Sco X-1 revisited withKepler, MAXI and HERMES: outflows, time-lags and echoes unveiled

Abstract

Sco X-1 has been the subject of many multi-wavelength studies in the past, being the brightest persistent extra-solar X-ray source ever observed. Here we revisit Sco X-1 with simultaneous short cadence Kepler optical photometry and MAXI X-ray photometry over a 78 day period, as well as optical spectroscopy obtained with HERMES. We find Sco X-1 to be highly variable in all our datasets. The optical fluxes are clearly bimodal, implying the system can be found in two distinct optical states. These states are generally associated with the known flaring/normal branch X-ray states, although the flux distributions associated with these states overlap. Furthermore, we find that the optical power spectrum of Sco X-1 differs substantially between optical luminosity states. Additionally we find rms-flux relations in both optical states, but only find a linear relation during periods of low optical luminosity. The full optical/X-ray discrete correlation function displays a broad ≈ 12.5 hour optical lag. However during the normal branch phase the X-ray and optical fluxes are anti-correlated, whilst being correlated during the flaring branch. We also performed a Cepstrum analysis on the full Kepler light curve to determine the presence of any echoes within the optical light curve alone. We find significant echo signals, consistent with the optical lags found using the discrete cross-correlation. We speculate that whilst some of the driving X-ray emission is reflected by the disk, some is absorbed and re-processed on the thermal timescale, giving rise to both the observed optical lags and optical echoes.

Accretion-induced variability links young stellar objects, white dwarfs, and black holes

“…We extracted light curves for 17 systems from this sample that did not have neighbors close by in the target pixel files provided by the Mikulski Archive for Space Telescopes (MAST). This allowed us to create light curves using relatively large target and background masks, which mitigated the effect of spacecraft jitter in the resulting light curves (41). We visually identified 6 of these 17 light curves as displaying clear aperiodic variability.…”

KeplerK2 observations of Sco X-1: orbital modulations and correlations withFermiGBM and MAXI

“…This has already been extensively discussed by Scaringi et al (2015) and attributed to a thermal time-scale lag. These authors reinforce its significance by de- Figure 14.…”

“…These asymmetries presumably arise from the intrinsic asymmetry of the failed transitions (brightenings in the low state and dips in the high state.) Scaringi et al (2015) used MAXI colours to identify FB and NB epochs. They found that the high state was usually associated with the FB and that the low state corresponded to the NB.…”

Parameters of X-ray binary system Scorpius X-1

“…A regular optical variability of Sco X-1 with a period ≈ 18.9 h was discovered using archival photographic plates by Gottlieb et al (1975) and was confirmed using spectroscopic observations (Cowley & Crampton 1975). Scaringi et al (2015); Hakala et al (2015); Hynes et al (2016) conducted the analysis of optical observations of Sco X-1 obtained by Kepler K2 mission and of X-ray observations obtained by Fermi GBM and MAXI. An average wavelength of wide wavelength range observations of Kepler K2 mission was in the middle of a band that corresponded to B, V, R filters.…”