The absence of rapid X-ray variability in active galaxies

Variations on time scales ranging from minutes to several hours in the X-ray flux from 54 observations of 38 active galaxies are identified. The sample is composed mostly of Seyfert I galaxies but also includes radio galaxies, NELG's BL Lacs and 3C 273. Only NGC 6814 varied on time scales as short as 100 sec. No other source was observed to vary with a time scale of less than 12 hours. Large amplitude short term variations are not a characteristic of the X-ray emission from active galaxies. Upper limits on sigma sub I/I ranged from 2% for Cen A, 5% for NGC 4151, to approximately 20% for sources giving 1 ct/sec in the detector. Three objects NGC 3227, NGC 4151 and MCG 5-23-16 show variability consistent with a time scale of approximately 1 day. Ways to reconcile the rapid variability seen for NGC 6814 (and NGC 4051) with the general stability observed for the other objects are considered

Spectral Evolution of Circinus X-1 Along its Orbit

We report on the spectral analysis of Circinus X-1 observed by the ASCA satellite in March 1998 along one orbital period. The luminosity of the source (in the 0.1-100 keV band) ranges from $2.5 \times 10^{38}$ erg s$^{-1}$ at the periastron (orbital phase 0.01) to $1.5 \times 10^{38}$ erg s$^{-1}$ at orbital phase 0.3. From the spectral analysis and the lightcurve we argue that Cir X-1 shows three states along the orbital evolution. The first state is at the orbital phase interval 0.97-0.3: the luminosity becames super-Eddington and a strong flaring activity is present. In this state a shock could form in the inner region of the system due to the super-Eddington accretion rate, producing an outflow of ionized matter whose observational signature could be the prominent absorption edge at $\sim 8.7$ keV observed in the energy spectrum at these phases. In the second state, corresponding to the orbital phase interval between 0.3 and 0.7, the accretion rate is sub-Eddington and we observe a weaker outflow, with smaller hydrogen column: the absorption edge is now at $\sim 8.3$ keV with an optical depth a factor of 2.5 to 6 smaller. The third state corresponds to the orbital phase interval 0.78-0.97. In this state the best fit model to the spectrum requires the presence of a partial covering component, indicating that the emission from the compact object is partially absorbed by neutral matter, probably the atmosphere of the companion star and/or the accreting matter from the companion.Comment: 18 pages, 3 figures. Accepted by Ap

Scattering and Iron Fluorescence Revealed During Absorption Dips in Circinus X-1

We show that dramatic spectral evolution associated with dips occurring near phase zero in RXTE observations of Cir X-1 is well-fit by variable and at times heavy absorption (N_H > 10^24 cm^-2) of a bright component, plus an underlying faint component which is not attenuated by the variable column and whose flux is ~10% of that of the unabsorbed bright component. A prominent Fe emission line at ~6.5 keV is evident during the dips. The absolute line flux outside the dips is similar to that during the dips, indicating that the line is associated with the faint component. These results are consistent with a model in which the bright component is radiation received directly from a compact source while the faint component may be attributed to scattered radiation. Our results are also generally consistent with those of Brandt et al., who found that a partial- covering model could explain ASCA spectra of a low-to-high transition in Cir X-1. The relative brightness of the two components in our model requires a column density of ~2*10^23 cm^-2 if the faint component is due to Thomson scattering in material that mostly surrounds the source. We find that illumination of such a scattering cloud by the observed direct component would produce an Fe K-alpha fluorescence flux that is in rough agreement with the flux of the observed emission line. We also conclude that if the scattering medium is not highly ionized, our line of sight to the compact source does not pass through it. Finally, we discuss simple pictures of the absorbers responsible for the dips themselves.Comment: Accepted for publication in The Astrophysical Journal (23 pages, including 11 figures

Rapid X-ray variability in the Seyfert galaxy NGC 6814

The HEAO-1 A-2 high time resolution X-ray observations of the X-ray emitting Seyfert I Galaxy NGC 6814 are reported. In sharp distinction with a sample of over 30 active galactic nuclei this object showed strong X-ray variability on timescales less than 3 hours. The mean flux on a timescale of 90 minutes varied by a factor of approximately 2.5 corresponding to Delta L sub x being approximately 1 x 10 to the 43rd power ergs/sec. An autocorrelation analysis shows a characteristic time for variability of 100 (+60 or -25) seconds. There is no indication of spectral variability with an upper limit on a change in the power law spectral index of the absolute value of Delta gamma .37, for a factor two change in intensity. The constraints of such rapid variability on a wide variety of X-ray source mechanisms are considered

