Monitoring campaign of 1RXS J171824.2-402934, the low-mass X-ray binary with the lowest mass accretion rate

Abstract

An X-ray monitoring campaign with Chandra and Swift confirms that 1RXS J171824.2-402934 is accreting at the lowest rate among the known persistently accreting low-mass X-ray binaries. A thermonuclear X-ray burst was detected with the all-sky monitor on RXTE. This is only the second such burst seen in 1RXS J171824.2-402934 in more than 20 Ms of observations done over 19 years. The low burst recurrence rate is in line with the low accretion rate. The persistent nature and low accretion rate can be reconciled within accretion disk theory if the binary system is ultracompact. An unprecedentedly short orbital period of less than approximately 7 minutes would be implied. An ultracompact nature, together with the properties of the type I X-ray burst, suggests, in turn, that helium-rich material is accreted. Optical follow-up of the Chandra error region does not reveal an unambiguous counterpart.

7 pages, 3 figures, accepted by A&A on August 23, 2009

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• An in-depth study of a neutron star accreting at low Eddington rate: On the possibility of a truncated disk and an outflow
• The nature of very faint X-ray binaries; hints from light curves
• Low-Mass X-ray Binaries
• The X-ray spectral properties of very-faint persistent neutron star X-ray binaries
• X-ray Reflection and An Exceptionally Long Thermonuclear Helium Burst from IGR J17062-6143
• A low-level accretion flare during the quiescent state of the neutron-star X-ray transient SAX J1750.8-2900
• New insights on the puzzling LMXB 1RXS J180408.9-342058: the intermediate state, the clocked type-I X-ray bursts and much more
• Outflows and spectral evolution in the eclipsing AMXP SWIFT J1749.4-2807 with NICER, XMM-Newton and NuSTAR
• Connecting the m-dots: accretion rates and thermonuclear burst recurrence times on neutron stars and white dwarfs