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Register a new userHEXTE Detections of Hard X-Ray Tails in Sco X-1
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Authors
F. DAmico
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We report the detection of a non-thermal hard X-ray component from Sco X-1 based upon the analysis of 20-220 keV spectra obtained with the HEXTE experiment on-board the RXTE satellite. We find that the addition of a power-law component to a thermal bremsstrahlung model is required to achieve a good fit in 5 of 16 observations analyzed. Using PCA data we were able to track the movement of the source along the Z diagram, and we found that the presence of the hard X-ray tail is not confined to a specific Z position. However, we do observe an indication that the power law index hardens with increasing mass accretion rate, as indicated from the position on the Z diagram. We find that the derived non-thermal luminosities are at order of 10% of that derived for the brightest of the atoll sources.
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We report RXTE results of spectral analyses of three (Sco X-1, GX 349+2, and Cyg X-2) out of the 6 known Z sources, with emphasis in the hard X-ray emission. No hard X-ray tails were found for Cyg X-2 (< 8.4E-5 photons cm**-2 s**-1, 50-100 keV, 3 sigma) and for GX 349+2 (< 7.9E-5 photons cm**-2 s**-1, 50-100 keV, 3 sigma). For Sco X-1 a variable hard X-ray tail (with an average flux of 2.0E-3 photons cm**-2 s**-1, 50-100 keV) has already been reported. We compare our results to reported detections of a hard component in the spectrum of Cyg X-2 and GX 349+2. We argue that, taking into account all the results on detections of hard X-ray tails in Sco X-1 and GX 349+2, the appearance of such a component is correlated with the brightness of the thermal component.
We study hard X-ray emission of the brightest accreting neutron star Sco X-1 with INTEGRAL observatory. Up to now INTEGRAL have collected ~4 Msec of deadtime corrected exposure on this source. We show that hard X-ray tail in time average spectrum of Sco X-1 has a power law shape without cutoff up to energies ~200-300 keV. An absence of the high energy cutoff does not agree with the predictions of a model, in which the tail is formed as a result of Comptonization of soft seed photons on bulk motion of matter near the compact object. The amplitude of the tail varies with time with factor more than ten with the faintest tail at the top of the so-called flaring branch of its color-color diagram. We show that the minimal amplitude of the power law tail is recorded when the component, corresponding to the innermost part of optically thick accretion disk, disappears from the emission spectrum. Therefore we show that the presence of the hard X-ray tail may be related with the existence of the inner part of the optically thick disk. We estimate cooling time for these energetic electrons and show that they can not be thermal. We propose that the hard X-ray tail emission originates as a Compton upscattering of soft seed photons on electrons, which might have initial non-thermal distribution.
Magnetars persistent emission above 10 keV was recently discovered thanks to the imaging capabilities of the IBIS coded mask telescope on board the INTEGRAL satellite. The only two sources that show some degree of long term variability are SGR 1806-20 and 1RXS J170849.0-400910. We find some indications that variability of these hard tails could be the driver of the spectral variability measured in these sources below 10 keV. In addition we report for the first time the detection at 2.8 sigma level of pulsations in the hard X-ray tail of SGR 1806-20.
We present the results from simultaneous radio (Very Large Array) and X-ray (Rossi-X-ray Timing Explorer) observations of the Z-type neutron star X-ray binary GX~17+2. The aim is to assess the coupling between X-ray and radio properties throughout its three rapidly variable X-ray states and during the time-resolved transitions. These observations allow us, for the first time, to investigate quantitatively the possible relations between the radio emission and the presence of the hard X-ray tails and the X-ray state of the source. The observations show: 1) a coupling between the radio jet emission and the X-ray state of the source, i.e. the position in the X-ray hardness-intensity diagram (HID); 2) a coupling between the presence of a hard X-ray tail and the position in the HID, qualitatively similar to that found for the radio emission; 3) an indication for a quantitative positive correlation between the radio flux density and the X-ray flux in the hard-tail power law component; 4) evidence for the formation of a radio jet associated with the Flaring Branch-to-Normal Branch X-ray state transition; 5) that the radio flux density of the newly-formed jet stabilizes when also the normal-branch oscillation (NBO) in the X-ray power spectrum stabilizes its characteristic frequency, suggesting a possible relation between X-ray variability associated to the NBO and the jet formation. We discuss our results in the context of jet models.
Sco X-1, the brightest low mass X-ray binary, is likely to be a source for gravitational wave emission. In one mechanism, emission of a gravitational wave arrests the increase in spin frequency due to the accretion torque in a low mass X-ray binary. Since the gravitational waveform is unknown, a detection method assuming no apriori knowledge of the signal is preferable. In this paper, we propose to search for a gravitational wave from Sco X-1 using a {{it source tracking}} method based on a coherent network analysis. In the method, we combine data from several interferometric gravitational wave detectors taking into account of the direction to Sco X-1, and reconstruct two polarization waveforms at the location of Sco X-1 in the sky as Sco X-1 is moving. The source tracking method opens up the possibility of searching for a wide variety of signals. We perform Monte Carlo simulations and show results for bursts, modeled, short duration periodic sources using a simple excess power and a matched filter method on the reconstructed signals.
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