Swift follow-up of a bright optical outburst from SS 433
ATel #11975; I. Khabibullin (MPA, IKI RAS), A. Semena, P. Medvedev, I. Mereminskiy (IKI RAS)
on 21 Aug 2018; 16:00 UT
Credential Certification: Ilya Mereminskiy (i.a.mereminskiy@gmail.com)
Referred to by ATel #: 11989
We present results of the Swift observation of SS 433 performed in follow-up of a bright optical outburst reported in ATel#11870.
The observation started at 2018-07-25 07:19:01T (MJD=58324.30567, ObsID 00035190036), i.e. 7.534d after the outburst. The observation consisted of two segments separated by 0.397d (34.3ks). Total exposure time with the Swift/XRT for two segments equals 1030.5s collected in Photon Counting mode, 353.5s for the first segment and 677s for the second one. The UVOT accumulated image in U-filter during the single exposure with 350s duration started 87s after the beginning of the first XRT segment.
The light curve and spectra were extracted from the 50'' circular aperture, the background was extracted from the annulus surrounding the source with 96'' inner radius and 480'' outer radius. Data reduction and processing have been performed using the standard Swift routines (XRTDAS) and XSELECT tool of the heasoft package (v6.24) and most recent calibration data.
The source has been detected in 0.3-10 keV band by XRT and in U band by UVOT with the mean background-corrected count rates 0.48±0.02 cts/s and 3.01±0.16 cts/s, respectively. The latter corresponds to the flux density Fλ=4.8±0.24 erg/s/cm2 /Å=180±1 µJy (17.51 Vega magnitude) in the U band, consistent with previous Swift observations of Dolan et al. 2007 and ATel#6364.
The 0.5-10 keV light curve shows significant variation between the two segments, with the first one being on average a factor of 1.5 brighter then the second one. There is no significant variation in relative contributions of narrower bands, which indicates absence of strong variation in the absorbing column density or relative contribution of the jets and the hard (cold reflection) component (see Khabibullin et al. 2016). The only exception might be a significant increase in [6-10] keV count rate during the second segment of the observation. This conclusion is confirmed by analysis of the spectra extracted from the full observation and individual segments.
We fit the spectra with with a model consisting of bremsstrahlung continuum and line emission from the pair of baryonic relativistic jets (as described in Medvedev et al. 2018). The spectral shape of all three datasets appeared to be consistent with the latest XMM-Newton observation, but with the overall normalization 0.23±0.03, NH=1.25±0.15×1022 cm-2, and redshift of the blue jet zb=-0.034±0.026 (redshift of the other jet is not constrained). This is consistent with the expectation for the SS 433's precession phase at the moment of the observation (0.22).
Importantly, fitting of the spectrum with simpler models (e.g. absorbed powerlaw or bremsstrahlung) results in significantly lower values of NH because of the dominant contribution of jets spectral line emission below 2 keV. The bremsstrahlung continuum has temperature 30 keV and unabsorbed 0.5-10 keV flux 4.0×10-11 erg/s/cm-2. The jets line emission gives unabsorbed 0.5-10 keV flux at level 2.6×10-11 erg/s/cm-2 (1.8×10-11 erg/s/cm-2 below 2 keV).
The detected X-ray emission corresponds to the class of SS 433's low X-ray states, which are 3-5 times dimmer then the more common normal out-of-eclipse states (Marshall et al. 2013). This is illustrated by comparison of the current observation with Chandra/HETGS data of another low-state spectrum (ObsID 1020). The latter indeed has similar overall intensity, however, with a slight excess at low energies.
Noteworthy, according to the RATAN-600 radio light curve, SS 433 experienced another bright radio outburst at the moment of the X-ray observation reported here.
We are grateful for Swift team for rapid scheduling of our observations.
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