240 research outputs found
The mass of the neutron star in the low-mass X-ray binary 2A 1822-371
Using phase resolved spectroscopic observations obtained with the Ultraviolet
and Visual Echelle Spectrograph on ESO's Kueyen Very Large Telescope
supplemented by spectroscopic observations obtained with the Boller and Chivens
Spectrograph on the Walter Baade Magellan telescope, we found sinusoidal
radial-velocity variations with a semi-amplitude 327+-17 km/s. From previous
observations and from the fact that the epoch of minimum velocity arrived early
with respect to the epoch calculated from pulse timing we know that the
companion star is suffering from irradiation. Since we most likely observed
primarily the side of the companion star facing the observer at phase ~0.75 the
velocity quoted above is not the true radial velocity semi-amplitude of the
companion star. Assuming a uniform contribution to the line profile from this
hemisphere yields a radial velocity semi-amplitude of 280+-26 km/s for a
systemic velocity of 54+-24 km/s; if the contribution is instead weighted
somewhat more towards the side of the companion facing the X-ray source then
the true semi-amplitude is larger than this value. Together with the well
constrained inclination (81<i<84 degrees) and the mass-function determined from
pulse-timing analysis (2.03+-0.03 x 10^-2 Msun), we derive a lower limit to the
mass of the neutron star and to that of the companion star of 0.97+-0.24 Msun
and 0.33+-0.05 Msun, respectively (1 sigma; including uncertainties in the
inclination). We briefly discuss other aspects of the spectrum and the
implications of our findings.Comment: 6 pages, 2 figures, accepted for publication by MNRA
Dependence of kHz QPO Properties on Normal-Branch Oscillation Phase in Scorpius X-1
We analysed RXTE data of Sco X-1 which show kHz quasi-periodic oscillations
(QPOs) and the 6--8 Hz normal-branch oscillation (NBO) simultaneously.
Using power spectra of 0.03--0.5 s data segments, we find that both the upper
kHz QPO frequency and the ratio of lower to upper kHz QPO amplitude are
anticorrelated to variations in the X-ray count-rate taking place on the NBO
time scale. The frequency dependence is similar to (but probably weaker than)
that found on longer time scales, but the power ratio dependence is opposite to
it. A model where radiative stresses on the disk material, modulated at the NBO
frequency, lead to changes in can explain the data; this implies some
of the NBO flux changes originate from inside the inner disk radius. We discuss
how these findings affect our understanding of kHz QPOs and of the
low-frequency variability of low-mass X-ray binaries.Comment: 13 pages including 3 figures, accepted for publlication in ApJ
Letter
Discovery of a Quasar with Double-Peaked Broad Balmer Emission Lines
Most massive galaxies contain a supermassive black hole (SMBH) at their
center. When galaxies merge, their SMBHs sink to the center of the new galaxy
where they are thought to eventually merge. During this process an SMBH binary
is formed. The presence of two sets of broad emission lines in the optical
spectrum of an active galactic nucleus (AGN) has been interpreted as evidence
for two broad line regions (BLR), one surrounding each SMBH in a binary. We
modeled the broad Balmer emission lines in SDSS spectra of 373 extreme
variability AGNs using one broad and several narrow Gaussian components. We
report on the discovery of SDSS J021647.53011341.5 (hereafter J0216) as a
double-peaked broad emission line source. Among the 373 AGNs there were five
sources that are known double-peaked emission line sources. Three of these have
been reported as candidate SMBH binaries in previous studies. We present all
six objects and their double-peaked broad Balmer emission lines, and discuss
the implications for a tidal disruption event (TDE) interpretation of the
extreme variability assuming the double-peaked sources are SMBH binaries.Comment: 6 pages, 1 figure, 1 table. Accepted for publication in MNRA
Hypercompact stellar clusters: morphological renditions and spectro-photometric models
Numerical relativity predicts that the coalescence of a black hole-binary
causes the newly formed black hole to recoil, and evidence for such recoils has
been found in the gravitational waves observed during the merger of
stellar-mass black holes. Recoiling (super)massive black holes are expected to
reside in hypercompact stellar clusters (HCSCs). Simulations of galaxy assembly
predict that hundreds of HCSCs should be present in the halo of a Milky
Way-type galaxy, and a fraction of those around the Milky Way should have
magnitudes within the sensitivity limit of existing surveys. However, recoiling
black holes and their HCSCs are still waiting to be securely identified. With
the goal of enabling searches through recent and forthcoming databases, we
improve over existing literature to produce realistic renditions of HCSCs bound
to black holes with a mass of 10 M. Including the effects of a
population of blue stragglers, we simulate their appearance in Pan-STARRS and
in forthcoming images. We also derive broad-band spectra and the
corresponding multi-wavelength colours, finding that the great majority of the
simulated HCSCs fall on the colour-colour loci defined by stars and galaxies,
with their spectra resembling those of giant K-type stars. We discuss the
clusters properties, search strategies, and possible interlopers.Comment: Accepted for publication on MNRAS, 17 pages, 7 figure
The infrared counterpart of the Z source GX5-1
We have obtained UKIRT infrared observations of the field of the bright
Galactic Z source GX5-1. From an astrometric plate solution tied to Tycho-ACT
standards we have obtained accurate positions for the stars in our field which,
combined with an accurate radio position, have allowed us to identify the
