Context.
The discovery of pulsations in several ultraluminous X-ray sources (ULXs) has demonstrated that a fraction of them are powered by super-Eddington accretion onto neutron stars (NSs). This has ...raised questions regarding the NS to black hole (BH) ratio within the ULX population and the physical mechanism that allows ULXs to reach luminosities well in excess of their Eddington luminosity. Is this latter the presence of strong magnetic fields or rather the presence of strong outflows that collimate the emission towards the observer?
Aims.
In order to distinguish between these scenarios, namely, supercritically accreting BHs, weakly magnetised NSs, or strongly magnetised NSs, we study the long-term X-ray spectral evolution of a sample of 17 ULXs with good long-term coverage, 6 of which are known to host NSs. At the same time, this study serves as a baseline to identify potential new NS-ULX candidates.
Methods.
We combine archival data from
Chandra
,
XMM-Newton
, and
NuSTAR
observatories in order to sample a wide range of spectral states for each source. We track the evolution of each source in a hardness–luminosity diagram in order to identify spectral changes, and show that these can be used to constrain the accretion flow geometry, and in some cases the nature of the accretor.
Results.
We find NS-ULXs to be among the hardest sources in our sample with highly variable high-energy emission. On this basis, we identify M 81 X-6 as a strong NS-ULX candidate, whose variability is shown to be akin to that of NGC 1313 X-2. For most softer sources with an unknown accretor, we identify the presence of three markedly different spectral states, which we interpret by invoking changes in the mass-accretion rate and obscuration by the supercritical wind/funnel structure. Finally, we report on a lack of variability at high energies (≳10 keV) in NGC 1313 X-1 and Holmberg IX X-1, which we argue may offer a means to differentiate BH-ULXs from NS-ULXs.
Conclusions.
We support a scenario in which the hardest sources in our sample might be powered by strongly magnetised NSs, meaning that the high-energy emission is dominated by the hard direct emission from the accretion column. Instead, softer sources may be explained by weakly magnetised NSs or BHs, in which the presence of outflows naturally explains their softer spectra through Compton down-scattering, their spectral transitions, and the dilution of the pulsed-emission should some of these sources contain NSs.
Context.
Most ultra-luminous X-ray sources (ULXs) are now thought to be powered by stellar-mass compact objects accreting at super-Eddington rates. While the discovery of evolutionary cycles have ...marked a breakthrough in our understanding of the accretion flow changes in the sub-Eddington regime in Galactic black hole binaries, their evidence in the super-Eddington regime has so far remained elusive. However, recent circumstantial evidence hinted at the presence of a recurrent evolutionary cycle in two archetypal ULXs: Holmberg II X–1 and NGC 5204 X–1.
Aims.
We aim to build on our previous work and exploit the long-term high-cadence monitoring of
Swift
-XRT in order to provide robust evidence of the evolutionary cycle in these two sources and investigate the main physical parameters inducing their spectral transitions.
Methods.
We studied the long-term evolution of both sources using hardness-intensity diagrams (HID) and by means of Lomb–Scargle periodograms and Gaussian process modelling to look for periodic variability. We also applied a physically motivated model to the combined
Chandra
,
XMM-Newton
,
NuSTAR
, and
Swift
-XRT data of each of the source spectral states.
Results.
We robustly show that both sources follow a clear and recurrent evolutionary pattern in the HID that can be characterised by the hard ultra-luminous (HUL) and soft ultra-luminous (SUL) spectral regimes, and a third state with characteristics similar to the super-soft ultra-luminous (SSUL) state. The transitions between the soft states seem consistent with aperiodic variability, as revealed by a timing analysis of the light curve of Holmberg II X–1; albeit, further investigation is warranted. The light curve of NGC 5204 X–1 shows a stable periodicity on a longer baseline of ∼200 days, possibly associated with the duration of the evolutionary cycle.
Conclusions.
The similarities between both sources provide strong evidence of both systems hosting the same type of accretor and/or accretion flow geometry. We support a scenario in which the spectral changes from HUL to SUL are due to a periodic increase of the mass-transfer rate and subsequent narrowing of the opening angle of the super-critical funnel. The narrower funnel, combined with stochastic variability imprinted by the wind, might explain the rapid and aperiodic variability responsible for the SUL–SSUL spectral changes. The nature of the longer periodicity of NGC 5204 X–1 remains unclear, and robust determination of the orbital period of these sources could shed light on the nature of the periodic modulation found. Based on the similarities between the two sources, a long periodicity should be detectable in Holmberg II X–1 with future monitoring.
Context.
