Swift J0243.6+6124: Swift discovery of an accreting NS transient

ATel #10809; J. A. Kennea (PSU), A. Y. Lien (NASA/GSFC/UMBC), H. A. Krimm (NSF), S. B. Cenko (NASA/GSFC), M. H. Siegel (PSU)
on 3 Oct 2017; 14:53 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 02:37UT on October 3, 2017, Swift/BAT triggered on a source which was identified as a possible GRB or Galactic transient (Cenko et al., GCN #21960). Continued Swift observations showed that the source was at a constant X-ray brightness over 6 orbits, ruling out the GRB nature (Kennea et al., GCN #21963). We therefore report that this is a new Galactic transient, Swift J0243.6+6124. Utilizing Photon Counting mode data, we localized the source to the following coordinates: RA/Dec(J2000) = 40.91806, 61.43412, which is equivalent to:

RA (J2000) =02h 43m 40.33s,
Dec(J2000) = +61d 26m 02.8s,

with an estimated uncertainty of 1.5 arc-seconds radius (90% confidence). This position lies 1.4 arc-seconds from a B=13 star, USNO-B1.0 1514-0083050, suggesting that this may be the optical counterpart of Swift J0243.6+6124. We also note that the MAXI reported enhancement seen from LSI +61 303 (Sugita et al., ATEL #10803), is likely to be from this new transient, which lies 25 arc-minutes from LSI +61 303.

Timing analysis of 3ks of WT data reveal a strongly detected periodicity at 9.86s (or 19.71s if the pulse is double peaked), suggesting that the new transient is in fact an X-ray pulsar, i.e. an X-ray binary containing a neutron star. Over the course of the observations in WT, the source maintains an average brightness of 14 XRT count/s.

Analysis of the WT spectrum reveals that the source is hard. A simple absorbed power-law fit gives N_H = 1.5 +/- 0.1 x 10²² cm^-2, with a photon index of 0.74 +/- 0.03, which adequately describes the data (reduced chi² = 1.34, 481 dof). The addition of a thermal (blackbody) component improves the fit (reduced chi^2=1.0, 479 dof), and modifies the fitted parameters N_H = 1.2 +/- 0.2 x 10²² cm^-2, photon index = 1.3 +/- 0.4, blackbody kT = 2.0 +/- 0.1 keV. The average X-ray flux from WT data was 1.3 x 10^-9 erg/s/cm² (0.5 - 10 keV, uncorrected for absorption).

The daily light curve in the 15-50 keV band from the BAT transient monitor (Krimm et al. 2013) shows some weak emission of ~ 0.004 counts/cm2/sec (~ 15 mCrab) that starts from around Sep. 21, 2017. The light curve starts rising significantly around Sep. 30, 2017, with the count rate reaching ~ 0.018 counts/cm2/sec (~ 80 mCrab) on Oct. 3, 2017.

The time-averaged spectrum from 0.0 sec to 328.0 sec from the initial BAT trigger (02:37:00 UT 3 October 2017) is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.80 +/- 0.42. The fluence in the 15-150 keV band is 6.7 +/- 1.6 x 10^-7 erg/cm². This spectrum is softer than that of a typical GRB.

The daily light curve from the BAT transient monitor is available at https://swift.gsfc.nasa.gov/results/transients/weak/SwiftJ0243.6p6124/

UVOT detects USNO-B1.0 1514-0083050 in all 6 filters, with the following magnitudes. There is no statistically strong evidence of optical variability.

Filter     T_start(s)  T_stop(s)    Exp(s)      Mag 
v    	    557     	1077         58     12.88+-0.04 
v    	   6317     	6517        196     12.89+-0.04 
b    	    656     	 848         38     13.86+-0.05 
b    	   5701     	24124       695     14.03+-0.04 
u    	    631     	1147         54     13.45+-0.05 
u    	 111672     	23661      1655     13.50+-0.04 
uvw1 	    606     	1126         58     14.27+-0.06 
uvw1 	   6728     	18380       887     14.44+-0.05 
uvm2 	    582     	1101         58     15.70+-0.12 
uvm2 	   6522     	16444       219     15.98+-0.08 
uvw2 	    706     	1052         38     15.41+-0.11 
uvw2 	   6112     	6312        196     15.57+-0.08 

Swift J0243.6+6124 BAT Transient Monitor Light Curve