arXiv subjects

A. Santangelo

Publications and source records attributed to A. Santangelo.

At least 217 records · Page 12Linked to original sources

arXiv2000

Broad band spectrum of Cygnus X-1 in two spectral states with BeppoSAX

We report on the 0.5--200 keV spectral properties of Cyg X-1 observed at different epochs with the Narrow Field Instruments of the BeppoSAX satellite. The source was in its soft state during the first observation of 1996 June. In the second observation of 1996 September, the source had parameters characteristic to its hard state. A soft X-ray excess, a broad Fe K$α$ line and Compton reflection are clearly detected in both states. The soft-state broad-band continuum is well modeled by a disk blackbody (accounting for the soft excess) and Compton upscattering of the disk photons by a hybrid, thermal/non-thermal, plasma, probably forming a corona above the disk (also giving rise to the Compton-reflection component). In the hard state, the primary hard X-ray spectrum can be well modeled by Compton upscattering of a weak blackbody emission by a thermal plasma at a temperature of $\sim 60$ keV. The soft excess is then explained by thermal Comptonization of the same blackbody emission by another hot plasma cloud characterized by a low value of its Compton parameter. Finally, we find the characteristic ratio of the bolometric flux in the soft state to that in the hard state to be about 3. This value is much more compatible with theories of state transitions than the previously reported (and likely underestimated) value of 1.5.

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arXiv2000

Detection of a broad iron emission line and sub-millisecond quasiperiodic oscillations from the type I X-ray burster 4U1728-34 in a high state

We report results from simultaneous RossiXTE and BeppoSAX observations of the neutron-star x-ray binary and type I X-ray burster 4U1728-34. The source was found in a high luminosity state, L_X near 0.1 L_Edd, and quasiperiodic oscillations were detected at 1284 +/- 6 Hz, the highest frequency ever observed from this source. The x-ray spectrum shows a broad, FWHM = 0.8 keV, iron K-alpha fluorescence line. We discuss interpretations of the broad line and the quasiperiodic oscillations and how future simultaneous spectral and timing observations can be used to test these interpretations and, potentially, to estimate the mass of the compact object.

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arXiv2000

BeppoSAX observation of the transient X-ray pulsar GS 1843+00

We present the results from both the timing and spectroscopic analysis of the transient X-ray pulsar GS 1843+00 observed by the BeppoSAX satellite on 1997 April 4, when the source was at a luminosity of ~10^{37} erg s^{-1}. GS 1843+00 shows a very hard spectrum that is well fitted by an absorbed power law (N_H \~2.3 10^{22} cm^{-2}) modified by a high energy cut-off above 6 keV. The source shows a small pulse amplitude in the whole energy band. The pulse profile evolves with energy from a double-peaked to single-peaked shape. The barycentric pulse period is 29.477+/-0.001 s.

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arXiv2000

ASCA and BeppoSAX observations of the peculiar X-ray source 4U1700+24/HD154791

The X-ray source 4U1700+24/HD154791 is one of the few galactic sources whose counterpart is an evolved M star. In X-rays the source shows extreme erratic variability and a complex and variable spectrum. While this strongly suggests accretion onto a compact object, no clear diagnosis of binarity was done up to now. We report on ASCA and BeppoSAX X-ray broad band observations of this source and on ground optical observations from the Loiano 1.5 m telescope.

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arXiv2000

X-ray/optical observations of XTE J0421+560/CI Cam in quiescence

We report on a BeppoSAX observation of the transient X-ray source XTE J0421+560 during quiescence performed ~150 days after the 1998 April outburst. The source had an unabsorbed 0.5-10 keV flux of 6.7x10^{-12} erg/cm2/s and was still remarkably soft with most of the emission below 2 keV. The X-ray spectrum can be fit with the same two-temperature model as the outburst observations. There is evidence for the presence of an Iron emission feature at ~7 keV. We report also on a series of optical observations performed using the 72 cm Teramo-Normale Telescope (TNT) of the Teramo Observatory, and the 1.5 m Loiano Telescope of the Bologna Observatory. The optical spectrum includes very strong Balmer emission lines, He I, [Fe II], and [O I] features. From the observed L_x/Lopt~10^{-3} the quiescent optical emission cannot be due to re-processing of the X-rays, but has to be generated in the optical companion or the circumstellar material. Moreover, the quiescent X-ray luminosity cannot be due to the optical star if it is of spectral type OB. Although the nature of the compact object present in the XTE J0421+560/CI Cam system cannot be firmly established, we speculate that it is most probably a white dwarf.

