Swift/BAT Detects Increase in Hard X-ray Emission from the Ultra-compact X-ray Binary 4U 1543-624

ATel #10690; Renee Ludlam (U of Michigan), Jon M. Miller (U of Michigan), James Miller-Jones (ICRAR-Curtin), Mark Reynolds (U of Michigan)
on 31 Aug 2017; 17:57 UT
Credential Certification: Renee Ludlam (rmludlam@umich.edu)

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The Swift/BAT detected an increase in hard X-ray emission (15-50 keV) coming from the ultra-compact X-ray binary 4U 1543-624 around 2017 August 9. The MAXI daily monitoring also shows a gradual increase in 2.0-20.0 keV X-ray intensity as of 2017 August 19. Swift/XRT ToO monitoring of the source was triggered and shows an increase in unabsorbed flux to 1.06E-9 ergs/cm2/s in the 0.3-10.0 keV energy band as of 2017 August 26. ATCA performed ToO observations for approximately 4 hours in the 5.5 GHz and 9.0 GHz bands while the antennas were in the 1.5A array configuration from 11:25-16:09 UTC on 2017 August 23. The source was not detected in either band. Stacking both the 5.5 GHz and 9.0 GHz data provides a 3-sigma upper limit of 27 uJy. The distance to the source is estimated to lie between 6.0-6.7 kpc (Wang et al. 2015, PASA, 32, 35), which implies an upper limit on the 8.4 GHz radio luminosity of 9.8E27-1.2E28 ergs/s. The Swift/XRT did not observe the source on 2017 August 23 so we use the Swift/BAT intensity to infer an X-ray luminosity of 6.3E36-7.9E36 ergs/s in the 3.0-9.0 keV energy band, assuming a photon index of 1.93 from XRT fits within days of the observation and neutral absorption column of 2.9E21 /cm2 (Dickey & Lockman 1990, ARA&A, 28, 215). This places the source well below the expected correlation for standard or radio-faint black holes on the radio-X-ray plane and closer to the expected trend for neutron stars. The nature of the compact object in the system is still unknown. No Type-I X-ray bursts have been seen to indicate a neutron star. Continued X-ray monitoring is encouraged. Swift/BAT: https://swift.gsfc.nasa.gov/results/transients/weak/4U1543-62/ MAXI: http://134.160.243.77/star_data/J1547-625/J1547-625.html