5,664 research outputs found
Black hole hunting in the Andromeda Galaxy
We present a new technique for identifying stellar mass black holes in low
mass X-ray binaries (LMXBs), and apply it to XMM-Newton observations of M31. We
examine X-ray time series variability seeking power density spectra (PDS)
typical of LMXBs accreting at a low accretion rate (which we refer to as Type A
PDS); these are very similar for black hole and neutron star LMXBs. Galactic
neutron star LMXBs exhibit Type A PDS at low luminosities (~10^36--10^37 erg/s)
while black hole LMXBs can exhibit them at luminosities >10^38 erg/s. We
propose that Type A PDS are confined to luminosities below a critical fraction
of the Eddington limit, that is constant for all LMXBs; we have examined
asample of black hole and neutron star LMXBs and find they are all consistent
with = 0.10+/-0.04 in the 0.3--10 keV band. We present luminosity and PDS
data from 167 observations of X-ray binaries in M31 that provide strong support
for our hypothesis. Since the theoretical maximum mass for a neutron star is
\~3.1 M_Sun, we therefore assert that any LMXB that exhibits a Type A PDS at a
0.3--10 keV luminosity greater than 4 x 10^37 erg/s is likely to contain a
black hole primary. We have found eleven new black hole candidates in M31 using
this method. We focus on XMM-Newton observations of RX J0042.4+4112, an X-ray
source in M31 and find the mass of the primary to be 7+/-2 M_Sun, if our
assumptions are correct. Furthermore, RX J0042.4+4112 is consistently bright in
\~40 observations made over 23 years, and is likely to be a persistently bright
LMXB; by contrast all known Galactic black hole LMXBs are transient. Hence our
method may be used to find black holes in known, persistently bright Galactic
LMXBs and also in LMXBs in other galaxies.Comment: 6 Pages, 6 figures. To appear in the conference proceedings of
"Interacting Binaries: Accretion, Evolution and Outcomes" (Cefalu, July 4-10
2004
Elliptic flow of thermal photons and dileptons
In this talk we describe the recently discovered rich phenomenology of
elliptic flow of electromagnetic probes of the hot matter created in
relativistic heavy-ion collisions. Using a hydrodynamic model for the
space-time dynamics of the collision fireball created in Au+Au collisions at
RHIC, we compute the transverse momentum spectra and elliptic flow of thermal
photons and dileptons. These observables are shown to provide differential
windows into various stages of the fireball expansion.Comment: 8 pages, including 9 figures. Invited talk at the Hard Probes 2006
Conference (Asilomar, June 9-16, 2006), to appear in the Proceedings
(Elsevier
Cumulative luminosity functions of the X-ray point source population in M31
We present preliminary results from a detailed analysis of the X-ray point
sources in the XMM-Newton survey of M31. These sources are expected to be
mostly X-ray binaries. We have so far studied 225 of the 535 sources found by
automated source detection. Only sources which were present in all three EPIC
images were considered. X-ray binaries are identified by their energy spectrum
and power density spectrum. Unlike in other surveys we have obtained source
luminosities from freely fit emission models. We present uncorrected luminosity
functions of the sources analysed so far.Comment: 2 pages, 1 figure. To appear in proceedings of IAUS23
Interpretation of the variability of the <i>β</i> Cephei star <i>λ</i> Scorpii. I. The multiple character
We derive accurate values of the orbital parameters of the close binary β Cephei star λ Scorpii. Moreover, we present the first determination of the properties of the triple system to which λ Scorpii belongs. Our analysis is based on a time series of 815 high-resolution spectra, covering a timespan of 14 years. We find a close orbit of 5d.9525days (e=0.26) and a wide orbit of approximately 1082d days (e=0.23). The orbital parameters of the triple star and a spectrum synthesis lead us to conclude that the system is composed of two early-type B stars and a low-mass pre-main-sequence star rather than containing an ultra-massive white dwarf as claimed before. Our proposed configuration is compatible with population synthesis. The radial velocity variations of the primary allow us to confirm the presence of at least one pulsation mode with frequency 4.679410 c d-1 which is subject to the light-time effect in the triple system. A detailed analysis of the complex line-profile variations is described in a subsequent paper
A mystery solved: the mass ratio of the dwarf nova EM Cygni
We have discovered that the spectrum of the well-known dwarf nova EM Cyg is
contaminated by light from a K2-5V star (in addition to the K-type mass donor
star). The K2-5V star contributes approximately 16 per cent of the light from
the system and if not taken into account has a considerable effect upon radial
velocity measurements of the mass donor star. We obtain a new radial velocity
amplitude for the mass donor star of K2 = 202 +/- 3 km/s, which compares with
the value of K2 = 135 +/- 3 km/s obtained in Stover, Robinson & Nather's
classic 1981 study of EM Cyg. The revised value of the amplitude combined with
a measurement of rotational broadening of the mass donor vsini = 140 +/- 6
km/s, leads to a new mass ratio of q = M2/M1 = 0.88 +/- 0.05. This solves a
long standing problem with EM Cyg because Stover et al.'s measurements
indicated a mass ratio q > 1, a value which should have led to dynamically
unstable mass transfer for the secondary mass deduced by Stover et al. The
revised value of the mass ratio combined with the orbital inclination i = 67
+/- 2 degrees leads to masses of 0.99 +/- 0.12 Msun and 1.12 +/- 0.08 Msun for
the mass donor and white dwarf respectively. The mass donor is evolved, since
it has a later spectral type (K3) than its mass would imply. We discuss whether
the K star could be physically associated with EM Cyg or not, and present the
results of the spectroscopic study.Comment: 10 pages, 12 figures, accepted for publication in MNRA
Discovery of disc precession in the M31 dipping X-ray binary Bo 158
We present results from three XMM-Newton observations of the M31 low mass
X-ray binary XMMU J004314.4+410726.3 (Bo 158), spaced over 3 days in 2004,
July. Bo 158 was the first dipping LMXB to be discovered in M31. Periodic
intensity dips were previously seen to occur on a 2.78-hr period, due to
absorption in material that is raised out of the plane of the accretion disc.
