Search for an optical and NIR counterpart of the newly discovered Accreting Millisecond X-ray Pulsar MAXI J1816-195

ATel #15445; M. C. Baglio, P. Saikia, D. M. Russell, K. Alabarta (NYU Abu Dhabi), F. Vincentelli (IAC), P. Casella (INAF-OAR), S. Campana, P. D'Avanzo (INAF-OAB), P. Goldoni (APC/IRFU), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), N. Masetti (INAF-OAS), T. Munoz-Darias (IAC)
on 20 Jun 2022; 08:59 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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MAXI J1816-195 is a newly discovered X-ray transient (ATel #15418). The source was first detected with the MAXI/GSC on 2022 June 7 (MJD 59737; ATel #15418); its X-ray position was then confirmed with higher precision by Swift/XRT observations (ATel #15421). On June 8 (MJD 59738), the source was observed with NICER, allowing the detection of coherent pulsations at 528.6 Hz (in the 1-10 keV range). Thanks to these observations, the source was cataloged as a neutron star X-ray binary, specifically a new accreting millisecond X-ray pulsar (ATel #15425). According to the latest reported Swift/XRT observations, the absorption is high (N_H = (1.82 +/- 0.04) x 10^22 cm^-2, corresponding to A_v=6.36+/-0.39 according to the empirical relation of Foight et al. 2016; ATel#15426), which makes the identification of an optical counterpart challenging (see ATel #15427, where upper limits of r~21.1 and z~19.7 are reported). NICER observations performed between June 8 (MJD 59738) and 10 (MJD 59740) allowed to estimate an orbital period of the source of 17402.27s (~4.8 hr), together with other orbital parameters (ATel#15431). The search for a radio counterpart with the Yamaguchi Interferometer resulted in 5 sigma upper limits of 1.5 mJy in C band (6856 MHz central frequency) and 1.9 mJy in X band (8448 MHz central frequency; ATel#15437).

We obtained observations at optical (g,r,i-bands) and near infrared (H-band) frequencies with the Rapid Eye Mount (REM) telescope located in La Silla, Chile, on 2022 June 17 (MJD 59747.22-59747.26). We performed 18 consecutive pointings, 120s integration time each, in the g,r,i-bands, and we acquired 18 simultaneous sets of 5 pointings (30s integration time each) in H-band. Once averaged, we obtained a 36 min integration time image for each of the optical filters, and a 45min integration time image in H-band. No target can be identified within the 3.7’’ Swift/XRT error circle (ATel #15421). We obtain the following 3-sigma upper limits (magnitudes are evaluated using aperture photometry and calibrated against the Pan-STARRS (AB magnitudes) and 2MASS (Vega magnitudes) catalog for the optical and NIR observations, respectively): g >18.98 mag, r > 18.66 mag, i > 18.50 mag, H>15.00 mag.

We also observed the source with the 1m telescopes of the Las Cumbres Observatory network, as part of a monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008). 3 pointings, 200s integration time each, in the g' and i' optical bands were obtained on June 7 (MJD 59737.8 and 59737.9) and 9 (MJD 59739.1). We averaged the images in each band, and we performed aperture photometry on the combined images; magnitudes were calibrated against the Pan-STARRS catalog (AB magnitudes). The target is not detected, with 3 sigma upper limits of g'>20.74, i'>20.90.

Multiwavelength observations following the evolution of the outburst are encouraged.

Near-infrared finding chart of MAXI J1816-195