Diapositiva 1

1

• Detecting Bare Strange Stars beyond The Milky Way
• E. Rubio1, D. Page1, V.V. Usov2
• Instituto de Astronomía, Universidad Nacional
  Autónoma de México México.
• Department of Condensed Matter Physics, Weizmann
  Institute of Science, Israel.

ABSTRACT We discuss the possibilities of
detecting bare strange stars using the observing
capabilities of the IBIS and SPI gamma ray
detectors on board INTEGRAL satellite. Our final
goal is make an observation proposal with this
satellite, in order to detect strange stars
beyond the Milky Way, increasing the
possibilities of detect this weird bodies. Our
conclusion here is that we have the opportunity,
of detect and observe very luminous (L gt 1039erg
s-1) sources beyond Milky Way, as far as the
Virgo Cluster for example.
INTRODUCTION Strange Stars, constituted of
deconfined quark matter bounded by color forces,
were proposed during the 80s (see 1, 2 and
3) as an alternative to neutron stars. Since
they are not bound by gravity, they can radiate
with luminosities exceeding the Eddington limit
(51038 erg s-1). A Bare Strange Star, in which
quark matter is present up to the stellar
surface, can produce super-Eddington luminosities
through pair production 4 and recent numerical
simulations of their thermal evolution 5 have
shown that such luminosities could be maintained
for very long periods of time (see Fig. 1 2).
Table 1. Luminosities L, mean photon energies
lte?gt, dL/de at lte?gt , and fluxes calculated for
the thermal spectra from 5 and 6.
L (erg s-1) lte?gt (keV) dL/de (photon s-1 keV-1) Flux/D2Mpc (photon s -1 cm-2 keV-1)
1036 500 5 1039 5 10-10
1037 400 6 1040 7 10-9
1038 300 2 1041 2.5 10-8
1039 170 7.5 1042 8 10-7
1040 100 2 1044 2 10-5
1041 60 5 1045 6 10-4
1042 50 7 1046 9 10-3
DETECTABILITY WITH INTEGRAL The International
Gamma Ray Astrophysics Laboratory known as
INTEGRAL, has the capability of detecting faint
?-ray sources. It is equipped whit two main
instruments, the SPI, a high resolution ?- ray
spectral imager and the IBIS, Imager on Board
Integral Satellite which is a ?-ray telescope
that provides precise images of the ?-ray
sources. In continuum mode and for a typical
observation time of 106 sec., these instruments
have threshold sensitivities of about 10-6 photon
s -1 cm-2 keV-1 for a 3s detection.
Fig. 2 Thermal luminosity for the same scenarios
as in Fig. 1. In these models it is assumed that
the stellar surface is convective (from Ref.
5).
Fig. 1 Time evolution of the luminosity of a bare
strange star, showing different scenarios for
quark color superconductivity (from Ref. 5).

• At L gt 1041 erg s-1 the pair plasma is optically
  thick and the outcoming photons
• follow a blackbody spectrum with temperature a
  few tens of keV
• - At L gt 1043 erg s-1 TBB T0
  (L/1043 erg s-1)1/4 with T0 2 108 K.
• - At 1041 erg s-1 lt L lt 1043 erg s-1
  TBB T0
• At L lt 1041 erg s-1 the photon spectrum differs
  significantly from a BB, as illustrated in Fig. 3
  4 from Ref. 6.
• In short, the spectrum hardens when the
  luminosity decreases.
• Both super-Eddington luminosities and the
  spectrum-luminosity characteristics differ from
  the thermal emission from neutron stars and
  provides a definite observational signature for a
  Bare Strange Star

L (erg s-1) DMax
1036 20 kpc
1037 80 kpc
1038 120 kpc
1039 1 Mpc
1040 4.5 Mpc
1041 25 Mpc
1042 100 Mpc
Table2. Maximum distances DMax for a 3s detection
of a Bare Strange Star of luminosity L with a
typical 106 sec. observation of INTEGRAL.
Fig. 3 (Left) Mean energies of the emerging
photons (thick solid line), as a fraction of the
total luminosity (see 6 for details).

• CONCLUSIONS
• A newborn Bare Strange Star (L ? 1042 erg s-1)
  could be
• detected, and identified, to a distance of
  about 100 Mpc,
• hence encompassing many large clusters of
  galaxies.
• (The birth event is expected to be a Gamma Ray
  Burst)
• A young Bare Strange Star (L ? 1039 erg s-1,
  age up to 1,000
• yrs) could be detected within the Local Group.

REFERENCES 1 Witten, E. 1984, Phys. Rev. D,
30, 272 2 Alcock, C., Farhi, E., Olinto, A.
1986, ApJ, 310, 261 3 Haensel, P., Zdunik,
J.L., Schaeffer, R. 1986, ApJ, 160, 121 4
Usov, V. V. 1998, Phys. Rev. Lett. 80, 230 5
Page, D. Usov, V. V. 2002, Phys. Rev. Lett. 89,
13 6 Aksenov, A.G., Milgrom, M., Usov, V.V.,
MNRAS, in press 7 Trams, N.R. SPI Observers
Manual Integral Science Operations, ESTEC.
2000 8 P. Barr. IBIS Observers Manual.
Integral Science Operations, ESTEC. 2000
Fig. 4 (Right) Energy spectra of the emerging
photons for different values of the total
luminosity. The dotted line shows the BB spectrum
for L 1042 erg s-1 (see 6 for details)