INTEGRAL non-detection of Cen X-4 in hard X-rays

The X-ray transient Cen X-4 harbors the nearest-known accreting neutron star in a low-mass X-ray binary. Emission above quiescence levels in optical, UV and soft X-rays has recently been reported (ATels #14254, #14302, #14303 and #14332), suggesting that Cen X-4 might be entering its third outburst after more than four decades in quiescence.

INTEGRAL performed a target of opportunity observation of the source between January 13th UTC 19:56 and January 14th UTC 21:46, 2021 (90 ksec elapsed time during INTEGRAL revolution 2320). Our analyses of JEM-X (3-30 keV) and IBIS/ISGRI (28-80 keV) data do not reveal any detection at the position of Cen X-4 for 37 ksec and 71 ksec effective exposures (corrected for dead time and vigneting), respectively.

We thus derive the following flux upper limits at 95% confidence level: 3-10 keV: 2 mCrab 10-25 keV: 5 mCrab 28-40 keV: 2 mCrab 40-80 keV: 3 mCrab The JEM-X light curves do not reveal any significant variations above the background level, which might have suggested thermonuclear bursting activity from the neutron star.

These results confirm the most recent observations in soft X-rays, UV and optical (ATel #14317) indicating that Cen X-4 has already returned to quiescent levels.