The X-Ray Variable Sky as Seen by MAXI: The Future of Dust-echo Tomography with Bright Galactic X-Ray Bursts

Abstract

Bright, short duration X-ray flares from accreting compact objects produce thin, dust scattering rings that enable dust-echo tomography: high-precision distance measurements and mapping of the line-of-sight distribution of dust. This work looks to the past activity of X-ray transient outbursts in order to predict the number of sight lines available for dust-echo tomography. We search for and measure the properties of 3σ significant flares in the 2–4 keV light curves of all objects available in the public MAXI archive. We derive a fluence sensitivity limit of 10−3 erg cm−2 for the techniques used to analyze the light curves. This limits the study mainly to flares from Galactic X-ray sources. We obtain the number density of flares and estimate the total fluence of the corresponding dust echoes. However, the sharpness of a dust-echo ring depends on the duration of a flare relative to quiescence. We select flares that are shorter than their corresponding quiescent period to calculate a number density distribution for dust-echo rings as a function of fluence. The results are fit with a power law of slope −2.3 ± 0.1. Extrapolating this to dimmer flares, we estimate that the next generation of X-ray telescopes will be 30 times more sensitive than current observatories, resulting in 10–30 dust ring echoes per year. The new telescopes will also be 10–100 times more sensitive than Chandra to dust ring echoes from the intergalactic medium.