arXiv subjects

N. R. Robba

Publications and source records attributed to N. R. Robba.

At least 37 records · Page 2Linked to original sources

arXiv2007

Spectral analysis of LMC X-2 with XMM/Newton: unveiling the emission process in the extragalactic Z-source

We present the results of the analysis of an archival observation of LMC X-2 performed with XMM/Newton. The spectra taken by high-precision instruments have never been analyzed before. We find an X-ray position for the source that is inconsistent with the one obtained by ROSAT, but in agreement with the Einstein position and that of the optical counterpart. The correlated spectral and timing behaviour of the source suggests that the source is probably in the normal branch of its X-ray color-color diagram. The spectrum of the source can be fitted with a blackbody with a temperature 1.5 keV plus a disk blackbody at 0.8 keV. Photoelectric absorption from neutral matter has an equivalent hydrogen column of 4 x 10^{20} cm^{-2}. An emission line, which we identify as the O VIII Lyman alpha line, is detected, while no feature due to iron is detected in the spectrum. We argue that the emission of this source can be straightforwardly interpreted as a sum of the emission from a boundary layer between the NS and the disc and a blackbody component coming from the disc itself. Other canonical models that are used to fit Z-sources do not give a satisfactory fit to the data. The detection of the O VIII emission line (and the lack of detection of lines in the iron region) can be due to the fact that the source lies in the Large Magellanic Cloud.

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arXiv2007

A complex environment around Cir X-1

We present the results of an archival 54 ks long Chandra observation of the peculiar source Cir X--1 during the phase passage 0.223-0.261. A comparative analysis of X-ray spectra, selected at different flux levels of the source, allows us to distinguish between a very hard state, at a low countrate, and a brighter, softer, highly absorbed spectrum during episodes of flaring activity, when the unabsorbed source luminosity is about three times the value in the hard state. The spectrum of the hard state clearly shows emission lines of highly ionized elements, while, during the flaring state, the spectrum also shows strong resonant absorption lines. The most intense and interesting feature in this latter state is present in the Fe K alpha region: a very broadened absorption line at energies ~ 6.5 keV that could result from a smeared blending of resonant absorption lines of moderately ionized iron ions (Fe XX - Fe XXIV). We also observe strong resonant absorption lines of Fe XXV and Fe XXVI, together with a smeared absorption edge above 7 keV. We argue that the emitting region during the quiescent/hard state is constituted of a purely photo-ionized medium, possibly present above an accretion disk, or of a photo-ionized plasma present in a beamed outflow. During the flaring states the source undergoes enhanced turbulent accretion that modifies both the accretion geometry and the optical depth of the gas surrounding the primary X-ray source.

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arXiv2007

Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram

We analyze a large collection of RXTE archive data from April 1997 to August 2003 of the bright X-ray source Scorpius X-1 in order to study the broadband spectral evolution of the source for different values of the inferred mass accretion rate by studying energy spectra from selected regions in the Z-track of its Color-Color Diagram. A two-component model, consisting of a soft thermal component interpreted as thermal emission from an accretion disk and a thermal Comptonization component, is unable to fit the whole 3--200 keV energy spectrum at low accretion rates. Strong residuals in the highest energy band of the spectrum require the addition of a third component that can be fitted with a power-law component, that could represent a second thermal Comptonization from a much hotter plasma, or a hybrid thermal/non-thermal Comptonization. We discuss the physical implications derived from the results of our analysis, with a particular emphasis on the hardest part of the X-ray emission and its possible origins.

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arXiv2007

Chandra Observation of the Big Dipper X 1624-490

We present the results of a 73 ks long Chandra observation of the dipping source X 1624-490. During the observation a complex dip lasting 4 hours is observed. We analyse the persistent emission detecting, for the first time in the 1st-order spectra of X 1624-490, an absorption line associated to \ion{Ca}{xx}. We confirm the presence of the \ion{Fe}{xxv} K$_α$ and \ion{Fe}{xxvi} K$_α$ absorption lines with a larger accuracy with respect to a previous XMM observation. Assuming that the line widths are due to a bulk motion or a turbulence associated to the coronal activity, we estimate that the lines have been produced in a photoionized absorber between the coronal radius and the outer edge of the accretion disk.

