SWIFT observations of the transient magnetar XTE J1810-197

ATel #16017; Elise Kesler, McKinley Brumback, Anna Gaishin, Nathan Morrow, Matthew Wanink (University of Michigan)
on 27 Apr 2023; 20:17 UT
Credential Certification: McKinley Brumback (brumback@umich.edu)

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XTE J1810-197, a transient magnetar with detected pulsations, has shown intermittent outbursts since its discovery in 2003 (Maan et al. 2019). We report on two recent observations of this source as obtained by the Neil Gehrels Swift Observatory.

The first observation, a 4 ks XRT exposure carried out on 23 March 2023, captured 78 photons in the 0.3-3.0 keV range (obsID 00014973014). This narrow energy range was motivated by the spectral shape, which was extremely soft and not detected above the background above 3 keV. We utilized XSPEC to fit the data with a combination of basic absorption and blackbody models. As the absorbing column was poorly constrained, we fixed its value to N_H=1.67x10^22 cm^-2, the average value for this source given by the HEASARC HI column density calculator (HI4PI Collaboration). These fits result in a blackbody temperature of kT=0.17 keV +/- 0.52z10^-2 keV and a normalization value of norm=5.3x10^-4 +/- 1.6x10^-4, with uncertainties at the 68% confidence level. From this fit, we determine a flux of 7.1x10^-13 +/- 2.2x10^-13 erg/cm^2/s, which implies a luminosity of 5.3x10^32 +/- 1.6x10^32 erg/s, assuming a distance of 2.5 +/- 1 kpc (Ding et al. 2020).

We also examine a more recent 3.3 ks XRT exposure beginning on 14 April 2023 (obsID 00014973016). This subsequent observation captured 148 photons in the 0.3-3.0 keV range. Due to low signal to noise, we were unable to attain good spectral fit in this range. Thus, we utilized the WebPIMMS tools, assuming the absorbed blackbody model and parameters from the previous analysis, to estimate a flux of 9.1x10^-13 +/- 0.75x10^-13 erg/cm^2/s. The resulting luminosity of 6.8x10^32 +/- 0.6x10^32 erg/s is consistent with our previous analysis within their errors.

Looking at the recent BAT light curves for XTE J1810-197 suggests that this source has not had a large outburst recently, though it does display small variations in brightness. Previous analyses of this transient magnetar (ATel #12689) yielded hotter blackbody temperatures than the ones discussed here, which may suggest a change in this source's properties over time. The soft nature of our spectra and different models likely also account for the discrepancies. We suggest further follow up measurements and observations in the X-ray range due to these factors and high errors on our measured luminosity.

References
Maan, Y., Joshi, B. C., Surnis, M. P., et al. (2019). ApJ, 882(1), L9
Ding, H. et al., MNRAS, Volume 498, Issue 3, November 2020, Pages 3736–3743
HI4PI Collaboration 2016, A&A, 594, A116
M. Balakrishnan, et al., 2019, The Astronomer's Telegram, 12689