probable infrared counterpart of GX5-1. Narrow-band photometry marginally
suggests excess Br-gamma emission in the counterpart, supporting its
association with an accretion-disc source. No significant variability is
observed in a limited number of observations. We compare the H and K magnitudes
with those of other Z sources, and briefly discuss possible sources of infrared
emission in these systems.Comment: 5 pages, 2 figures, accepted by MNRA
The hard quiescent spectrum of the neutron-star X-ray transient EXO 1745-248 in the globular cluster Terzan 5
We present a Chandra observation of the globular cluster Terzan 5 during
times when the neutron-star X-ray transient EXO 1745-248 located in this
cluster was in its quiescent state. We detected the quiescent system with a
(0.5-10 keV) luminosity of ~2 x 10^{33} ergs/s. This is similar to several
other neutron-star transients observed in their quiescent states. However, the
quiescent X-ray spectrum of EXO 1745--48 was dominated by a hard power-law
component instead of the soft component that usually dominates the quiescent
emission of other neutron-star X-ray transients. This soft component could not
conclusively be detected in EXO 1745-248 and we conclude that it contributed at
most 10% of the quiescent flux in the energy range 0.5-10 keV. EXO 1745-248 is
only the second neutron-star transient whose quiescent spectrum is dominated by
the hard component (SAX J1808.4-3658 is the other one). We discuss possible
explanations for this unusual behavior of EXO 1745-248, its relationship to
other quiescent neutron-star systems, and the impact of our results on
understanding quiescent X-ray binaries. We also discuss the implications of our
results on the way the low-luminosity X-ray sources in globular clusters are
classified.Comment: Accepted by ApJ Main Journal, September 22, 2004. Figure 2 is a color
figur
Mass models of NGC 6624 without an intermediate-mass black hole
An intermediate-mass black hole (IMBH) was recently reported to reside in the
centre of the Galactic globular cluster (GC) NGC 6624, based on timing
observations of a millisecond pulsar (MSP) located near the cluster centre in
projection. We present dynamical models with multiple mass components of NGC
6624 - without an IMBH - which successfully describe the surface brightness
profile and proper motion kinematics from the Hubble Space Telescope (HST) and
the stellar mass function at different distances from the cluster centre. The
maximum line-of-sight acceleration at the position of the MSP accommodates the
inferred acceleration of the MSP, as derived from its first period derivative.
With discrete realizations of the models we show that the higher-order period
derivatives - which were previously used to derive the IMBH mass - are due to
passing stars and stellar remnants, as previously shown analytically in
literature. We conclude that there is no need for an IMBH to explain the timing
observations of this MSP.Comment: 8 pages, 7 figures, MNRAS. Updated to match final journal styl
Spectroscopic monitoring of the candidate tidal disruption event in F01004-2237
We present results of spectroscopic monitoring observations of the
Ultra-Luminous Infra Red Galaxy F01004-2237. This galaxy was observed to
undergo changes in its optical spectrum, detected by comparing a spectrum from
2015 with one from 2000. These changes were coincident with photometric
brightening. The main changes detected in the optical spectrum are enhanced He
II 4686 emission and the appearance of He I
3898,5876 emission lines. The favoured interpretation of
these changes was that of a tidal disruption event (TDE) happening in 2010.
However, subsequent work suggested that these changes are caused by another
hitherto unknown reason related to variations in the accretion rate in the
active galactic nucleus (AGN). Our optical spectroscopic monitoring
observations show that the evolution of the He lines is in line with the
evolution seen in TDEs and opposite of what observed from reverberation mapping
studies of AGNs, renewing the discussion on the interpretation of the flare as
a TDE.Comment: 9 pages, 3 figures, 2 tables. Accepted for publication on ApJ on
January 22, 202
Black hole masses of tidal disruption event host galaxies
The mass of the central black hole in a galaxy that hosted a tidal disruption
event (TDE) is an important parameter in understanding its energetics and
dynamics. We present the first homogeneously measured black hole masses of a
complete sample of 12 optically/UV selected TDE host galaxies (down to
22 mag and =0.37) in the Northern sky. The mass estimates
are based on velocity dispersion measurements, performed on late time optical
spectroscopic observations. We find black hole masses in the range
310 MM210 M.
The TDE host galaxy sample is dominated by low mass black holes (10
M), as expected from theoretical predictions. The blackbody peak
luminosity of TDEs with M10 M is consistent
with the Eddington limit of the SMBH, whereas the two TDEs with M10 M have peak luminosities below their SMBH
Eddington luminosity, in line with the theoretical expectation that the
fallback rate for M10 M is sub-Eddington. In
addition, our observations suggest that TDEs around lower mass black holes
evolve faster. These findings corroborate the standard TDE picture in 10
M black holes. Our results imply an increased tension between
observational and theoretical TDE rates. By comparing the blackbody emission
radius with theoretical predictions, we conclude that the optical/UV emission
is produced in a region consistent with the stream self-intersection radius of
shallow encounters, ruling out a compact accretion disk as the direct origin of
the blackbody radiation at peak brightness.Comment: 16 pages, 9 figures. Submitted to MNRAS; including minor revisions
suggested by the refere
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