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arXiv2000

Broad-band BeppoSAX observation of the low-mass X-ray binary X1822-371

Results of a 1997 September 9-10 BeppoSAX observation of the 5.57 hr low-mass X-ray binary (LMXRB) X1822-371 are presented. The 0.3-40 keV spectrum is unusually complex and cannot be fit by any of the standard models applied to other LMXRB. At least two components are required. One component has a shape consistent with that expected from the Comptonization of an input soft (Wein) spectrum while the other, contributing ~40% of the 1-10 keV flux, is consistent with being a blackbody. In addition, there is a ``dip'' in the spectrum which can be modeled by a 1.33 +0.05 -0.11 keV absorption edge with an optical depth, tau, of 0.28 +/- 0.06. If the same model is fit to ASCA Solid-State Imaging Spectrometer spectra obtained in 1993 and 1996, then reasonable fits are also obtained, with a similar absorption feature required. The nature of this feature is highly uncertain; its energy corresponds to the K-edges of highly ionized Ne x and neutral Mg, or to an L-edge of moderately ionized Fe. Surprisingly, no strong (tau > 0.05) Fe-K or (tau > 0.18) O-K edges are visible. The folded lightcurve of X1822-371 is similar to previous observations, except that no strong softening is seen near the eclipse. An updated orbital ephemeris is provided.

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arXiv1999

Discovery of a faint 437 s X-ray pulsar 1SAX J1452.8-5949

A new pulsar, 1SAX J1452.8-5949, was discovered during a BeppoSAX galactic plane survey on 1999 July 20 at R.A.=14h 52m 49s, Dec=-59 49' 18'' (J2000) with a 90% confidence uncertainty radius of 50''. Coherent pulsations were detected with a barycentric period of a 437.4 +/- 1.4 s. The X-ray spectrum can be modeled by a power-law with a photon index of 1.4 +/- 0.6 and absorption consistent with the galactic value in the direction of the source ($1.9 \times 10^{22}$ atom cm$^{-2}$). An Fe K line with a equivalent width of $\approxgt$1.3 keV may be present in the spectrum. The unabsorbed 2-10 keV flux is $9 \times 10^{-13}$ erg cm$^{-2}$s$^{-1}$. The X-ray properties and lack of an obvious optical counterpart are consistent with a Be star companion at a distance of between approximately 6 and 12 kpc which implies a luminosity of (4-15)$\times 10^{33}$ erg s$^{-1}$.

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arXiv1999

BeppoSAX spectroscopy of the Hercules X-1 short-on state

We present results of a 5.7 day duration BeppoSAX observation of the short-on state of Her X-1 and a short observation during the decline of the preceding main-on state. The 0.1-10 keV spectra can be fit with a power-law and blackbody model together with Fe emission features at 1.0 keV and 6.5 keV. During the later stages of the short-on state there are long intervals when the absorption is $\approxgt$$5 \times 10^{22}$ atom cm$^{-2}$. These intervals become longer and occur ~0.3 day earlier in each orbital cycle as the short-on state progresses. During the intervals of high absorption the 0.1 keV blackbody is still clearly detected. This may indicate the presence of separate scattered and absorbed spectral components, although other explanations such as partial covering or a partially ionized absorber cannot be excluded. During the rest of the short-on state the ratio of flux in the blackbody compared to the power-law is consistent with that in the main-on state. This supports the view that much of the 35 day modulation is caused by an energy independent process, such as electron scattering. The discovery of strong absorption late in the short-on state is consistent with the predictions of the warped disk model (Petterson 1977, see also Schandl & Meyer 1994) where the end of the short-on state is caused by the accretion disk moving into the line of sight to the neutron star. The pulse phase difference between the blackbody and the power-law maxima is 250 +/- 20 degrees in both observations (separated by 0.43 of a 35 day cycle). This constant phase difference is consistent with the blackbody originating at the inner edge of a precessing accretion disk.

