arXiv subjects

Roberto Soria

Publications and source records attributed to Roberto Soria.

174 records · Page 10Linked to original sources

arXiv2002

X-ray sources in the starburst spiral galaxy M83: nuclear region and discrete source population

Chandra has resolved the starburst nuclear region of the face-on grand-design spiral M83. Eighty-one point sources are detected above 3.5 sigma in the ACIS S3 image, and 15 of them are within the inner 16-arcsec region of the galaxy. A point source with L_x ~ 3 x 10^(38) erg/s in the 0.3--8.0 keV band is found to coincide with the infra-red nuclear photometric peak, one of the two dynamical nuclei of the galaxy. No point-like sources are resolved (at a 2.5-sigma level) at the centre of symmetry of the outer optical isophote ellipses, suspected to be another dynamical nucleus. About 50% of the total emission in the nuclear region is unresolved; of this, about 70% can be attributed to hot thermal plasma, and the rest is probably due to unresolved point sources (eg, faint X-ray binaries). The azimuthally-averaged radial distribution of the unresolved emission has a King-like profile, with no central cusp. Strong emission lines are seen in the spectrum of the optically thin plasma component. The high abundances of C, Ne, Mg, Si and S with respect to Fe suggest that the interstellar medium in the nucleus is enriched and heated by type-II supernova explosions and winds from massive stars. The cumulative luminosity distribution of the discrete X-ray sources is neither a single nor a broken power law. Separating the sources in the nuclear region (within a distance of 60 arcsec from the X-ray centre) from the rest reveals that the two groups have different luminosity distributions. The log[N(>S)]--log S curve of the sources in the inner region (nucleus and stellar bar) is a single power law, which we interpret as due to continuous, ongoing star formation. Outside the central region, there is a smaller fraction of sources brighter than the Eddington limit for an accreting neutron star.

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arXiv2001

Low and High Angular Momentum Accretion Flows in BHCs: Case Study of XTE J1550-564

The 1998 outburst of XTE J1550-564 started with a hard X-ray spike, rising in less than a day and declining after 3-4 days; at the same time, the soft X-ray flux was rising with a longer timescale (~ 10 days). We suggest that the soft and the initial hard X-ray emission are produced by two different components of the accretion flow: a higher angular momentum flow, which forms the disk, and a lower angular momentum flow feeding the hot inner region. Thus, we argue that the onset of the outburst is determined by an increased mass transfer rate from the companion star, but the outburst morphology is also determined by the distribution of specific angular momentum in the accreting matter. In XTE J1550-564, we attribute the initial, impulsive low angular momentum accretion to the breaking down of magnetic confinement by the magnetically active secondary star. We show that a hard X-ray spike is seen at the onset of an outburst in other BHCs.

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arXiv2001

Formation of optical lines in black-hole binaries

The HI Balmer emission lines of black-hole binaries show double-peaked profiles during the high-soft state and the quiescent state. In the high-soft state the profiles are asymmetric with a stronger red peak, but the profiles are symmetric in the quiescent state. We suggest that in the high-soft state the emission lines originate from the temperature-inversion layer caused by irradiative heating of an optically thick accretion disk. Irradiative heating also causes the formation of a disk wind, which mildly absorbs the blue peak of the lines. The double-peaked lines seen in the quiescent state arise from an optically thin disk. In the absence of a disk wind, the lines are unabsorbed and so the symmetry of the line profiles is preserved.

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arXiv2001

The optical emission lines as a probe of state transitions in black-hole candidates

Optical spectroscopic studies of emission lines in black-hole candidates can help us investigate state transitions in those systems. Changes in the optical line profiles reflect changes in the geometry of the accretion flow, usually associated with X-ray state transitions in the inner region. We identify at least four optical states in the black-hole candidate GRO J1655-40 in outburst, and two optical states in GX339-4.

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arXiv2000

XMM-Newton EPIC and RGS observations of LMC X-3

We report the results of preliminary analysis of the XMM_Newton EPIC and RGS observations of the candidate black-hole binary LMC X-3 between February and June 2000. The observations covered both the soft and the hard X-ray spectral states. The hard-state spectra were dominated by a power-law component with a photon index Gamma = 1.9 +/- 0.1. The soft-state spectra consisted of a thermal component with a multi-colour disk temperature T_in = 0.9 keV and a power-law tail with Gamma ~ 2.5--2.7. The model in which the X-rays from LMC X-3 in the high-soft state are powered by a strong stellar wind from a massive companion is not supported by the small line-of-sight absorption (n_H <~ 10^{21} cm^{-2}) deduced from the RGS data. The transition from the soft to the hard state appears to be a continuous process associated with the changes in the mass-transfer rate.

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arXiv2000

XMM-Newton Optical Monitor observations of LMC X-3

We study the optical counterpart of the black-hole X-ray binary LMC X-3, by using XMM-Newton/OM observations carried out during a low-hard X-ray state. We derive a better constraint for the temperature, mass and radius of the companion star, and we show that the star is likely to be a ~ B5 subgiant filling its Roche lobe. Taking into account the effect of X-ray irradiation, we suggest a value f_M = (1.5 +/- 0.3) M_sun for the mass function in this system, lower than previously thought; we provide a more accurate lower limit to the mass of the compact object.

