Swift observes an outburst of the symbiotic star system 4U 1700+24 (= V934 Her)

ATel #6482; H. A. Krimm (CRESST/GSFC/USRA),J. A. Kennea (PSU), D. N. Burrows (PSU), M. H. Siegel (PSU), J. R. Cummings, V. Mangano (PSU)
on 18 Sep 2014; 02:17 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

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Swift has detected an outburst from the symbiotic X-ray binary system 4U 1700+24 (= V934 Her). This system triggered the BAT onboard (Burrows et al, GCN Circ. #16829) and the Swift satellite responded with an autonomous repointing.

XRT began observations at 13:50:12.7 UT, 1177.2 seconds after the BAT trigger. We have analysed observations taken over 3 orbits, consisting of 3.8ks of data, taking in a combination of PC and WT modes. The PC mode data allows us to accurately localize the transient, and we find a UVOT enhanced position of RA/Dec(J2000) = 256.64377, 23.97135, which is equivalent to:

RA(J2000) = 17h 06m 34.50s
Dec(J2000) = +23d 58’ 16.9’’

with an estimated uncertainty of 1.9 arc-seconds radius (90% confidence). This position lies 1.7 arc-seconds from the Simbad position of 4U 1700+24, consistent with this transient source being 4U 1700+24.

The light-curve displays a high degree of variability over the three orbits with a dynamic range of more than an order of magnitude, with variability timescales on the order of hundreds of seconds.

The best fit model to the mean WT mode spectrum (PC mode data are affected by pile-up, so we do not include them here), is a blackbody + power law model (reduced chi^2 = 1.01 for 203 d.o.f.). Simpler models (e.g. power law or blackbody only) produce significantly poorer fits (red chi^2 ~ 1.6 for power law, 2.8 for blackbody). Fitted absorption is very low, essentially zero with an upper limit of 8.2 x 10^21 cm^-2 (90% confidence). Fitted blackbody temperature is 1.6 +/- 0.1 keV, and photon index = 1.4 +/- 0.2. There is no strong evidence of spectral variability throughout the observation.

The BAT light curve does not show significant variability on the time scale of the event data from T-150 to T+963 sec. The spectrum for the most significant interval, from T+528.32 to T+711.32 sec, is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.52 +- 0.32. The fluence in the 15-150 keV band is 6.8 +- 1.2 x 10^-7 erg/cm2.Quoted errors are at the 90% confidence level.

The BAT transient monitor light curve shows that the outburst of 4U 1700+24 actually began early in 2014, with significant emission starting around 5 February 2014 (MJD 56693), with a 16-day average rate of 0.0003 +/- 0.00006 ct/s/cm^2 (~10 mCrab, 15-50 keV). The count rate has remained roughly at this level for the past seven months, before starting to rise more significantly starting on 12 September 2014, as reported by Burrows et al. (GCN Circ. #16829). It is currently seen in the one-day average for 17 September at 0.011 +/- 0.0015 ct/s/cm^2(~50 mCrab).

Swift/BAT transient monitor light curve for 4U 1700+24