The neutron star LMXB 1RXS J180408.9-342058 transitioned to a soft X-ray state
ATel #7352; N. Degenaar (Cambridge), D. Altamirano (Southampton), A. Deller (ASTRON), C. Heinke (Alberta), J. Hessels (Amsterdam/ASTRON), A. Joadand (ASTRON), J. Miller-Jones (Curtin), J. Moldon (ASTRON), R. Wijnands (Amsterdam)
on 7 Apr 2015; 14:53 UT
Credential Certification: Nathalie Degenaar (degenaar@ast.cam.ac.uk)
Subjects: X-ray, Neutron Star, Transient
The neutron star low-mass X-ray binary 1RXS J180408.9-342058 started an accretion outburst in 2015 January (ATel #6997) and has remained active since. Around April 3, a significant drop in the hard (15-50 keV) X-rays is seen by Swift/BAT, whereas an increase in the softer X-rays (2-10 keV) is seen by MAXI. This suggests that the source has transitioned to a soft X-ray spectral state.
Pointed Swift/XRT observations show that in February-March the source was in a hard X-ray spectral state. Radio jets were detected and the 0.5-10 keV X-ray spectrum was adequately described by a simple absorbed power-law model with a hydrogen column density of N_H~5E21 cm-2, a photon index of Gamma~1.1 and an unabsorbed flux of ~(0.5-3)E-9 erg cm-2 s-1 (ATels #7039, #7255). At the estimated distance of 5.8 kpc (ATel #4050), this translates into a 0.5-10 keV luminosity of ~(0.2-1)E37 erg s-1. However, Swift/XRT observations obtained on April 3 (1.1 ks) and April 6 (1.0 ks) show that the source brightened and that its X-ray spectrum softened. The 0.5-10 keV spectrum can now be described by a combination of a kT_disk=0.09+/-0.02 keV disk black body, a hotter kT_bb=0.9+/-0.1 keV black body, and a Gamma=1.75+/-0.15 power law (with N_H=(0.8+/-0.2)E21 cm-2). The inferred unabsorbed flux of ~(1-1.5)E-8 erg cm-2 s-1 translates into a luminosity of ~(4-6)E37 erg s-1 at 5.8 kpc (0.5-10 keV). These two Swift/XRT observations show two absorption features around ~7 and ~8 keV that could possibly correspond to ionized iron and may indicate the presence of an accretion disk wind. These properties are indeed characteristic of a soft X-ray spectral state.
Swift/XRT monitoring of 1RXS J180408.9-342058 is ongoing. Multi-wavelength observations are encouraged.
