* Astronomy

Title: Monitoring campaign of 1RXS J171824.2-402934, the low-mass X-ray binary with the lowest mass accretion rate
Authors: J.J.M. in 't Zand (SRON), P.G. Jonker (SRON & Cfa), C.G. Bassa (SRON & Radboud U. Nijmegen), C.B. Markwardt (UMD & NASA-GSFC), A.M. Levine (MKI)

We carried out X-ray and optical observations of 1RXS J171824.2-402934. A monitoring campaign with Chandra and Swift confirms that it is the persistently accreting low-mass X-ray binary with the lowest known mass accretion rate. The second thermonuclear X-ray flash ever was detected, with RXTE-ASM. It testifies to the low accretion rate. Accretion disk theory predicts that the persistent nature and low accretion rate result from a short orbital period. For a helium-rich accretion disk, as suggested from the short orbital period and the flash, an unprecedented small period is required of less than about 7 min. Optical follow-up of the Chandra error region does not reveal an unambiguous counterpart. Deeper imaging under good seeing condition, followed by photometry are needed for further study.

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