The report of these observations stated that the dip depth was anti-correlated
with source intensity. However, our new observations do not favour a strict
intensity dependance, but rather suggest that the dip variation is due to
precession of the accretion disc. This is to be expected in LMXBs with a mass
ratio <~ 0.3 (period <~ 4 hr), as the disc reaches the 3:1 resonance with the
binary companion, causing elongation and precession of the disc. A smoothed
particle hydrodynamics simulation of the disc in this system shows retrograde
rotation of a disc warp on a period of ~11 P_orb, and prograde disc precession
on a period of ~29 P_orb. This is consistent with the observed variation in the
depth of the dips. We find that the dipping behaviour is most likely to be
modified by the disc precession, hence we predict that the dipping behaviour
repeats on a 81+/-3 hr cycle.Comment: 9 pages, 6 figures, accepted for publication by MNRAS, changed
conten
XMM-Newton reveals ~100 new LMXBs in M31 from variability studies
We have conducted a survey of X-ray sources in XMM-Newton observations of
M31, examining their power density spectra (PDS) and spectral energy
distributions (SEDs). Our automated source detection yielded 535 good X-ray
sources; to date, we have studied 225 of them. In particular, we examined the
PDS because low mass X-ray binaries (LMXBs) exhibit two distinctive types of
PDS. At low accretion rates, the PDS is characterised by a broken power law,
with the spectral index changing from ~0 to ~1 at some frequency in the range
\~0.01--1 Hz; we refer to such PDS as Type A. At higher accretion rates, the
PDS is described by a simple power law; we call these PDS Type B. Of the 225
sources studied to date, 75 exhibit Type A variability, and are almost
certainly LMXBs, while 6 show Type B but not Type A, and are likely LMXBs. Of
these 81 candidate LMXBs, 71 are newly identified in this survey; furthermore,
they are mostly found near the centre of M31. Furthermore, most of the X-ray
population in the disc are associated with the spiral arms, making them likely
high mass X-ray binaries (HMXBs). In general these HMXBs do not exhibit Type A
variability, while many central X-ray sources (LMXBs) in the same luminosity
range do. Hence the PDS may distinguish between LMXBs and HMXBs in this
luminosity range.Comment: 4 pages, 2 figures. To appear in proceedings of IAUS230: "Populations
of High Energy Sources in Galaxies", 14-19 August 2005, Dublin, Eds E.J.A.
Meurs and G. Fabbian
Energy-dependent evolution in IC10 X-1: hard evidence for an extended corona and implications
We have analyzed a ~130 ks XMM-Newton observation of the dynamically confirmed black hole + Wolf-Rayet (BH+WR) X-ray binary (XB) IC10 X-1, covering ~1 orbital cycle. This system experiences periodic intensity dips every ~35 hr. We find that energy-independent evolution is rejected at a >5σ level. The spectral and timing evolution of IC10 X-1 are best explained by a compact disk blackbody and an extended Comptonized component, where the thermal component is completely absorbed and the Comptonized component is partially covered during the dip. We consider three possibilities for the absorber: cold material in the outer accretion disk, as is well documented for Galactic neutron star (NS) XBs at high inclination; a stream of stellar wind that is enhanced by traveling through the L1 point; and a spherical wind. We estimated the corona radius (r ADC) for IC10 X-1 from the dip ingress to be ~106 km, assuming absorption from the outer disk, and found it to be consistent with the relation between r ADC and 1-30 keV luminosity observed in Galactic NS XBs that spans two orders of magnitude. For the other two scenarios, the corona would be larger. Prior BH mass (M BH) estimates range over 23-38 M ☉, depending on the inclination and WR mass. For disk absorption, the inclination, i, is likely to be ~60-80°, with M BH ~ 24-41 M ☉. Alternatively, the L1-enhanced wind requires i ~ 80°, suggesting ~24-33 M ☉. For a spherical absorber, i ~ 40°, and M BH ~ 50-65 M ☉
- …