NICER follow-up observations of 4U 1705-32
ATel #14669; P. M. Bult, K. C. Gendreau (NASA/GSFC), W. Iwakiri (Chuo Univ), M. Ng, D. Chakrabarty (MIT), Z. Arzoumanian (NASA/GSFC), T. Guver (Istanbul Univ.), H. Negoro (Nihon Univ.)
on 29 May 2021; 14:32 UT
Credential Certification: Peter Bult (p.m.bult@nasa.gov)
Subjects: X-ray, Neutron Star, Transient
Referred to by ATel #: 14670
On 2021 May 25 the MAXI/GSC nova alert system triggered on increased X-ray emission from the direction of the known neutron star low-mass X-ray binary 4U 1705-32 / 1RXS J170854.4-321857 (ATel #14663). Following up on this report, we executed pointed NICER observations of 4U 1705-32 on May 28, collecting 2.9 ks exposure between 09:00 and 18:39 UTC. The source was detected in the 0.5-10 keV band with a count-rate that gradually decreased from 25 ct/s to 23 ct/s over the course of the observations. We performed a periodicity search and found no X-ray pulsations in the 0.3-10 keV band.
The 0.3-9.0 keV energy spectrum could be approximated with an absorbed power-law model, yielding an absorption column density of 0.41 +/- 0.02 x 1022 cm-2, a photon index of 1.6 +/- 0.1 and an unabsorbed 1-10 keV X-ray flux of 9.7 x 10-11 erg/s/cm2 at a best-fit chi-square of 377 for 202 degrees of freedom. Assuming a distance of 13 kpc (in 't Zand et al. 2005; A&A, 440, 287), this flux converts to an X-ray luminosity of 2 x 1036 erg/s.
Our luminosity measurement for 4U 1705-32 is substantially below the 1037 erg/s luminosity measured by MAXI on May 25. An updated MAXI light curve shows that on May 28 the source had dropped below the MAXI/GSC detection threshold, consistent with our NICER measurement. Further, we note that the best-fit spectrum model parameters, the intensity, and the residuals relative to the power-law model are all roughly consistent with archival XMM-Newton observations of 4U 1705-32 (Armas Padilla et al., 2019; MNRAS 488, 4). As such it is unclear if the reported outburst was indeed associated with 4U 1705-32, or if it was a new transient in the same direction (and possibly outside the NICER field of view).
NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
