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Post Info TOPIC: 4U 1820-30
Blobrana


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Title: XMM-Newton observation of 4U 1820-30: Broad band spectrum and the contribution of the cold interstellar medium
Authors: E. Costantini (1), C. Pinto (1), J. S. Kaastra (1 and 2), J. J. M. in't Zand (1), M. J. Freyberg (3), L. Kuiper (1), M. Mendez (4), C. P. de Vries (1), L. B. F. M. Waters (1 and 5) ((1) SRON, (2) Univ. of Utrecht, (3) MPE, (4) Univ. of Groningen, (5) UvA)

We present the analysis of the bright X-ray binary 4U 1820-30, based mainly on XMM-Newton-RGS data, but using complementary data from XMM-Epic, Integral, and Chandra-HETG, to investigate different aspects of the source. The broad band continuum is well fitted by a classical combination of black body and Comptonised emission. The continuum shape and the high flux of the source (L/L_Edd ~0.16) are consistent with a "high state" of the source. We do not find significant evidence of iron emission at energies >=6.4 keV. The soft X-ray spectrum contain a number of absorption features. Here we focus on the cold-mildly ionised gas. The neutral gas column density is N_H ~1.63x10^21 cm^-2. The detailed study of the oxygen and iron edge reveals that those elements are depleted, defined here as the ratio between dust and the total ISM cold phase, by a factor 0.20 ±0.02 and 0.87 ±0.14, respectively. Using the available dust models, the best fit points to a major contribution of Mg-rich silicates, with metallic iron inclusion. Although we find that a large fraction of Fe is in dust form, the fit shows that Fe-rich silicates are disfavoured. The measured Mg:Fe ratio is 2.0 ±0.3. Interestingly, this modelling may point to a well studied dust constituent (GEMS), sometimes proposed as a silicate constituent in our Galaxy. Oxygen and iron are found to be slightly over- and under-abundant, respectively (1.23 and 0.85 times the solar value) along this line of sight. We also report the detection of two absorption lines, tentatively identified as part of an outflow of mildly ionised gas (\xi ~ -0.5) at a velocity of ~1200 km/s.

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Blobrana


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Title: On the Dynamics and Tidal Dissipation Rate of the White Dwarf in 4U 1820-30
Authors: Snezana Prodan, Norman Murray

It has been suggested that the 170 day period in the light curve of the low mass X-ray binary 4U 1820-30 arises from the presence of a third body with a large inclination to the binary orbit. We show that this long period motion arises if the system is librating around the stable fixed point in a Kozai resonance. We demonstrate that mass transfer drives the system toward this fixed point, and calculate, both analytically and via numerical integrations, that the period of libration is of order 170 days when the mutual inclination is near the Kozai critical value. The non-zero eccentricity of the binary, combined with tidal dissipation, implies that the rate of change of the binary period would be slower than, or even of opposite sign to, that implied by standard mass transfer models. If the 170 day period results from libration, then, contrary to appearances, the orbital period of the inner binary is increasing with time; in that case, (e/0.009)^2Q/k_2 > 2.5 x 10^9, where k_2 = 0.01 is the tidal Love number and e = 0.009 is the fiducial eccentricity of the inner binary. It appears unlikely that the observed negative period derivative results from the smaller than expected (but positive) value of \dot P combined with the previously suggested acceleration of the system in the gravitational field of the host globular cluster NGC 6624. The discrepancy between the observed and expected period derivative requires further investigation.

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Blobrana


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Title: The Mass and Radius of the Neutron Star in 4U 1820-30
Authors: Tolga Guver, Patricia Wroblewski, Larry Camarota, Feryal Ozel
(Version v2)

We report on the measurement of the mass and radius of the neutron star in the low-mass X-ray binary 4U 1820-30. The analysis of the spectroscopic data on multiple thermonuclear bursts yields well-constrained values for the apparent emitting area and the Eddington flux, both of which depend in a distinct way on the mass and radius of the neutron star. The distance to the source is that of the globular cluster NGC 6624, where the source resides. Combining these measurements, we uniquely determine the probability density over the stellar mass and radius. We find the mass to be M = 1.58 ± 0.06 M_sun and the radius to be R = 9.11 ± 0.40 km.

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Blobrana


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Posts: 131433
Date:
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Title: The Mass and Radius of the Neutron Star in 4U 1820-30
Authors: Tolga Guver, Patricia Wroblewski, Larry Camarota, Feryal Ozel

We report on the measurement of the mass and radius of the neutron star in the low-mass X-ray binary 4U 1820-30. The analysis of the spectroscopic data on multiple thermonuclear bursts yields well-constrained values for the apparent emitting area and the Eddington flux, both of which depend in a distinct way on the mass and radius of the neutron star. The distance to the source is that of the globular cluster NGC 6624, where the source resides. Combining these measurements, we uniquely determine the probability density over the stellar mass and radius. We find the mass to be M = 1.58 ±0.06 M_sun and the radius to be R = 9.11 ±0.40 km.

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