Revisiting the expected Micro-X signal from the 3.5 keV line

TL;DR

This paper reevaluates the expected detection of the 3.5 keV line by Micro-X, showing that previous estimates were optimistic and that detection is unlikely with the original setup, but possible with increased field-of-view.

Contribution

It provides a revised estimate of the Micro-X instrument's ability to detect the 3.5 keV line using recent off-centre region data and suggests modifications to improve detection prospects.

Findings

Original Micro-X estimate overestimated detection counts.

Detection likelihood is low with the original payload.

Enhanced field-of-view improves detection chances to marginal levels.

Abstract

One of the future instruments to resolve the origin of the unidentified 3.5 keV emission line is the Micro-X sounding rocket telescope. According to the estimate made in 2015, Micro-X will be able to detect on average about 18.2 photons from the 3.5 keV line during its 300-second-long planned observation. However, this estimate is based on the extrapolation of the 3.5 keV line signal from the innermost Galactic Centre (GC) region available in 2015. With newly available reports on the 3.5 keV line emission in five off-centre regions, we found that similar Micro-X payload will result in 3.4-4.3 counts on average, depending on the dark matter distribution. Therefore, we show that the 3.5 keV line is unlikely to be detected with a single Micro-X launch using an original Micro-X payload. Increasing its field-of-view from 20$^\circ$ to 33$^\circ$ and its repointing out of GC (to avoid the brightest X-ray point source on the sky, Sco X-1) will increase the expected number of counts from 3.5 keV line to 7.5-7.9, which corresponds to its expected marginal ($\sim 2\sigma$) detection within a single Micro-X observation.