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Chandra Observation of the MAXI J1749-200 Field

ATel #10891; Deepto Chakrabarty (MIT), Peter G. Jonker (SRON), Craig B. Markwardt (NASA/GSFC)
on 25 Oct 2017; 05:54 UT
Credential Certification: Deepto Chakrabarty (deepto@space.mit.edu)

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

As part of an ongoing Chandra X-ray Observatory program for precise localization of X-ray transients in low-mass X-ray binaries (LMXBs), we obtained a short Chandra/ACIS-S observation of the neutron star transient and type-1 X-ray burster MAXI J1749-200 in the globular cluster NGC 6440, following reports in early October that it was in outburst (ATEL #10821, #10826, #10827, #10832). Our observation was made on 2017 October 23, 16:23 TT (MJD 58049.683), which was 24 days after the outburst was initially detected by MAXI (ATEL #10821). The exposure time was 969.7 s, using a (1/8)-subarray of the ACIS S3 chip with 0.4 s readout time.

MAXI J1749-200 had returned to quiescence by the time of our Chandra observation. We detected at least two faint X-ray sources within the 3.5 arcsec Swift error circle (ATEL #10826), consistent with the sources CX1 and CX3 from Pooley et al. (2002, ApJ, 573, 184). We also detected a slight count excess in the vicinity of CX15 at the southern edge of the error circle. CX1 is the recurrent X-ray transient and accreting millisecond pulsar SAX J1748.9-2021, and the only known type-1 X-ray burster among these three sources. CX3 is a suspected quiescent LMXB, and CX15 is a suspected cataclysmic variable. The flux of each of these three sources in our observation implies a luminosity of order 10^32 erg/s, consistent with a quiescent LMXB. We are thus unable to identify which of these sources was the MAXI J1749-200 transient in early October.