The presence of large ionised gaseous nebulae found around some ultraluminous X-ray sources (ULXs) provides the means to assess the mechanical and radiative feedback of the central source, ...and hence constrain the efficiency and impact on the surroundings of the super-Eddington regime powering most of these sources. NGC 1313 X–1 is an archetypal ULX, reported to be surrounded by abnormally high O
I
λ
6300/H
α
> 0.1 ratios, and for which high-resolution spectroscopy in X-rays has hinted at the presence of powerful outflows.
Aims.
We report observations taken with the integral field unit Multi-Unit Spectroscopic Explorer (MUSE) mounted at the Very Large Telescope of NGC 1313 X–1 in order to confirm the presence of a nebula inflated by the winds, investigate its main sources of ionisation and estimate the mechanical output of the source.
Methods.
We investigated the morphology, kinematics, and sources of ionisation of the bubble through the study of the main nebular lines. We compared the main line ratios with spatially resolved Baldwin–Phillips–Terlevich diagrams and with the prediction from radiative shock libraries, which allows us to differentiate regions excited by shocks from those excited by extreme ultraviolet and X-ray radiation.
Results.
We detect a bubble of 452 × 266 pc in size, roughly centred around the ULX, which shows clear evidence of shock ionisation in the outer edges. We estimate shock velocities to be in the ≈160 − 180 km s
−1
range based on the line ratios. This suggests that an average and continuous outflow power of ∼(2 − 4.5)×10
40
erg s
−1
over a timescale of (4.5 − 7.8)×10
5
yr is required to inflate the bubble. In the interior of the bubble and closer to the ULX we detect an extended (∼140 pc) X-ray ionised region. Additionally, we detect two supernova remnants coincidentally close to the ULX bubble of which we also report age and explosion energy estimates.
Conclusions.
The elongated morphology and the kinematics of the bubble strongly suggest that the bubble is being inflated by winds and/or jets emanating from the central source, supporting the presence of winds found through X-ray spectroscopy. The estimated mechanical power is comparable to or higher than the X-ray luminosity of the source, which provides additional evidence in support of NGC 1313 X–1 harbouring a super-Eddington accretor.
Measurement of performance of the NectarCAM photodetectors Tsiahina, A.; Jean, P.; Olive, J.-F. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
08/2021, Letnik:
1007
Journal Article
Recenzirano
Odprti dostop
NectarCAM is a camera for the medium-sized telescopes of the Cherenkov Telescope Array (CTA), which covers the energy range of 100 GeV to 30 TeV. The camera is equipped with 265 focal plane modules ...(FPMs). Each FPM comprises 7 pixels, each consisting of a photo-multiplier tube, a preamplifier, an independently controlled power supply, and a common control system. We developed a dedicated test bench to validate and qualify the industrial FPM production and to measure the performance of each FPM in a dark room before its integration in the camera. We report the measured performance of 61 FPM prototypes obtained with our experimental setup. We demonstrate that the gains of the photo multiplier tubes are stable and that pulse widths, transit time spreads, afterpulse rates and charge resolutions are within the specifications for NectarCAM.
Gamma-ray bursts (GRBs) fall into two classes: short-hard and long-soft bursts. The latter are now known to have X-ray and optical afterglows, to occur at cosmological distances in star-forming ...galaxies, and to be associated with the explosion of massive stars. In contrast, the distance scale, the energy scale and the progenitors of the short bursts have remained a mystery. Here we report the discovery of a short-hard burst whose accurate localization has led to follow-up observations that have identified the X-ray afterglow and (for the first time) the optical afterglow of a short-hard burst; this in turn led to the identification of the host galaxy of the burst as a late-type galaxy at z = 0.16 (ref. 10). These results show that at least some short-hard bursts occur at cosmological distances in the outskirts of galaxies, and are likely to be caused by the merging of compact binaries.
Higgs and supersymmetry Buchmueller, O.; Cavanaugh, R.; De Roeck, A. ...
The European physical journal. C, Particles and fields,
06/2012, Letnik:
72, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Global frequentist fits to the CMSSM and NUHM1 using the MasterCode framework predicted
M
h
≃119 GeV in fits incorporating the (
g
−2)
μ
constraint and ≃126 GeV without it. Recent results by ATLAS ...and CMS could be compatible with a Standard Model-like Higgs boson around
M
h
≃125 GeV. We use the previous MasterCode analysis to calculate the likelihood for a measurement of any nominal Higgs mass within the range of 115 to 130 GeV. Assuming a Higgs mass measurement at
M
h
≃125 GeV, we display updated global likelihood contours in the (
m
0
,
m
1/2
) and other parameter planes of the CMSSM and NUHM1, and present updated likelihood functions for
, BR(
B
s
→
μ
+
μ
−
) and the spin-independent dark matter cross section
. The implications of dropping (
g
−2)
μ
from the fits are also discussed. We furthermore comment on a hypothetical measurement of
M
h
≃119 GeV.