The Quasi-1D S=1/2 Antiferromagnet Cs2CuCl4 in a Magnetic Field

Magnetic excitations of the quasi-1D S=1/2 Heisenberg antiferromagnet (HAF) Cs2CuCl4 have been measured as a function of magnetic field using neutron scattering. For T<0.62 K and B=0 T the weak inter-chain coupling produces 3D incommensurate ordering. Fields greater than Bc =1.66 T, but less than the field (~8 T) required to fully align the spins, are observed to decouple the chains, and the system enters a disordered intermediate-field phase (IFP). The IFP excitations are in agreement with the predictions of Muller et al. for the 1D S=1/2 HAF, and Talstra and Haldane for the related 1/r^2 chain (the Haldane-Shastry model). This behaviour is inconsistent with linear spin-wave theory.Comment: 10 pages, 4 encapsulated postscript figures, LaTeX, to be published in PRL, e-mail comments to [email protected]

Measurements with the Chandra X-Ray Observatory's flight contamination monitor

NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor (FCM), a system of 16 radioactive calibration sources mounted to the inside of the Observatory's forward contamination cover. The purpose of the FCM is to verify the ground-to-orbit transfer of the Chandra flux scale, through comparison of data acquired during the ground calibration with those obtained in orbit, immediately prior to opening the Observatory's sun-shade door. Here we report results of these measurements, which place limits on the change in mirror--detector system response and, hence, on any accumulation of molecular contamination on the mirrors' iridium-coated surfaces.Comment: 7pages,8figures,for SPIE 4012, paper 7

Evidence for a parsec scale X-ray jet from the accreting neutron star Circinus X-1

We analyzed the zero-order image of a 50 ks Chandra gratings observation of Circinus X-1, taken in 2005 during the source's low-flux state. Circinus X-1 is an accreting neutron star that exhibits ultra-relativistic arcsecond-scale radio jets and diffuse arcminute-scale radio jets and lobes. The image shows a clear excess along the general direction of the north-western counter-jet, coincident with the radio emission, suggesting that it originates either in the jet itself or in the shock the jet is driving into its environment. This makes Circinus X-1 the first neutron star for which an extended X-ray jet has been detected. The kinetic jet power we infer is significantly larger than the minimum power required for the jet to inflate the large scale radio nebula.Comment: Added journal reference, corrected on reference and typo in labels for Fig. 1; 5 pages, 3 figures, ApJ Letter, in pres

Evolution of Hard X-Ray Spectra Along the Branches in Cir X-1

Using the data from the PCA and HEXTE on board the RXTE satellite, we investigate the evolution of the 3-200 keV spectra of the peculiar low mass X-ray binary (LMXB) Cir X-1 along the branches on its hardness-intensity diagram (HID) from the vertical horizontal branch (VHB), through the horizontal horizontal branch (HHB) and normal branch (NB), to the flaring branch (FB). We detect a power-law hard component in the spectra. It is found that the derived photon indices ($\Gamma$) of the power-law hard component are correlated with the position on the HID. The power-law component dominates the X-ray emission of Cir X-1 in the energy band higher than $\sim 20$ keV. The fluxes of the power-law component are compared with those of the bremsstrahlung component in the spectra. A possible origin of the power-law hard component is discussed.Comment: 14 pages, 5 figures, ApJ Letter accepte

Optical Spectroscopy of the environment of a ULX in NGC 7331

Optical photometric and spectroscopic data are presented that show an association of an ultraluminous X-ray source in NGC 7331 with a young star cluster of mass 1.1e5 solar masses and age 4.25 Myr. If the ULX is part of the bright stellar cluster, then the mass of the progenitor of the compact accretor must have been greater than about 40-50 solar masses in order to already have evolved through the supernova stage to a compact object. The companion star is also likely an evolved massive star. The emission line spectrum of the nebula surrounding the cluster can be interpreted as a result of photoionization by the cluster OB stars with an additional source of shock excitation producing strong [SII], [OI] and NII lines. This additional source appears to be as much as five times more powerful than the supernovae and stellar winds in the cluster can provide. Additional mechanical energy input associated with the ULX itself can help explain the residual shock excited line luminosities of the emission region.Comment: 17 pages, accepted to Ap