Long Term X-ray Monitoring Of The TeV Binary LS I +61 303 with RXTE

TL;DR

This study presents the longest and most sensitive X-ray monitoring of LS I +61 303, revealing flux-hardness correlation, large flares, and rapid variability, providing new insights into the system's emission behavior.

Contribution

It provides the first extensive, high-sensitivity X-ray dataset covering multiple binary cycles, capturing large flares and rapid variability in LS I +61 303.

Findings

No strong orbital modulation detected in flux or photon index.

Flux and photon index are strongly correlated, with spectra hardening at higher fluxes.

Observed the fastest X-ray variability from LS I +61 303, with doubling times as short as 2 seconds.

Abstract

We report on the results of a long term X-ray monitoring campaign of the galactic binary LS I +61 303 performed by the Rossi X-ray Timing Explorer. This dataset consists of 1 ks pointings taken every other day between 2007 August 28 until 2008 February 2. The observations covered six full cycles of the 26.496 day binary period and constitute the largest continuous X-ray monitoring dataset on LS I +61 303 to date with this sensitivity. There is no statistically strong detection of modulation of flux or photon index with orbital phase; however, we do find a strong correlation between flux and photon index, with the spectrum becoming harder at higher fluxes. The dataset contains three large flaring episodes, the largest of these reaching a flux level of 7.2 (+0.1,-0.2)*10^-11 erg cm^-2 s^-1 in the 3-10 keV band, which is a factor of three times larger than flux levels typically seen in the system. Analysis of these flares shows the X-ray emission from LS I +61 303 changing by up to a factor of six over timescales of several hundred seconds as well as doubling times as fast as 2 seconds. This is the fastest variability ever observed from LS I +61 303 at this wavelength and places constraints on the size of the X-ray emitting region.