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arXiv2006

Chandra Observation of the Dipping Source XB 1254-690

We present the results of a 53 ks long Chandra observation of the dipping source XB 1254--690. During the observation neither bursts or dips were observed. From the zero-order image we estimated the precise X-ray coordinates of the source with a 90% uncertainty of 0.6\arcsec. Since the lightcurve did not show any significant variability, we extracted the spectrum corresponding to the whole observation. We confirmed the presence of the \ion{Fe}{xxvi} K$_α$ absorption lines with a larger accuracy with respect to the previous XMM EPIC pn observation. Assuming that the line width were due to a bulk motion or a turbulence associated to the coronal activity, we estimate that the lines were produced in a photoionized absorber between the coronal radius and the outer edge of the accretion disk.

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arXiv2006

Constraints to the EOS of ultradense matter with model-independent astrophysical observations

The recent discovery of burst oscillations at 1122 Hz in the x-ray transient XTE J1739-285, together with the measurement of the mass of the binary millisecond pulsar PSR J0751+1807 (2.1 +- 0.2 solar masses) can finally allow us to put strong, model-independent observational constraints to the equation of state of compact stars. We show that the measurement of the moment of inertia of PSR J0737+3039A, together with these constraints, could allow to discriminate further the details of the inner structure of neutron stars. Moreover, we show that if XTE J1739-285 is constituted of nucleonic matter, any equation of state allows only a narrow range of very high masses, and this could explain why up to now compact stars spinning faster than a millisecond have been so difficult to detect.

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arXiv2006

A Hard X-ray View on Scorpius X-1 with INTEGRAL: non-Thermal Emission ?

We present here simultaneous INTEGRAL/RXTE observations of Sco X-1, and in particular a study of the hard X-ray emission of the source and its correlation with the position in the Z-track of the X-ray color-color diagram. We find that the hard X-ray (above about 30 keV) emission of Sco X-1 is dominated by a power-law component with a photon index of ~3. The flux in the power-law component slightly decreases when the source moves in the color-color diagram in the sense of increasing inferred mass accretion rate from the horizontal branch to the normal branch/flaring branch vertex. It becomes not significantly detectable in the flaring branch, where its flux has decreased by about an order of magnitude. These results present close analogies to the behavior of GX 17+2, one of so-called Sco-like Z sources. Finally, the hard power law in the spectrum of Sco X-1 does not show any evidence of a high energy cutoff up to 100 - 200 keV, strongly suggesting a non-thermal origin of this component.

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arXiv2006

Chandra Observation of the Persistent Emission from the Dipping Source XB 1916-053

We present the results of a 50 ks long Chandra observation of the dipping source XB 1916-053. During the observation two X-ray bursts occurred and the dips were not present at each orbital period. From the zero-order image we estimate the precise X-ray coordinates of the source with a 90% uncertainty of 0.6''. In this work we focus on the spectral study of discrete absorption features, during the persistent emission, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. We detect, for the first time in the 1st-order spectra of XB 1916-053, absorption lines associated to Ne X, Mg XII, Si XIV, and S XVI, and confirm the presence of the Fe XXV and Fe XXVI absorption lines with a larger accuracy with respect to the previous XMM EPIC pn observation. Assuming that the line widths are due to a bulk motion or a turbulence associated to the coronal activity, we estimate that the lines are produced in a photoionized absorber distant from the neutron star 4 x 10^{10} cm, near the disk edge.

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arXiv2005

The Iron K-shell features of MXB 1728-34 from a Simultaneous Chandra-RXTE Observation

We report on a simultaneous Chandra and RossiXTE observation of the low-mass X-ray binary atoll bursting source MXB 1728-34 performed on 2002 March 3-5. We fit the 1.2-35 keV continuum spectrum with a blackbody plus a Comptonized component. Large residuals at 6-10 keV can be fitted by a broad (FWHM ~ 2 keV) Gaussian emission line or, alternatively, by two absorption edges associated with lowly ionized iron and Fe XXV/XXVI at ~7.1 keV and ~9 keV, respectively. In this interpretation, we find no evidence of broad, or narrow, emission lines between 6 and 7 keV. We test our alternative modelling of the iron K shell region by reanalysing a previous BeppoSAX observation of MXB 1728-34, finding a general agreement with our new spectral model.

