Hard X-ray observation of Scorpius X - 1

Ubertini, P.; Bazzano, A.; Cocchi, M.; Padula, C. La; Sood, R. K.

doi:10.1086/171051

Cited by 7 publications

(4 citation statements)

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“…Buselli et al 1968;Riegler, Boldt, & Serlemitsos 1970;Agrawal et al 1971;Haymes et al 1972), but in most cases they were not found, perhaps due to pronounced variations (e.g. Miyamoto & Matsuoka 1977, and references therein;Soong & Rothschild 1983;Jain et al 1984;Ubertini et al 1992). Recently a hard tail was detected in another Z source, GX 17+2, observed by BeppoSAX in a broad energy range (0.1-200 keV).…”

Section: Introductionmentioning

confidence: 98%

Detection of a Hard Tail in the X‐Ray Spectrum of the Z Source GX 349+2

Salvo

Robba

Iaria

et al. 2001

ApJ

110

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We present the results of a BeppoSAX observation of the Z source GX 349+2 covering the energy range 0.1-200 keV. The presence of flares in the light curve indicates that the source was in the flaring branch during the BeppoSAX observation. We accumulated energy spectra separately for the non-flaring intervals and the flares. In both cases the continuum is well described by a soft blackbody (kT BB ∼ 0.5 keV) and a Comptonized spectrum corresponding to an electron temperature of kT e ∼ 2.7 keV, optical depth τ ∼ 10 (for a spherical geometry), and seed photon temperature of kT W ∼ 1 keV. All temperatures tend to increase during the flares. In the non-flaring emission a hard tail dominates the spectrum above 30 keV. This can be fit by a power law with photon index ∼ 2, contributing ∼ 2% of the total source luminosity over the BeppoSAX energy range. A comparison with hard tails detected in some soft states of black hole binaries suggests that a similar mechanism could originate these components in black hole and neutron star systems.

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Section: Introductionmentioning

confidence: 98%

Detection of a Hard Tail in the X‐Ray Spectrum of the Z Source GX 349+2

Salvo

Robba

Iaria

et al. 2001

ApJ

110

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“…Since its discovery (Giacconi et al 1962) Sco X‐1 has been extensively observed in many different wavelength bands (e.g., correlated optical/X‐ray photometry: Mook et al 1975; Petro et al 1981; Augusteijn et al 1992; Mc Namara et al 1994, optical spectroscopy: La Sala & Thorstensen 1985, radio: Hjellming et al 1990, UV: Vrtilek et al 1990, 1991, hard X‐rays: Jain et al 1984; Ubertini et al 1992). The discovery (van der Klis et al 1985; Hasinger et al 1986; Middleditch & Priedhorsky 1986) of rapid (∼15–55 Hz) and slow (∼6 Hz) quasi‐periodic oscillations (QPO) in the X‐ray flux of the brightest LMXBs opened a way to a greater understanding of the accretion flow close to the compact object in these systems.…”

