Swift/BAT discovery of a new transient source Swift J1729.5-3223

ATel #14791; Alberto Segreto, Melania Del Santo, Antonino D'Ai', Thomas David Russell, Elena Ambrosi (INAF/IASF Palermo, Italy), Alessio Marino (Palermo University, Italy)
on 16 Jul 2021; 09:17 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Melania Del Santo (melania@ifc.inaf.it)

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We report on the recent discovery of a new X-ray transient, which we name Swift J1729.5-3223, discovered in the BAT survey data using the Palermo BATIMAGER code (Segreto et al. 2010, A&A, 510A, 47S). This new X-ray source has continued to be detected by BAT since MJD 59373. Using the BAT data, we determine the position to be R.A.=262.399 deg, Dec=-32.39 deg (3 arcmin error radius). On 2021 June 15 (MJD 59380) the 20-50 keV flux was 2.4x10^-10 erg/cm2/s and the corresponding spectrum is well fitted by a simple power-law with Gamma=1.6+/-0.1.

We followed up this discovery with a Swift/XRT Target of Opportunity observation, performed on MJD 59409 (2021-07-14) from 14:38:02 UT (ObsID 14435001) for 1 ks of exposure time. XRT was operating in Photon Counting mode. An uncatalogued X-ray source consistent with the BAT position was detected with the XRT at the source position:

RA (J2000): 17h29m34s.7 (262.395 deg)
Dec (J2000): -32deg22'20.6'' (-32.372389 deg),
with a 90% error radius of 3.7''.

The XRT spectrum is well fitted by an absorbed power-law model (reduced chi2 = 0.85, 20 dof) with N_H=(2.1 +/- 1.1) x 10^22 cm-2, Gamma = 1.1 +/- 0.4 and a 0.3-10 keV absorbed flux of (5.5 +/- 0.7) x 10^-11 erg/cm2/s. This spectrum is consistent with a low mass X-ray binary, either with a black hole or with a neutron star, in a hard state.
We are planning follow up X-ray and radio observations of this new source. Additional multi-wavelength observations are encouraged.

We thank the Neil Gehrels Swift Observatory team for making this observation possible, the duty scientists as well as the science planners.