A Soft X-Ray Spectral Episode for the Clocked Burster, GS 1826-24 as Measured by Swift and NuSTAR

Creators: Chenevez, J.; Galloway, D. K.; in 't Zand, J. J. M.; Tomsick, J. A.; Barret, D.; Chakrabarty, D.; Fürst, F.; Boggs, S. E.; Christensen, F. E.; Craig, W. W.; Hailey, C. J.; Harrison, F. A.; Romano, P.; Stern, D.; Zhang, W. W.

Abstract

We report on NuSTAR and Swift observations of a soft state of the neutron star low-mass X-ray binary GS 1826–24, commonly known as the "clocked" burster. The transition to the soft state was recorded in 2014 June through an increase of the 2–20 keV source intensity measured by MAXI, simultaneous with a decrease of the 15–50 keV intensity measured by Swift/BAT. The episode lasted approximately two months, after which the source returned to its usual hard state. We analyze the broadband spectrum measured by Swift/XRT and NuSTAR and estimate the accretion rate during the soft episode to be ≈ 13% m_(Edd), within the range of previous observations. However, the best-fit spectral model, adopting the double Comptonization used previously, exhibits significantly softer components. We detect seven type-I X-ray bursts, all significantly weaker (and with shorter rise and decay times) than observed previously. The burst profiles and recurrence times vary significantly, ruling out the regular bursts that are typical for this source. One burst exhibited photospheric radius expansion and we estimate the source distance as (5.7 ± 0.2)ξ_b^(-1/2) kpc, where ξ_b parameterizes the possible anisotropy of the burst emission. The observed soft state may most likely be interpreted as a change in accretion geometry at about similar bolometric luminosity as in the hard state. The different burst behavior can therefore be attributed to this change in accretion flow geometry, but the fundamental cause and process for this effect remain unclear.

Additional Information

© 2016. The American Astronomical Society. Received 2015 September 3; accepted 2015 December 24; published 2016 February 17. J.C. would like to thank Niels Jørgen Westergaard for useful discussions. J.C. acknowledges financial support from ESA/PRODEX Nr. 90057. P.R. acknowledges financial contribution from contract ASI-INAF I/004/11/0 and ASI-INAF I/037/12/0. This work made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR and Swift Operations teams for executing the ToO observations and the Software and Calibration teams for analysis support. This research has used the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). The MAXI data are provided by RIKEN, JAXA, and the MAXI team. Swift/BAT transient monitor results are provided by the Swift/BAT team. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. This paper utilizes preliminary analysis results from the Multi-INstrument Burst ARchive (MINBAR), which is supported under the Australian Academy of Science's Scientific Visits to Europe program, and the Australian Research Council's Discovery Projects and Future Fellowship funding schemes. Facilities: NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission, Swift - Swift Gamma-Ray Burst Mission, MAXI - JAXA Monitor of All-sky X-ray Image experiment.

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