Section: Introductionmentioning

confidence: 99%

The timing properties of Sco X-1 along its Z track with EXOSAT

Dieters

Klis

2000

Monthly Notices of the Royal Astronomical Society

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A B S T R A C TWe present a systematic, homogeneous analysis of all the EXOSAT ME, high time resolution data on Sco X-1. We investigate, for the first time, all power spectral properties of the ,100 Hz quasi-periodic oscillations (QPO) and noise of Sco X-1 as a function of position on the Z-shaped track traced out in the X-ray colour±colour diagram. For this purpose, we introduce a new generally applicable method for parametrizing the position of a source on its track in a colour±colour or hardness±intensity diagram. Generally, the properties of Sco X-1 vary smoothly as a function of position along the Z track. However, some variability parameters change abruptly at either of the vertices of the Z track, indicating that the branches of the Z track represent distinct source states not only in spectral state but also in rapid variability characteristics. All variability components are found to have energy spectra harder than the average flux. We show that the very low-frequency noise (VLFN) is consistent with being solely the result of motion along the Z track. The power spectra of the X-ray intensity as well as source position along the Z track extend, unbroken, to time-scales of nearly 1 day. We study the high-frequency noise (HFN) component for the first time in sufficient detail to show that there are changes in the HFN cut-off frequency with position on the Z track. It changes abruptly from ,75 to ,35 Hz at the normal/flaring branch vertex. The HFN is found to extend out to about 300 Hz. The QPO show a remarkably rapid change in frequency at or just before the normal-branch/flaring-branch (NB/FB) vertex. This transition happens within 1.5 per cent of the entire extent of the Z track. The QPO themselves are visible for 17 per cent of the Z. We find a new type of behaviour near the NB/FB vertex, i.e., rapid excursions from the NB into the FB and back again taking only a few minutes. We find several indications that position on the Z track is not the only parameter governing the behaviour of Sco X-1. The most dramatic examples of this are two brief episodes where the QPO frequency changed rapidly without the usual changes in colours and intensity that accompany a change of source state. In one case the frequency rapidly rose from 6 to 16 Hz, and then returned to 6 Hz, while Sco X-1 apparently remained unmoved on the normal branch in the colour±colour diagram. In the second case the QPO frequency changed from 16 to 7 Hz without the usual indications of a passage through the normal/ flaring branch vertex (simultaneous dips in the count rate and colours as the frequency changes through 8 Hz). Thus it seems that deviations from the usual one-to-one correspondence between QPO behaviour and spectral state, perhaps caused QPO frequency mode switching, do occasionally occur.

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“…A variable hard component dominating the spectrum of Sco X-1 above ∼ 40 keV was detected as early as 1966 (Peterson & Jacobsen 1966; see also Riegler et al 1970;Agrawal et al 1971;Haymes et al 1972). In other occasions the hard tail in Sco X-1 was not found (e.g., Miyamoto & Matsuoka 1977, and references therein;Soong & Rothschild 1983;Jain et al 1984;Ubertini et al 1992), perhaps owing to pronounced variations. Evidence for a hard component was also found in Cyg X-2 (Peterson 1973), GX 349+2 (Greenhill et al 1979), and in Ginga data of GX 5-1 (although in this case a contribution from a contaminating source could not be excluded, Asai et al 1994).…”

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confidence: 99%

Disappearance of Hard X‐Ray Emission in the LastBeppoSAXObservation of the Z Source GX 349+2

Iaria

Salvo

Robba

et al. 2004

ApJ

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We report on the results from two BeppoSAX observations of the Z source GX 349+2 performed in February 2001 and covering the broad energy range 0.12-200 keV. The light curve obtained from these observations shows a large flaring activity, the count rate varying from ∼ 130 to ∼ 260 counts s −1 , indicating that the source was in the flaring branch during these observations. The average spectrum is well described by a soft blackbody (kT BB ∼ 0.5 keV) and a Comptonized component having a seed-photon temperature of kT 0 ∼ 1 keV, an electron temperature of kT e ∼ 2.7 keV, and optical depth τ ∼ 11. To well fit the energy spectrum three gaussian lines are needed at 1.2 keV, 2.6 keV, and 6.7 keV with corresponding equivalent widths of 13 eV, 10 eV, and 39 eV, probably associated to L-shell emission of Fe XXIV, Lyα S XVI, and Fe XXV, respectively. These lines may be produced at different distances from the neutron star, which increase when the count rate of the source increases. An absorption edge is also needed at 9 keV with an optical depth of ∼ 3 × 10 −2 . From the Color-Color Diagram (CD) we selected five zones from which we extracted the corresponding energy spectra. The temperatures of the blackbody and of the Comptonized component tend to increase when the intensity of the source increases. We discuss our results comparing them to those obtained from a previous BeppoSAX observation, performed in March 2000, during which the source was a similar position of its Z-track. In particular we find that, although the source showed similar spectral states in the 2000 and the 2001 observations, a hard tail, that was significantly detected in March 2000, is not observed in these recent observations.

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Hard X-ray observation of Scorpius X - 1

Cited by 7 publications

References 0 publications

Detection of a Hard Tail in the X‐Ray Spectrum of the Z Source GX 349+2

Detection of a Hard Tail in the X‐Ray Spectrum of the Z Source GX 349+2

The timing properties of Sco X-1 along its Z track with EXOSAT

Disappearance of Hard X‐Ray Emission in the LastBeppoSAXObservation of the Z Source GX 349+2

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