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arXiv1999

BeppoSAX Observations of the Atoll X-Ray Binary 4U0614+091

We report the first simultaneous measurement of the broad band X-ray (0.3-150 keV) spectrum of the neutron star x-ray binary 4U0614+091. Our data confirm the presence of a hard x-ray tail that can be modeled as thermal Comptonization of low-energy photons on electrons having a very high temperature, greater than 220 keV, or as a non-thermal powerlaw. We detected a spectral feature that can be interpreted as reprocessing, via Compton reflection, of the direct emission by an optically-thick disk and found a correlation between the photon index of the power-law tail and the fraction of radiation reflected which is similar to the correlation found for black hole candidate x-ray binaries and Seyfert galaxies.

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arXiv1999

X-ray observations during a Her X-1 anomalous low-state

Results of a 1999 July 8-10 BeppoSAX observation during an anomalous low-state of Her X-1 are presented. The standard on-state power-law and blackbody continuum model is excluded at high confidence unless partial covering is included. This gives a power-law photon index of 0.63 +/- 0.02 and implies that 0.28 +/- 0.03 of the flux undergoes additional absorption of (27 +/- 7) 10^22 atom/cm2. 11% of the observed 0.1-10 keV flux is from the 0.068 +/- 0.015 keV blackbody. 1.237747(2) s pulses with a semi-amplitude of 2.1 +/- 0.8% are detected at >99.5% confidence and confirmed by RXTE measurements. This implies that Her X-1 underwent substantial spin-down close to the start of the anomalous low-state. The spectral and temporal changes are similar to those recently reported from 4U1626-67. These may result from a strongly warped disk that produces a spin-down torque. The X-ray source is then mostly viewed through the inner regions of the accretion disk. A similar mechanism could be responsible for the Her X-1 anomalous low-states. Shadowing by such an unusually warped disk could produce observable effects in the optical and UV emission from the companion star.

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arXiv1999

The broad-band spectrum of OAO1657-415 with BeppoSAX: In search of cyclotron lines

We report on a 30 ks observation of the high-mass X-ray binary pulsar OAO1657-415 performed by BeppoSAX on September 1998. The wide band spectrum is well fit by both a cutoff power law, or a power law modified by a high energy cutoff, plus a fluorescence Iron line at 6.5 keV. The two models are statistically equivalent. The inclusion of a cyclotron resonance feature at ~36 keV -- corresponding to a magnetic field strength 3.2 (1+z) x 10^12 G, where z is the gravitational redshift -- improves significantly the X^2 for the cutoff model but only marginally for the power law plus high energy cutoff model. The statistical significance of our data is not adequate to discriminate between the two models, and even the "Normalized Crab Ratio" technique, successfully used to pinpoint cyclotron features in the spectra of other X-ray pulsars, is not conclusive in answering the question whether the feature is real or it is an artifact due to an improper modeling of the continuum used to fit the data.

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arXiv1999

The broad band spectral properties of binary X-ray pulsars

The X-ray telescopes on board BeppoSAX are an optimal set of instruments to observe bright galactic binary pulsars. These sources emit very hard and quite complex X-ray spectra that can be accurately measured with BeppoSAX between 0.1 and 200 keV. A prototype of this complexity, the source Her X-1, shows at least seven different components in its spectrum. A broad band measure is therefore of paramount importance to have a thorough insight into the physics of the emitting region. Moreover the detection of cyclotron features, when present, allows a direct and highly significant measure of the magnetic field intensity in the emission region. In this paper we briefly report the results obtained with BeppoSAX on this class of sources, with emphasis on the detection and on the measured properties of the cyclotron lines.

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arXiv1999

Strong Field Gravity and X-Ray Observations of 4U1820-30

The behavior of quasi-periodic oscillations (QPOs) at frequencies near 1 kHz in the x-ray emission from the neutron star x-ray binary 4U1820-30 has been interpreted as evidence for the existence of the marginally stable orbit, a key prediction of strong-field general relativity. The signature of the marginally stable orbit is a saturation in QPO frequency, assumed to track inner disk radius, versus mass accretion rate. Previous studies of 4U1820-30 have used x-ray count rate as an indicator of mass accretion rate. However, x-ray count rate is known to not correlate robustly with mass accretion rate or QPO frequency in other sources. Here, we examine the QPO frequency dependence on two other indicators of mass accretion rate: energy flux and x-ray spectral shape. Using either of these indicators, we find that the QPO frequency saturates at high mass accretion rates. We interpret this as strong evidence for the existence of the marginally stable orbit.

