburst of the XTE J1810-197 in X-rays (ATels #12600, #12291, #12297, #12484). We have been monitoring its evolution for two months, since February 8th (OSID 00031335006 - OBSID 00031335018).

Analysis of the most recent observation on April 19th (2 ks in WT mode) revealed that the spectrum continues to be well fit by a blackbody with a chi squared value of 1.1 for 267 degrees of freedom. We held a fixed absorption column of 1.0 E+22 cm^-2 in accordance with ATel #12297. With an energy range of 0.3 keV to 10 keV, we found that the temperature was 0.69 +- 0.01 keV and the flux was 1.37 +- 0.12 E-10 erg/s/cm^2. Using a power law fit (with chi squared of 1.7 for 268 degrees of freedom), we found a photon index of 3.04 +- 0.07. (Uncertainties are at the 90% confidence level).

Examining the values for flux, temperature, and photon index for the past two and a half months yields no sudden changes. We observed a flux of 2.2 +/- 0.2 E-10 erg/s/cm^2 on February 8th and a temperature of 0.72 +/- 0.1 keV. The flux shows an overall decreasing trend. However, the temperature and photon index do not show this trend. The temperature started at 0.72 keV, decreased to 0.69 keV on March 29th, and increased again to 0.715 keV on April 12th. The photon index has decreased and increased in the last two months, hovering around 3.0 +- 0.2. In addition to monitoring temperature, flux, and photon index, we also calculate emission region sizes (assuming an average distance of 3.5 kpc) for the different observation dates using the bbodyrad model of XSPEC. This measurement of the emission region size likely represents the size of the magnetic pole emission region (Kaspi et al. 2017). On February 8th, the effective source size was 1.6 km. The size has decreased only slightly since then; and the most recent observation gives an effective source size of 1.55 km. These variations are within the error margins.

We thank the Neil Gehrels Swift Observatory for providing the data used in our analysis.

References

A. I. Ibrahim, C. B. Markwardt, J. H. Swank, S. Ransom, M. Roberts, V. Kaspi, P. M. Woods, S. Safi-Harb, S. Balman, W. C. Parke, C. Kouveliotou, K. Hurley, and T. Cline, 2004, ApJ , 609, L21.

L. Levin, A. G. Lyne, G. Desvignes, R. P. Eatough, R. Karuppusamy, M. Kramer, B. W. Stappers, and P. Weltevrede, 2019, arxiv:1903.02660.

V. M. Kaspi and A. M. Beloborodov, “Magnetars,” 2017, ARA&A, 55, 261.