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arXiv2005

Resolving the Fe xxv Triplet with Chandra in Cen X-3

We present the results of a 45 ks Chandra observation of the high-mass X-ray binary Cen X--3 at orbital phases between 0.13 and 0.40 (in the eclipse post-egress phases). Here we concentrate on the study of discrete features in the energy spectrum at energies between 6 and 7 keV, i.e. on the iron K$_α$ line region, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. We clearly see a K$_α$ neutral iron line at $\sim 6.40$ keV and were able to distinguish the three lines of the \ion{Fe}{25} triplet at 6.61 keV, 6.67 keV, and 6.72 keV, with an equivalent width of 6 eV, 9 eV, and 5 eV, respectively. The equivalent width of the K$_α$ neutral iron line is 13 eV, an order of magnitude lower than previous measures. We discuss the possibility that the small equivalent width is due to a decrease of the solid angle subtended by the reflector.

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arXiv2005

The broad-band spectrum of Cyg X-2 with INTEGRAL

We study the broad band (3-100 keV) spectrum of Cygnus X-2 with INTEGRAL. We find that the spectrum is well fitted by a Comptonized component with a seed-photons temperature of ~1 keV, an electron temperature of ~3 keV and an optical depth tau ~ 8. Assuming spherical geometry, the radius of the seed-photons emitting region is ~17 km. The source shows no hard X-ray emission; it was detected only at a 3 sigma level above 40 keV. We also analyzed public ISGRI data of Cyg X--2 to investigate the presence of a hard X-ray component. We report the possible presence of hard X-ray emission in one data set.

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arXiv2005

Timing an Accreting Millisecond Pulsar: Measuring the Accretion Torque in IGR J00291+5934

We present here a timing analysis of the fastest accreting millisecond pulsar IGR J00291+5934 using RXTE data taken during the outburst of December 2004. We corrected the arrival times of all the events for the orbital (Doppler) effects and performed a timing analysis of the resulting phase delays. In this way we find a clear parabolic trend of the pulse phase delays showing that the pulsar is spinning up as a consequence of accretion torques during the X-ray outburst. The accretion torque gives us for the first time an independent estimate of the mass accretion rate onto the neutron star, which can be compared with the observed X-ray luminosity. We also report a revised value of the spin period of the pulsar.

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arXiv2005

Revised orbital parameters of the accreting millisecond pulsar SAX J1808.4-3658

We present temporal analysis of the three outbursts of the X-ray millisecond pulsar SAX J1808.4-3658 that occurred in 1998, 2000 and 2002. With a technique that uses the chi^2 obtained with an epoch folding search to discriminate between different possible orbital solutions, we find an unique solution valid over the whole five years period for which high temporal resolution data are available. We revise the estimate of the orbital period, P_orb =7249.1569(1) s and reduce the corresponding error by one order of magnitude with respect to that previously reported. Moreover we report the first constraint on the orbital period derivative, -6.6 x 10^-12 < Pdot < +0.8 x 10^-12 s/s. These values allow us to produce, via a folding technique, pulse profiles at any given time. Analysis of these profiles shows that the pulse shape is clearly asymmetric in 2002, occasionally showing a secondary peak at about 145 degrees from the main pulse, which is quite different from the almost sinusoidal shape reported at the beginning of the 1998 outburst.

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arXiv2005

A broad iron line in the Chandra/HETG spectrum of 4U 1705-44

We present the results of a Chandra 30 ks observation of the low mass X-ray binary and atoll source 4U 1705-44. Here we concentrate on the study of discrete features in the energy spectrum at energies below 3 keV, as well as on the iron Kalpha line, using the HETG spectrometer on board of the Chandra satellite. Below 3 keV, three narrow emission lines are found at 1.47, 2.0, and 2.6 keV. The 1.47 and 2.6 keV are probably identified with Ly-alpha emission from Mg XII and S XVI, respectively. The identification of the feature at 2.0 keV is uncertain due to the presence of an instrumental feature at the same energy. The iron Kalpha line at ~6.5 keV is found to be intrinsically broad (FWHM ~ 1.2 keV); its width can be explained by reflection from a cold accretion disk extending down to 15 km from the neutron star center or by Compton broadening in the external parts of a hot (~2 keV) Comptonizing corona. We finally report here precise X-ray coordinates of the source.

