INTEGRAL Picture Of the Month
October 2021

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Mysterious 6.6 keV emission line structure after the 2011 superburst of EXO 1745-248 revealed by MAXI, RXTE, and INTEGRAL

A neutron star low-mass X-ray binary (NS-LMXB), which consists of a mass-donating low-mass star and a mass-accreting neutron star, often produce X-ray bursts by unstable thermonuclear burning of accreted H/He, and are hence thought to be a factory of elements up to or heavier than iron. Since such heavy-element ashes sink beneath an optically thick atmosphere on the NS surface, they will not be easily detected in astronomical observations. One possible way to search for such heavy elements is to obtain information on the material ejected outward by the radiation pressure of the X-ray burst.

On 24 October 2011, the Monitor of All-sky X-ray Image (MAXI) detected a very long X-ray burst, a so-called superburst, from EXO 1745-248. Furthermore, after the superburst ended, an outburst due to an increase in the accretion rate occurred. The left image shows the MAXI X-ray light curve of EXO 1745-248. INTEGRAL and RXTE happened to be observing this object right after the superburst. The dotted lines and arrows in the figure indicate the timing of the INTEGRAL and RXTE observations, respectively. The RXTE observation timings can be classified into three epochs; in the rising phase (A), during (B, C, and D), and after (E, F) the outburst. The right figure shows the result of each observed X-ray intensity spectrum. Surprisingly, in the RXTE/PCA spectrum (A) observed about 40 hours after the superburst peak, a strong broad emission line structure is clearly seen around 6.6 keV (hereafter, referred to as "unusual emission structure", or UES). INTEGRAL had observed this source 10 hours before the time when the mysterious UES was seen, but no significant X-ray signal by JEM-X was detected. The JEM-X upper limit makes the peculiarity of the UES even more striking.

Superbursts are very rare phenomena and such UES features have never been observed from this object (or other similar objects). Considering that the probability of these rare events occurring by chance within 40 hours of each other is very low, it would be natural to assume that the UES was associated with the superburst and probably related to heavy elements produced by the X-ray burst. Since the UES is consistent with gravitationally redshifted charge exchange emission from Ti, Cr, Fe, and Co, it is suggested that the emission results from a charge-exchange interaction between a highly metal-enriched fall-back ionized burst wind and an accretion disk around the neutron star.

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