Support ATel At Patreon

[ Previous | Next | ADS ]

Swift observations of 1RXS J180408.9-342058; return to quiescence

ATel #4085; R. Kaur (University of Amsterdam), C. Heinke (University of Alberta)
on 2 May 2012; 13:50 UT
Credential Certification: Ramanpreet Kaur (raman.pk@gmail.com)

Subjects: X-ray, Binary, Neutron Star, Transient, Variables

Referred to by ATel #: 7008

INTEGRAL detected a X-ray burst on April 16, 2012 at a position consistent with the position of an unidentified source 1RXS J180408.9-342058, and tentatively it was assumed that the burst is associated with the same source (Atel # 4050). To further monitor the X-ray activity of the source, we performed a follow-up observation using the Swift satellite.

1RXS J180408.9-342058 was observed using Swift in photon counting mode for an exposure time of 1.3 ks, on April 30, 2012 at UT 19:12. The source was detected at RA, Dec (J2000) = 18:04:08.05,-34:20:49.4 (90% confidence error 3.8"), consistent with the prior Swift detection (Atel # 4050). The background corrected count rate was 1.1E-02 in the energy range 0.2-10 keV. An absorbed power-law spectrum gave a power-law index of 3.1 and a low flux, motivating neutron star atmosphere fits. We fitted the ungrouped spectrum using an absorbed NSATMOS hydrogen-atmosphere model, assuming a 5.8 kpc distance, and a 1.4 Msun, 10-km radius neutron star. We find a reasonable fit (72% of realizations of the model have a smaller C-statistic), with N_H=3.8+5.8-2.5e21 cm^-2, log Teff=6.24+0.06-0.08 K and an unabsorbed 0.5-10 keV flux of 7.2e-13 ergs/cm^2/s. This gives an implied L_X=2.9E+33 ergs/s for a 5.8 kpc distance, consistent with observations of other quiescent neutron star low-mass X-ray binaries. We conclude that 1RXS J180408.9-342058 was probably responsible for the Type 1 X-ray burst observed with INTEGRAL, and that it has now returned to quiescence.

To compare the spectral parameters from our observations with the Swift observations obtained on April 17, 2012 (Atel # 4050), we re-analysed the previous observations and fitted the extracted X-ray spectrum with NSATMOS hydrogen-atmosphere model as well. The spectral fit gave N_H = 2.6-0.18+0.23E+21 cm^-2 and log Teff = 6.3-0.05+0.04 K by using the same input parameters as used for our observations (58% of 1E+4 Monte Carlo realizations have smaller C-statistics). We obtained an unabsorbed 0.5-10 keV flux of 1.2E-12 ergs/cm^2/s which translates to L_X = 4.8E+33 ergs/s for 5.8 kpc distance to the source.

We thank the Swift team for making this observation possible. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.