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arXiv2005

High-Energy pulse profile of the Transient X-ray Pulsar SAX J2103.5+4545

In two recent INTEGRAL papers, Lutovinov et al. (2003) and Blay et al. (2004) report a timing and spectral analysis of the transient Be/X-ray pulsar SAX J2103.5+4545 at high energies (5--200 keV). In this work we present for the first time a study of the pulse profile at energies above 20 keV using INTEGRAL data. The spin-pulse profile shows a prominent (with a duty cycle of 14%) and broad (with a FWHM of ~ 51 s) peak and a secondary peak which becomes more evident above 20 keV. The pulsed fraction increases with energy from ~ 45% at 5--40 keV to ~ 80% at 40--80 keV. The morphology of the pulse profile also changes as a function of energy, consistent with variations in the spectral components that are visible in the pulse phase resolved spectra. A study of the double peaked profile shows that the difference in the two peaks can be modeled by a different scattering fraction between the radiation from the two magnetic poles.

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arXiv2004

High Resolution Spectroscopy of 4U 1728-34 from a Simultaneous Chandra-RXTE Observation

We report on a simultaneous Chandra and RossiXTE observation of the LMXB atoll bursting source 4U 1728-34 performed on 2002 March 3-5. We fitted the 1.2-35 keV continuum spectrum with a blackbody plus a Comptonized component. An overabundance of Si by a factor of ~2 with respect to Solar abundance is required for a satisfactory fit. Large residuals at 6-10 keV can be fitted by a broad (FWHM ~ 1.6 keV) Gaussian emission line, or, alternatively, by absorption edges associated with Fe I and Fe XXV at ~7.1 keV and ~9 keV, respectively. In this interpretation, we find no evidence of a broad, or narrow Fe Kalpha line, between 6 and 7 keV. We tested our alternative modeling of the iron Kalpha region by reanalyzing a previous BeppoSAX observation of 4U 1728-34, finding a general agreement with our new spectral model.

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arXiv2004

On the Soft Excess in the X-ray Spectrum of Cir X-1: Revisitation of the Distance to Cir X-1

We report on a 300 ks BeppoSAX (0.12-200 keV) observation of Circinus X-1 (Cir X-1) at phases between 0.62 and 0.84 and on a 90 ks BeppoSAX observation of Cir X--1 at phases 0.11-0.16. Using the canonical model adopted until now to fit the energy spectrum of this source large residuals appear below 1 keV. These are well fitted using an equivalent hydrogen column of $0.66 \times 10^{22}$ cm$^{-2}$, adding absorption edges of O VII, O VIII and Ne IX in the spectra extracted from the observation at phases 0.62-0.84 and adding absorption edges of O VII, O VIII, Mg XI and Mg XII and absorption lines of O VIII and Mg XII in the spectra extracted from the observation at phases 0.11-0.16. During the observation at phases 0.62--0.84 the electron density associated to the ionized matter is $\sim 10^{13}$ cm$^{-3}$ remaining quit constant going away from the compact object. During the observation at phases 0.11-0.16 the electron density profile varies along the distance going from $\sim 6 \times 10^{13}$ cm$^{-3}$ at $\sim 10^{11}$ cm to $\sim 9 \times 10^{10}$ cm$^{-3}$ at $\sim 10^{13}$ cm. The equivalent hydrogen column towards Cir X-1 is thre times lower than the value obtained from previous models. This low value would imply that Cir X-1 is at a distance of 4.1 kpc.

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arXiv2004

To accrete or not to accrete: the dilemma of the recycling scenario

We study the evolution of a low-mass X-ray binary by coupling a binary stellar evolution code with a general relativistic code that describes the behaviour of the neutron star. We find that non-conservative mass transfer scenarios are required to prevent the formation of submillisecond pulsars and/or the collapse to a black hole. We discuss the sweeping effects of an active magneto-dipole rotator on the transferred matter as a promising mechanism to obtain highly non-conservative evolutions.

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