Radio Emission and the Timing Properties of the Hard X‐Ray State of GRS 1915+105

Abstract

We combine a complete sample of 113 pointed observations taken with the Rossi X-ray Timing Explorer between 1996-1999, monitoring observations taken with the Ryle telescope and the Green Bank Interferometer, and selected observations with the Very Large Array to study the radio and X-ray properties of GRS 1915+105 when its X-ray emission is hard and steady. We establish that radio emission always accompanies the hard-steady state of GRS 1915+105, but that the radio flux density at 15.2 GHz and the X-ray flux between 2-200 keV are not correlated. Therefore we study the X-ray spectral and timing properties of GRS 1915+105 using three approaches: first, by describing in detail the properties of three characteristic observations, then by displaying the time evolution of the timing properties during periods of both faint and bright radio emission, and lastly by plotting the timing properties as a function of the the radio flux density. We find that as the radio emission becomes brighter and more optically thick, 1) the frequency of a ubiquitous 0.5-10 Hz QPO decreases, 2) the Fourier phase lags between hard (11.5-60 keV) and soft (2-4.3 keV) in the frequency range of 0.01-10 Hz change sign from negative to positive, 3) the coherence between hard and soft photons at low frequencies decreases, and 4) the relative amount of low frequency power in hard photons compared to soft photons decreases. We discuss how these results reflect upon basic models from the literature describing the accretion flow around black holes and the possible connection between Comptonizing electrons and compact radio jets.

High‐ResolutionChandraHETGS andRossi X‐Ray Timing ExplorerObservations of GRS 1915+105: A Hot Disk Atmosphere and Cold Gas Enriched in Iron and Silicon

“…Despite their use of a very complicated continuum model, it is evident from Figure 1 of Kotani et al (2000) that the observations showing strong absorption features (1994 and 1995) display steep X-ray spectra near 10 keV, in contrast with the 1996 observation, where the spectrum is harder and no absorption features were found with the ASCA data. We conÐrm with the RXT E PCA data archive that GRS 1915]105 was in a bright mode (1.2 crab) of the hard steady state on 1996 October 23 (see Muno et al 2001). One might therefore suspect from the reported ASCA observations that the absorption features from highly ionized atoms are conÐned to the soft X-ray states of GRS 1915]105 ; however, the Chandra results presented here clearly show such absorption features in the low hard state (0.4 crab) as well.…”

“…Radio observations of GRS 1915]105 with the Greenbank Interferometer6 on 2000 April 24.54 indicate a Ñux of 20^4 mJy at 2.25 GHz. The combined radio and X-ray properties place GRS 1915]105 in the low hard state, which is associated with the presence of a steady jet (Dhawan, Mirabel, & Rodriguez 2000 ;Muno et al 2001).…”

“…We then normalized the power spectrum to units of (rms Ñuctuations mean~1)2 Hz~1 in the source, applying a small correction for the contribution of the di †use X-ray background between 2 and 9 keV. The results show a quasiperiodic oscillation (QPO) and a broadband power continuum typical of the low hard state in GRS 1915]105 (see, e.g., Muno et al 2001). The strongest QPO feature is at 3.69 Hz with an integrated amplitude of 8.5% and a weak Ðrst harmonic.…”

A simple model for the complex lag structure of microquasars

“…This however, seems improbable because the passage through 2 Hz is smooth in all variables (see Fig. 6 of Muno et al 2001).…”

“…In this section we will discuss Figs. 7 and 8 in Muno et al (2001), in order to emphasize the observational constraints on the behaviour we are trying to explain here.…”