OAN-SPM VR observations during the optical rise of Aql X-1

ATel #16841; Raul Michel, Angel Castro (UNAM), Kevin Alabarta (NYU Abu Dabhi)
on 3 Oct 2024; 22:15 UT
Credential Certification: Angel Castro (acastro@astro.unam.mx)

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Aql X-1 is a low-mass X-ray binary with a neutron star that undergoes regular outbursts. The most recent outburst was reported in X-rays in ATel#16821 and the optical in ATel#16822, with the probable start of the rise occurring between September 11 (MJD 60564.78) and September 14 (MJD 60567.86). The return to quiescence after its last period of activity was reported in October 2023 (ATel#16284). The binary system has an orbital period of 18.9479 hours (García et al., 1999). We present photometric observations in the V and R bands of Aql X-1 using the 1.5m telescope at the San Pedro Martir National Astronomical Observatory (OAN-SPM) between September 21 (MJD 60574.13) and 30 (MJD 60583.29), 2024. Aql X-1 was observed for just over four consecutive hours on average, using an alternating sequence of filters with integration times of 30 and 50 seconds in the V and R bands, respectively. During the nights of monitoring, a rise of more than one magnitude was observed in both bands. The data were reduced using the IRAF package, while VR photometry of the field stars was obtained from the GAIA synthetic photometry catalogue. The mean photometric error is 0.017 mag for the V filter and 0.021 mag for the R filter. Subsequent X-ray observations with the NuSTAR instrument (ATel#16829; MJD 60576.5) reported the detection of a thermonuclear burst lasting a few seconds, possibly similar to those observed during the 2020 outburst (Guver et al., 2021) with NICER. However, previous observations with Swift/XRT (ATel#16826) did not report these bursts in their preliminary analysis. In the radio band (ATel#16823), a point source was identified, and a spectral index consistent with a self-absorbed compact jet was fitted (Diaz-Trigo et al., 2028). Additional R-band photometry is reported for five different epochs on ATel#16840. Based upon observations carried out at the Observatorio Astronomico Nacional on the Sierra San Pedro Martir (OAN-SPM), Baja California, Mexico.

OAN-SPM optical light curve of Aql X-1