24 research outputs found
The Puzzles of RX J1856.5-3754: Neutron Star or Quark Star?
We discuss recent Chandra and XMM-Newton observations of the bright isolated
neutron star RX J1856.5-3754 and suggest that the absence of any line features
is due to effects of a high magnetic field strength (~10^13 G). Using different
models for the temperature distribution across the neutron star surface
assuming blackbody emission to fit the optical and X-ray spectrum and we derive
a conservative lower limit of the "apparent" neutron star radius of 16.5 km x
(d/117 pc). This corresponds to the radius for the "true" (de-redshifted)
radius of 14 km for a 1.4 Msun neutron star, indicating a stiff equation of
state at high densities. A comparison of the result with mass-radius diagrams
shows that quark stars and neutron stars with quark matter cores can be ruled
out with high confidence.Comment: 6 page, 2 figures, "The Restless High-Energy Universe" Proceedings of
the symposium dedicated to six years of successful BeppoSAX operations
Amsterdam, May 5-8, 200
The contribution of Narrow-Line Seyfert 1 galaxies to the soft X-ray background
The ROSAT Ultradeep HRI survey in the Lockman Hole contains a complete sample
of 91 X-ray sources with fluxes in the 0.5-2 keV band larger than 1.2 times
10e-15 erg cm-2 s-1, where over about 75 per cent of the sources are quasars or
Seyfert galaxies. During the course of our optical identification work, we have
obtained optical spectra of 67 narrow emission line galaxies (NELG), which are
physically not associated with the X-ray sources. We have derived the
equivalent width (EW) and the full width at half maximum (FWHM) for the most
prominent emission lines of 41 quasars and Seyfert galaxies taken from the
ROSAT Deep Survey (RDS), which has a flux limit of 5.5 times 10e-15 erg cm-2
s-1 in the 0.5-2.0 keV band. Furthermore we have obtained the EW and FWHM
values of the field NELGs. Here we present the spectroscopic discrimination
between RDS Seyfert galaxies and field galaxies (NELG). The analysis of the
emission lines has revealed that a single object out of 69 spectroscopically
identified AGN fits the optical criteria of Narrow-Line Seyfert 1 galaxies
(NLS1). This may indicate that NLS1 contribute only marginally to the soft
X-ray background, but we can not exclude a possible larger contribution.Comment: Invited talk presented at the Joint MPE,AIP,ESO workshop on NLS1s,
Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at
http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
The Magnificent Seven: Magnetic fields and surface temperature distributions
Presently seven nearby radio-quiet isolated neutron stars discovered in ROSAT
data and characterized by thermal X-ray spectra are known. They exhibit very
similar properties and despite intensive searches their number remained
constant since 2001 which led to their name ``The Magnificent Seven''. Five of
the stars exhibit pulsations in their X-ray flux with periods in the range of
3.4 s to 11.4 s. XMM-Newton observations revealed broad absorption lines in the
X-ray spectra which are interpreted as cyclotron resonance absorption lines by
protons or heavy ions and / or atomic transitions shifted to X-ray energies by
strong magnetic fields of the order of 10^13 G. New XMM-Newton observations
indicate more complex X-ray spectra with multiple absorption lines. Pulse-phase
spectroscopy of the best studied pulsars RX J0720.4-3125 and RBS 1223 reveals
variations in derived emission temperature and absorption line depth with pulse
phase. Moreover, RX J0720.4-3125 shows long-term spectral changes which are
interpreted as due to free precession of the neutron star. Modeling of the
pulse profiles of RX J0720.4-3125 and RBS 1223 provides information about the
surface temperature distribution of the neutron stars indicating hot polar caps
which have different temperatures, different sizes and are probably not located
in antipodal positions.Comment: 10 pages, 8 figures, to appear in Astrophysics and Space Science, in
the proceedings of "Isolated Neutron Stars: from the Interior to the
Surface", edited by D. Page, R. Turolla and S. Zan
RX J1856.5-3754 as a possible Strange Star candidate
RX J1856.5-3754 has been proposed as a strange star candidate due to its very
small apparent radius measured from its X-ray thermal spectrum. However, its
optical emission requires a much larger radius and thus most of the stellar
surface must be cold and undetectable in X-rays. In the case the star is a
neutron star such a surface temperature distribution can be explained by the
presence of a strong toroidal field in the crust (Perez-Azorin et al. 2006,
Geppert et al. 2006). We consider a similar scenario for a strange star with a
thin baryonic crust to determine if such a magnetic field induced effect is
still possible.Comment: 4 pages, 4 figures; to appear in proceedings of the conference
"Isolated Neutron Stars: From the Interior to the Surface", eds. S. Zane, R.
Turolla, D. Page; Astrophysics & Space Science in pres
Quark matter imprint on Gravitational Waves from oscillating stars
We discuss the possibility that the detection of gravitational waves emitted
by compact stars may allow to constrain the MIT bag model of quark matter
equation of state. Our results show that the combined knowledge of the
frequency of the emitted gravitational wave and of the mass, or the radiation
radius, of the source allows one to discriminate between strange stars and
neutron stars and set stringent bounds on the bag constants.Comment: Eight pages, four figures. Revised version, to appear on General
Relativity and Gravitatio