tokens: tailed and, more importantly, (b) that the chosen statistical sample represents the global statistics well, at least, in terms of statistical power. Therefore, if we randomly select a pixel inside our sampling area from one observation, the probability to observe a value above the background mean is or . 10 10 is the probability to observe values outside the limits. Half of this should account for the probability to observe values that exceed only the upper limit of . Therefore, a is adopted as a realistic occurrence probability of a stochastic observation in a single experiment. Considering also that the signal at the position of the A HST star is the only consistently present signal in all our NIR images (i.e., three independent experiments), then the probability that it results from random background noise fluctuations should be or . Since this is negligible, we can safely consider that the presence of an underlying point source, which systematically “drives” the flux within the green circle above the limit in repeated observations, is the origin of the observed NIR signal of Figure 5 . However, are the photometric properties of the NIR detections consistent with the visual photometric properties of the A HST star? 4.2. Photometric Calibration of the NIR Detections The V and I magnitudes of the A HST star are measured with aperture photometry on the and HST images (Section 2.2 ). They are and mag, respectively. On the contrary, due to the low S/N of the Subaru data, aperture photometry is not an option for estimating the NIR magnitudes of the detections. Alternatively, their , and K S magnitudes are estimated by taking the ratio of the sum of the peak fluxes of G.S.1 and G.S.2 to the peak fluxes of the NIR detections, 11 11 Point sources of different brightness observed simultaneously with the same telescope are convolved with the same PSF as long as their separation is smaller than ∼30 arcsec (i.e., isoplanatic angle). Consequently, the ratio of the fluxes at any point across their PSFs and the ratio of their integrated fluxes will (ideally) be approximately equal and, therefore, any of these measurements should be representative of their true brightness difference. usingwhere is the peak flux of each guide star, is the peak flux of each detection, and is the Subaru magnitude of the G.S. that we derived in Section 2.1.1 . This yields , and mag for the detections in each NIR band. The uncertainties are calculated according to the relationwhere we assume a 10% uncertainty in the measurement of and a 30% uncertainty in the measurement of the, more uncertain, . Having magnitude estimations, we can attempt to constrain the V − I , visual–NIR, 12 12 I.e., , and . and NIR 13 13 I.e., , and . colors of the NIR detections/A HST star and, therefore, its spectral type and luminosity class. 4.3. An Extragalactic Supergiant or a Galactic Dwarf? There are two possibilities for the nature of the A HST star. It can be either a distant supergiant
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