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INTEGRAL Observations of the Ultra-compact X-ray Binary 4U 1543-624 in Outburst

ATel #10719; J. M. Miller, R. M. Ludlam, M. T. Reynolds (Univ. of Michigan), E. Kuulkers (ESA/ESTEC), C. Ferrigno, E. Bozzo (ISDC, Univ. of Geneva)
on 7 Sep 2017; 12:24 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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

4U 1543-624 is an X-ray binary that typically emits at a steady but low X-ray flux. To the best of our knowledge, there are no reports of millisecond variability, nor Type-1 X-ray bursts, nor other clear hallmarks of a neutron star primary in 4U 1543-624. Optical light curves obtained by Wang & Chakrabarty (2004) established a period of P = 18.2 minutes for 4U 1543-624, signaling that it is an ultra-compact binary wherein the primary accretes from a degenerate donor. Patterns of enhanced abundances in low-energy X-ray spectra also appear to point to a white dwarf companion star (e.g., Juett & Chakrabarty 2003, Madej & Jonker 2011).

Ludlam et al. (2017, ATEL #10690) report enhanced activity in 4U 1543-624, based on Swift/BAT monitoring of the source starting on around 2017 August 09. To study the high energy extent of this peculiar hard outburst, INTEGRAL observed 4U 1543-624 on 2017 August 23-24, for a total exposure time of 42.5 ks.

The following fluxes (and, 1-sigma errors) were measured:

Band, Flux (mCrab)

3-10 keV, 31 +/- 2
10-20 keV, 23 +/- 4
20-40 keV, 23 +/- 1
40-80 keV, 18 +/- 2

The clear detection in the 40-80 keV is notable and signals that the outburst is reaching to very high energy, though with a modest soft X-ray increase and no detected radio emission (see ATEL #10690). The trend of declining Crab-scaled flux with energy signals that the source spectrum is softer than that of the Crab. This indicates a power-law index steeper than Gamma ~ 2.1 (Toor & Seward 1974), or a power-law with a break or exponential cut-off.

An extraction of the JEM-X1 and IBIS/ISGRI data using OSA 10.2 confirms this. The 3-80~keV specrum can be satisfactorily fit with a simple power-law, with an index of Gamma = 2.34 +/- 0.05 and a 3-100 keV flux of 9.8 E-10 erg/cm2/s.

We thank the INTEGRAL Science Operations Center (ISOC) for the effort in scheduling the TOO, and the INTEGRAL Science Data Center (ISDC) for rapid processing of the data obtained via this TOO request.

Madej, O., & Jonker, P., 2011, MNRAS, 412, L11
Juett, A. & Chakrabarty, D., 2003, ApJ, 599, 498
Toor, A., & Seward, F., 1974, AJ, 79, 995
Wang, Z. & Chakrabarty, D., 2004, ApJ, 616, L139