The X-ray flaring properties of Sgr A* during six years of monitoring with Swift

TL;DR

This study analyzes six years of Swift X-ray observations of Sgr A*, detecting six bright flares, and constrains their occurrence rate, spectral properties, and potential changes due to environmental interactions.

Contribution

It provides the longest continuous monitoring of Sgr A*'s X-ray flares with Swift, doubling the known bright flare detections and offering insights into flare rates and spectral variability.

Findings

Detected six bright X-ray flares from Sgr A*.

Estimated flare occurrence rate of 0.1-0.2 per day.

Observed potential spectral differences among flares.

Abstract

Starting in 2006, Swift has been targeting a region of ~21'X21' around Sagittarius A* (Sgr A*) with the onboard X-ray telescope. The short, quasi-daily observations offer an unique view of the long-term X-ray behavior of the supermassive black hole. We report on the data obtained between 2006 February and 2011 October, which encompasses 715 observations with a total accumulated exposure time of ~0.8 Ms. A total of six X-ray flares were detected with Swift, which all had an average 2-10 keV luminosity of Lx (1-4)E35 erg/s (assuming a distance of 8 kpc). This more than doubles the number of such bright X-ray flares observed from Sgr A*. One of the Swift-detected flares may have been softer than the other five, which would indicate that flares of similar intensity can have different spectral properties. The Swift campaign allows us to constrain the occurrence rate of bright (Lx > 1E35 erg/s) X-ray flares to be ~0.1-0.2 per day, which is in line with previous estimates. This analysis of the occurrence rate and properties of the X-ray flares seen with Swift offers an important calibration point to asses whether the flaring behavior of Sgr A* changes as a result of its interaction with the gas cloud that is projected to make a close passage in 2013.