346 research outputs found
Swift J1357.2-0933: the faintest black hole?
Swift J1357.2-0933 is the first confirmed very faint black hole X-ray
transient and has a short estimated orbital period of 2.8 hr. We observed Swift
J1357.2-0933 for ~50 ks with XMM-Newton in 2013 July during its quiescent
state. The source is clearly detected at a 0.5-10 keV unabsorbed flux of
~3x10^-15 erg cm-2 s-1. If the source is located at a distance of 1.5 kpc (as
suggested in the literature), this would imply a luminosity of ~8x10^29 erg
s-1, making it the faintest detected quiescent black hole LMXB. This would also
imply that there is no indication of a reversal in the quiescence X-ray
luminosity versus orbital period diagram down to 2.8 hr, as has been predicted
theoretically and recently supported by the detection of the 2.4 hr orbital
period black hole MAXI J1659-152 at a 0.5-10 keV X-ray luminosity of ~ 1.2 x
10^31 erg s-1. However, there is considerable uncertainty in the distance of
Swift J1357.2-0933 and it may be as distant as 6 kpc. In this case, its
quiescent luminosity would be Lx ~ 1.3 x 10^31 erg s-1, i.e., similar to MAXI
J1659-152 and hence it would support the existence of such a bifurcation
period. We also detected the source in optical at r' ~22.3 mag with the
Liverpool telescope, simultaneously to our X-ray observation. The X-ray/optical
luminosity ratio of Swift J1357.2-0933 agrees with the expected value for a
black hole at this range of quiescent X-ray luminosities.Comment: 5 pages, 3 figures, Accepted for publication in MNRA
NuSTAR + XMM-Newton monitoring of the neutron star transient AX J1745.6-2901
AX J1745.6-2901 is a high-inclination (eclipsing) transient neutron star (NS)
Low Mass X-ray Binary (LMXB) showcasing intense ionised Fe K absorption. We
present here the analysis of 11 XMM-Newton and 15 NuSTAR new data-sets
(obtained between 2013-2016), therefore tripling the number of observations of
AX J1745.6-2901 in outburst. Thanks to simultaneous XMM-Newton and NuSTAR
spectra, we greatly improve on the fitting of the X-ray continuum. During the
soft state the emission can be described by a disk black body (
keV and inner disc radius km), plus hot ( keV)
black body radiation with a small emitting radius ( km)
likely associated with the boundary layer or NS surface, plus a faint
Comptonisation component. Imprinted on the spectra are clear absorption
features created by both neutral and ionised matter. Additionally, positive
residuals suggestive of an emission Fe K disc line and consistent with
relativistic ionised reflection are present during the soft state, while such
residuals are not significant during the hard state. The hard state spectra are
characterised by a hard () power law, showing no evidence
for a high energy cut off ( keV) and implying a small optical
depth (). The new observations confirm the previously witnessed trend
of exhibiting strong Fe K absorption in the soft state, that significantly
weakens during the hard state. Optical (GROND) and radio (GMRT) observations
suggest for AX J1745.6-2901 a standard broad band SED as typically observed in
accreting neutron stars.Comment: Accepted for publication in MNRA
The "K-Correction" for Irradiated Emission Lines in LMXBs: Evidence for a Massive Neutron Star in X1822-371 (V691 CrA)
We study the K-correction for the case of emission lines formed in the X-ray
illuminated atmosphere of a Roche lobe filling star. We compute the
K-correction as function of the mass ratio 'q' and the disc flaring angle
'alpha' using a compact binary code where the companion's Roche lobe is divided
into 10^5 resolution elements. We also study the effect of the inclination
angle in the results. We apply our model to the case of the neutron star
low-mass X-ray binary X1822-371 (V691 CrA), where a K-emission velocity
K_em=300 +-8 km/s has been measured by Casares et al. (2003). Our numerical
results, combined with previous determination of system parameters, yields
1.61Msun < M_NS < 2.32Msun and 0.44Msun < M_2 < 0.56Msun for the two binary
components(i. e. 0.24 < q < 0.27), which provide a compelling evidence for a
massive neutron star in this system. We also discuss the implications of these
masses into the evolutionary history of the binary.Comment: 6 pages, 5 figures. Accepted for publication in Ap
Discovery of two simultaneous non-harmonically related Quasi-Periodic Oscillations in the 2005 outburst of the black-hole binary GRO J1655-40
We studied the low-frequency quasi-periodic oscillations (LFQPOs) in the
black hole GRO J1655-40 during the 2005 outburst, using data from the Rossi
X-ray Timing Explorer. All LFQPOs could be identified as either type B or type
C using previously proposed classification schemes. In the soft state of the
outburst the type-C LFQPOs reached frequencies that are among the highest ever
seen for LFQPOs in black holes. At the peak of the outburst, in the
ultra-luminous state, the power spectrum showed two simultaneous,
non-harmonically related peaks which we identified as a type-B and a type-C
QPO. The simultaneous presence of a type-C and type-B QPO shows that at least
two of the three known LFQPO types are intrinsically different and likely the
result of distinct physical mechanisms. We also studied the properties of a
broad peaked noise component in the power spectra of the ultra-luminous state.
This noise component becomes more coherent with count rate and there are strong
suggestions that it evolves into a type-B QPO at the highest observed count
rates.Comment: 14 pages, 6 figures, 2 tables, accepted for publication in MNRA
Ask The Machine: Systematic detection of wind-type outflows in low-mass X-ray binaries
The systematic discovery of outflows in the optical spectra of low-mass X-ray
binaries opened a new avenue for the study of the outburst evolution in these
extreme systems. However, the efficient detection of such features in a
continuously growing database requires the development of new analysis
techniques with a particular focus on scalability, adaptability, and
automatization. In this pilot study, we explore the use of machine learning
algorithms to perform the identification of outflows in spectral line profiles
observed in the optical range. We train and test the classifier on a simulated
database, constructed through a combination of disc emission line profiles and
outflow signatures, emulating typical observations of low-mass X-ray binaries.
The final, trained classifier is applied to two sets of spectra taken during
two bright outbursts that were particularly well covered, those of V404 Cyg
(2015) and MAXI J1820+070 (2018). The resulting classification gained by this
novel approach is overall consistent with that obtained through traditional
techniques, while it simultaneously provides a number of key advantages over
the latter, including the access to low velocity outflows. This study sets the
foundations for future studies on large samples of spectra from low-mass X-ray
binaries and other compact binaries.Comment: Accepted for publication in MNRAS. 13 pages, 6 figure
The optical counterpart of the bright X-ray transient Swift J1745-26
We present a 30-day monitoring campaign of the optical counterpart of the
bright X-ray transient Swift J1745-26, starting only 19 minutes after the
discovery of the source. We observe the system peaking at i' ~17.6 on day 6
(MJD 56192) to then decay at a rate of ~0.04 mag/day. We show that the optical
peak occurs at least 3 days later than the hard X-ray (15-50 keV) flux peak.
Our measurements result in an outburst amplitude greater than 4.3 magnitudes,
which favours an orbital period < 21 h and a companion star with a spectral
type later than ~ A0. Spectroscopic observations taken with the GTC-10.4 m
telescope reveal a broad (FWHM ~ 1100 km/s), double-peaked H_alpha emission
line from which we constrain the radial velocity semi-amplitude of the donor to
be K_2 > 250 km/s. The breadth of the line and the observed optical and X-ray
fluxes suggest that Swift J1745-26 is a new black hole candidate located closer
than ~7 kpc.Comment: 5 pages, 4 figures, accepted for publication in MNRA
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