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arXiv1999

Hard X-Ray Radiation in the Coma Cluster Spectrum

Hard X-ray radiation has been detected for the first time in the Coma cluster by BeppoSAX. Thanks to the unprecedented sensitivity of the Phoswich Detection System (PDS) instrument, the source has been detected up to ~80 keV. There is clear evidence (4.5 sigma) for non-thermal emission in excess of thermal above ~25 keV. The hard excess is very unlikely due to X Comae, the Seyfert 1 galaxy present in the field of view of the PDS. A hard spectral tail due to inverse Compton on CMB photons is predicted in clusters, like Coma, with radio halos. Combining the present results with radio observations, a volume-averaged intracluster magnetic field of ~0.15 micro G is derived, while the electron energy density of the emitting electrons is ~7x10**-14 erg/cm**3.

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arXiv1998

BeppoSAX observations of an orbital cycle of the X-ray binary pulsar GX301-2

We present preliminary results on our campaign of observations of the X-ray binary pulsar GX301-2. BeppoSAX observed this source six times in January/February 1998: at the periastron and apoastron, and at other four, intermediate, orbital phases. We present preliminary results on the GX301-2 spectral and temporal behaviour as a function of orbital phase.

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arXiv1998

The extraordinary X-ray spectrum of XTE J0421+560

We report results of two BeppoSAX observations of the transient X-ray source XTE J0421+560 during the outburst that started in March 1998. The source exhibits radio jets and coincides with the binary system CI Cam. The 0.1--50 keV spectrum is unlike those of other X-ray transients, and cannot be fit with any simple model. The spectra can be represented by an absorbed two component bremsstrahlung model with narrow Gaussian emission features identified with O, Ne/Fe-L, Si, S, Ca and Fe-K. During the second observation (TOO2) the energies of the O and the Ne/Fe-L features decreased smoothly by ~9% over an interval of 30 hrs. No significant energy shift of the other lines is detected with e.g, a 90% confidence upper limit to any Fe-K line shift of 3.5%. No moving lines were detected during the first observation (TOO1) with e.g., an upper limit of <1.4% to any shift of the Fe-K line. The low-energy absorption decreased by a factor >=1.8 from ~6 10E21 atom cm/2 between TOO1 and TOO2. We propose that the time variable emission lines arise in precessing relativistic jets, while the stationary lines originate in circumstellar material.

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arXiv1998

The 1998 outburst of the X-ray transient 4U 1630-47 observed with BeppoSAX

We report results of five pointed BeppoSAX observations of the black hole candidate 4U 1630-47 during part of its 1998 outburst when the 2-10 keV source luminosity decreased from 2.1x10^38 erg/s (for an assumed distance of 10 kpc) by a factor of ~7. The 2-200 keV spectrum can be modeled by a soft multi-temperature disk blackbody and a hard power-law. During four of the observations, there is evidence for deviations from a pure power-law shape >~20 keV which may be due to reflection. As the outburst decayed a number of spectral trends are evident: (1) the amount of photoelectric absorption decreased, (2) the spectrum became harder (i.e. the power-law index became smaller), (3) the temperature of the disk blackbody decreased and (4) the inferred inner disk radius increased by a factor of ~4. The change in the accretion disk inner radius during the outburst is in contrast to results from other soft X-ray transients. We find that the disk blackbody temperature depends on the inner disk radius roughly as T=r^(-3/4). For a standard Shakura-Sunyaev disk model this implies that the inferred mass accretion rate at the inner disk radius did not change appreciably during the part of the outburst observed by BeppoSAX.

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arXiv1998

BeppoSAX Observations of Bright Seyfert 2 Galaxies: Measuring the Intrinsic Continuum Emission

We report broad band (0.1-200 keV) X-ray observations, made by BeppoSAX, of a sample of bright Seyfert 2 galaxies: NGC7172, NGC2110, NGC4507, Mkn 3 and NGC7674. These spectra provide a better understanding of the effects of X-ray reprocessing by cold material in the source and allow to put tighter constraints on the various spectral parameters involved. In particular, the data are used to determine, with less ambiguities than in the past, the shape of the intrinsic continuum emission by means of the high energy data. Within the small sample both Compton thin and Compton thick sources are found according to the expectations of the unified theory.

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