Optical confirmation of the outburst of Swift J1922.7-1716 observed with LCO and REM

ATel #16461; D. M. Russell (NYU Abu Dhabi), M. C. Baglio (INAF-OAB), N. Degenaar (U. Amsterdam), K. Alabarta (NYUAD), M. Armas Padilla (IAC), A. Bahramian (ICRAR-Curtin), S. Campana, S. Covino, P. D'Avanzo (INAF-OAB), S. Fijma (U. Amsterdam), P. Goldoni (APC/IRFU), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), T. Maccarone (TTU), N. Masetti (INAF-OAS), J. C.A. Miller-Jones (ICRAR-Curtin), T. Munoz-Darias (IAC), S. Rout (NYUAD), T. D. Russell (INAF/IASF Palermo), P. Saikia (NYUAD), A. Shaw (Butler U.), G. R. Sivakoff (U. Alberta), M. Stoop (U. Amsterdam), J. van den Eijnden (U. Warwick), R. Wijnands (U. Amsterdam)
on 20 Feb 2024; 17:41 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

Tweet

Swift J1922.7-1716 is a low-mass X-ray binary (LMXB) discovered as an X-ray transient by BAT on Swift in 2004 (ATel #668, #669, Falanga et al. 2006). A second outburst was detected in 2011 by Swift (BAT, XRT and UVOT; GCN #12522, Degenaar et al. 2012) and MAXI (ATel #3548). During the 2011 outburst, the optical spectrum was found to be typical of an LMXB (ATel #3740, #3742), and a Type-I X-ray burst was detected with Swift (BAT and XRT; ATel #3741), demonstrating that the compact object in the system is a neutron star. Recently, a new outburst was reported after an X-ray brightening was detected by MAXI on 2024 Feb 10 (ATel #16451).

Here, we report optical observations of the field of Swift J1922.7-1716 with Las Cumbres Observatory (LCO) network. Images were taken with the 1-m node at Teide Observatory, Tenerife, Spain on 2024 Feb 16, 17, 18 and 19. As the source is emerging from Sun constraint, observations were made at high airmass (3.4-3.6), and there were poor seeing conditions during the first two nights. We detected an optical source coincident with the position of Swift J1922.7-1716 (GCN #12522; for a finding chart see Degenaar et al. 2012). The following preliminary magnitudes in SDSS i' and Bessel V filters were estimated using aperture photometry, from field stars in the PanSTARRS DR2 and APASS DR10 catalogs:

MJD 60356.28: i' = 17.27 +/- 0.13,

MJD 60357.28: i' = 17.29 +/- 0.06; V = 17.33 +/- 0.04,

MJD 60358.28: i' = 17.38 +/- 0.04; V = 17.32 +/- 0.03,

MJD 60359.27: i' = 17.54 +/- 0.04; V = 17.43 +/- 0.02.

There is evidence for a slight fading over the four dates. The above magnitudes are within the range of those reported in UVOT v-band during the 2005 and 2011 outbursts of Swift J1922.7-1716 (the brightest was v = 16.98 +/- 0.05 in 2005 and v = 17.40 +/- 0.14 in 2011; Degenaar et al. 2012). Since there is no source at this position in quiescence, to a limiting magnitude of >23.4 in g and i bands (ATel #3742), we can confirm that Swift J1922.7-1716 is indeed in outburst.

The field of Swift J1922.7-1716 was also observed with the Robotic Eye Mount (REM) 60cm telescope in La Silla (Chile) strictly simultaneously at NIR (H-band, 1.64 micron) and optical (g',r',i') filters. Observations took place on 2024 February 17 (MJD 60357.37) with bad airmass conditions (3.8). The source was detected in the optical with the following magnitudes (extracted using aperture photometry): g'=17.51 +/- 0.09 , r'=17.64 +/- 0.09 , i'=17.21 +/- 0.10. The results are consistent with the magnitudes obtained with the LCO observations.

The source was not significantly detected in the acquired 30sx5 H-band averaged image. We derived a 3-sigma upper limit of H>16.6 mag. We will conduct new observations with REM once the visibility of the target improves and we are able to observe under better airmass conditions.

We encourage multi-wavelength observations of this neutron star LMXB. We will continue to observe the system with LCO and REM.

This material is based upon work supported by Tamkeen under the NYU Abu Dhabi Research Institute grant CASS (Center for Astrophysics and Space Science).