Exploring LSST's capabilities for early detection of outbursts in low-mass X-ray binaries

Abstract

Following long periods of quiescence, low-mass X-ray binaries can exhibit intense X-ray outbursts triggered by instabilities within the accretion disk. These outbursts can sometimes be detected in optical wavelengths before being detected in X-rays, acting as an early onset warning and enabling a deep study of accretion disk properties informed by the lag between optical and X-ray rise. We explore the potential of Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) to detect these outbursts early through optical observations. We evaluate the capabilities of LSST based on currently planned survey cadence, filter-specific depth, and other observational factors that affect early detection. We develop and apply an extended metric to assess outburst detectability and recovery fraction. We find that despite inherent potential for early detection of XRB outbursts, the currently planned survey strategy makes it challenging to detect early onset of XRBs. Lastly, we demonstrate how this estimate can be used to infer the wider LMXB population in the Galaxy as the LSST progresses.

Figures (5)

• Figure 1: Figure highlights the intrinsic Galactic distribution of sources and the associated extinction structure. The top panel shows the sample density of simulated LMXBs used as input for the simulations, illustrating their assumed distribution relative to the Galactic plane and bulge. The bottom panel shows the corresponding line-of-sight extinction, based on the 3D extinction model and extinction map.
• Figure 2: Figure shows the light curves for two sources detected in the 10 year survey. The top panel is for a source in a non deep drilling field and the bottom plot is of a source in a deep drilling field. Every coloured dot in the plot corresponds to one magnitude measurement for the light curve. The x-axis shows the time since the start of the observation in days and the y-axis shows the magnitude of the observations. The different filters shown in the figure have different coverage and extinction which affects the detectability and magnitude of the outburst.
• Figure 3: Sky maps illustrating the detection statistics of LMXB outbursts in the LSST baseline v4.3 10-year survey, shown in equatorial coordinates to reflect the LSST observational footprint. The top panel shows the spatial distribution of sources that are theoretically detectable above the nominal detection threshold, while the bottom panel highlights the subset of sources detected early, defined as being detected at least twice within 7 days of the outburst onset. The color bar indicates the fraction of events detected at each sky location. The reduced detectability toward the Galactic center reflects the combined effects of extinction and survey sensitivity rather than an imposed spatial exclusion.
• Figure 4: Cumulative probability distribution showing the time delay between the detection of an outburst ($\rm T_{detect}$) and its onset ($\rm T_{outburst}$) for sources observed in deep drilling fields (red curve) and non-deep drilling fields (blue curve). The plot illustrates the relative detection efficiency of the LSST survey strategy, with deep drilling fields showing a higher probability of earlier detection compared to non-deep drilling fields. This highlights the advantage of deep drilling fields in identifying outbursts more promptly, which is critical for time-sensitive astrophysical studies.
• Figure 5: Pairwise plots demonstrating the posterior samples for the model described in §\ref{['sec:lmxbpop']} in the hypothetical scenario where a total of 50 LMXB outbursts are identified over the entire life of LSST. The red contours and histograms represent scenario "a" (most LMXBs having shorter reoccurrence times), while the blue ones represent scenario "b" (a wider distribution of reoccurrence times).