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arXiv2000

Optical Spectroscopy of GX339-4 during the High-Soft and Low-Hard States II: Line Ionisation and Emission Region

We have carried out observations of the X-ray transient GX339-4 during its high-soft and low-hard X-ray spectral states. Our high-resolution spectroscopic observation in 1999 April suggests that the H-alpha line has a single-peaked profile in the low-hard state as speculated in our previous paper. The HeII 4686 line, however, has a double-peaked profile in both the high-soft and low-hard states. This suggests that the line-emission mechanism is different in the two states. Our interpretation is that double-peaked lines are emitted from a temperature-inversion layer on the accretion-disk surface when it is irradiatively heated by soft X-rays. Single-peaked lines may be emitted from outflow/wind matter driven by hard X-ray heating. We have constructed a simple plane-parallel model and we use it to illustrate that a temperature-inversion layer can be formed at the disk surface under X-ray illumination. We also discuss the conditions required for the formation of temperature inversion and line emission. Based on the velocity separations measured for the double-peaked lines in the high-soft state, we propose that GX339-4 is a low-inclination binary system. The orbital inclination is about 15 deg if the orbital period is 14.8 hours.

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arXiv1999

Optical Spectroscopy of GRO J1655-40

We have obtained optical spectra of the soft X-ray transient GRO J1655-40 during different X-ray spectral states (quiescence, high-soft and hard outburst) between 1994 Aug and 1997 Jun. Characteristic features observed during the 1996-97 high-soft state were: a) broad absorption lines at Halpha and Hbeta, probably formed in the inner disk; b) double-peaked HeII 4686 emission lines, formed in a temperature-inversion layer on the disk surface, created by the soft X-ray irradiation; c) double-peaked Halpha emission, with a strength associated with the hard X-ray flux, suggesting that it was probably emitted from deeper layers than He II 4686. The observed rotational velocities of all the double-peaked lines suggest that the disk was extended slightly beyond its tidal radius. Three classes of lines were identified in the spectra taken in 1994 Aug-Sep, during a period of low X-ray activity between two strong X-ray flares: broad absorption, broad (flat-topped) emission and narrow emission. We have found that the narrow emission lines (single-peaked or double-peaked) cannot be explained by a thin-disk model. We propose that the system was in a transient state, in which the accretion disk might have had an extended optically thin cocoon and significant matter outflow. After the onset of a hard X-ray flare the disk signatures disappeared, and strong single-peaked Halpha and Paschen emission was detected, suggesting that the cocoon became opaque to optical radiation. High-ionisation lines disappeared or weakened. Two weeks after the end of the flare, the cocoon appeared to be once again optically thin.

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arXiv1999

Optical Spectroscopy of GX339-4 During the High-Soft and Low-Hard States (Paper I)

We carried out spectroscopic observations of the candidate black-hole binary GX339-4 during its low-hard and high-soft X-ray states, in 1997 and 1998 respectively. We have found that the spectrum is dominated by emission lines of neutral elements with asymmetric, round-topped profiles in the low-hard state (probably due to a dense matter outflow from an inflated outer disk). In the high-soft state, however, the emission lines from both neutral and ionized elements have unambiguously resolved double-peaked profiles; this indicates the presence of an irradiatively-heated thin accretion disk. Higher-ionization lines have larger peak-to-peak velocity separations. Our data do not show velocity modulations of the line centres due to the orbital motion of the compact object; there are no detectable lines from the companion star. These features are consistent with those of a system with a low-mass companion star and low orbital inclination. More results (from the 1999 low-hard state) coming soon in Paper II, stay tuned...

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arXiv1999

A Study of the New X-ray Transient RXTE J2123-058 during its Post-Outburst State

We carried out I, R, V and B photometric observations of the neutron star X-ray binary RXTE J2123-058 shortly after the end of the X-ray outburst in mid-1998. We adopt the low mass binary model to interpret our observations. After folding our data on the 0.24821-d orbital period, and correcting for the steady brightness decline following the outburst, we observe sinusoidal oscillations with hints of ellipsoidal modulations which became progressively more evident. Our data also show that the decline in brightness was faster in the V band than in the R and I bands. This suggests both the cooling of an irradiation-heated secondary star and the fading of an accretion disc over the nights of our observations.

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arXiv1998

The Optical Counterpart of the X-ray Transient RX J0117.6-7330: Spectroscopy and Photometry

We conducted spectroscopic and photometric observations of the optical counterpart of the X-ray transient RX J0117.6-7330 in the Small Magellanic Cloud, during a quiescent state. The primary star is identified as a B0.5 IIIe, with mass M = (18 +/- 2) M(sun) and bolometric magnitude M(bol) = -7.4 +/- 0.2. The main spectral features are strong H-alpha emission, H-beta and H-gamma emission cores with absorption wings, and narrow HeI and OII absorption lines. Equivalent width and FWHM of the main lines are listed. The average systemic velocity over our observing run is v(r) = (184 +/- 4) km/s; measurements over a longer period of time are needed to determine the binary period and the K velocity of the primary. We determine a projected rotational velocity v sin i = (145 +/- 10) km/s for the Be star, and we deduce that the inclination angle of the system is i = (21 +/- 3)deg.

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arXiv1998

Measuring the Motion of the Black Hole in GRO J16550-40

We report optical spectroscopic observations of the Low Mass X-ray Binary GRO J1655-40 during outbursts in August-September 1994 and in June 1996. The spectra show emission and absorption lines from the accretion disk surrounding the X-ray source, which have enabled us to measure the motion of the black-hole candidate in this system. It is the first time among black-hole candidates that an emission-line radial velocity curve is seen in phase with the expected motion of the primary. The projected radial velocity semi-amplitude determined for the primary [K(X) = 76.2 +/- 7.5 km/s], combined with estimates of the projected radial velocity semi-amplitude of the companion star from previous observations [K(C) = 228.2 +/- 3.0 km/s], yields a 95% confidence limit, derived purely from kinematics, of M(X) > 5.1 M(sun) for the mass of the compact object in GRO J1655-40, confirming its black hole nature.

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