The intense radiation flux of Type I X-ray bursts is expected to interact with the accretion flow around neutron stars. High frequency quasiperiodic oscillations (kHz QPOs), observed at frequencies ...matching orbital frequencies at tens of gravitational radii, offer a unique probe of the innermost disk regions. In this paper, we follow the lower kHz QPOs, in response to Type I X-ray bursts, in two prototypical QPO sources, namely 4U 1636-536 and 4U 1608-522, as observed by the Proportional Counter Array of the Rossi X-ray Timing Explorer. We have selected a sample of 15 bursts for which the kHz QPO frequency can be tracked on timescales commensurable with the burst durations (tens of seconds). We find evidence that the QPOs are affected for over ~200 s during one exceptionally long burst and ~100 s during two others (although at a less significant level), while the burst emission has already decayed to a level that would enable the pre-burst QPO to be detected. On the other hand, for most of our burst-kHz QPO sample, we show that the QPO is detected as soon as the statistics allow and in the best cases, we are able to set an upper limit of ~20 s on the recovery time of the QPO. This diversity of behavior cannot be related to differences in burst peak luminosity. We discuss these results in the framework of recent findings that accretion onto the neutron star may be enhanced during Type I X-ray bursts. The subsequent disk depletion could explain the disappearance of the QPO for ~100 s, as possibly observed in two events. However, alternative scenarios would have to be invoked for explaining the short recovery timescales inferred from most bursts. Heating of the innermost disk regions would be a possibility, although we cannot exclude that the burst does not affect the QPO emission at all. Clearly the combination of fast timing and spectral information of Type I X-ray bursts holds great potential in the study of the dynamics of the inner accretion flow around neutron stars. However, as we show, breakthrough observations will require a timing instrument providing at least ten times the effective area of the RXTE/PCA.
The CMSSM and NUHM1 after LHC Run 1 Buchmueller, O.; Cavanaugh, R.; Roeck, A. De ...
The European physical journal. C, Particles and fields,
2014/6, Letnik:
74, Številka:
6
Journal Article
Recenzirano
Odprti dostop
We analyze the impact of data from the full Run 1 of the LHC at 7 and 8 TeV on the CMSSM with
μ
>
0
and
<
0
and the NUHM1 with
μ
>
0
, incorporating the constraints imposed by other experiments such ...as precision electroweak measurements, flavour measurements, the cosmological density of cold dark matter and the direct search for the scattering of dark matter particles in the LUX experiment. We use the following results from the LHC experiments: ATLAS searches for events with
E
/
T
accompanied by jets with the full 7 and 8 TeV data, the ATLAS and CMS measurements of the mass of the Higgs boson, the CMS searches for heavy neutral Higgs bosons and a combination of the LHCb and CMS measurements of
BR
(
B
s
→
μ
+
μ
-
)
and
BR
(
B
d
→
μ
+
μ
-
)
. Our results are based on samplings of the parameter spaces of the CMSSM for both
μ
>
0
and
μ
<
0
and of the NUHM1 for
μ
>
0
with 6.8
×
10
6
, 6.2
×
10
6
and 1.6
×
10
7
points, respectively, obtained using the MultiNest tool. The impact of the Higgs-mass constraint is assessed using FeynHiggs 2.10.0, which provides an improved prediction for the masses of the MSSM Higgs bosons in the region of heavy squark masses. It yields in general larger values of
M
h
than previous versions of FeynHiggs, reducing the pressure on the CMSSM and NUHM1. We find that the global
χ
2
functions for the supersymmetric models vary slowly over most of the parameter spaces allowed by the Higgs-mass and the
E
/
T
searches, with best-fit values that are comparable to the
χ
2
/
dof
for the best Standard Model fit. We provide 95 % CL lower limits on the masses of various sparticles and assess the prospects for observing them during Run 2 of the LHC.
Photon detection is a major issue of high-energy astronomy instrumentation. One classical setup that has proven successful in space missions is the combination of photomultiplier tubes (PMTs) with ...scintillators, converting incoming high-energy photons into visible light, which is converted in an electrical impulse. Although being extremely sensitive and rapid, PMTs have the drawback of being bulky, fragile, and requiring a high-voltage power supply of thousands volts. The silicon photomultipliers (SiPM) appear to be a promising alternative to PMTs in many applications such as small satellites. We have started a R&D program to assess the possibility of using SiPMs for space-based applications in the high-energy astronomy domain. We present here the results of our characterization of SiPMs coming from several manufacturers. Each detector has been tested at low temperature and pressure to study its performance in a representative space environment. For this, we developed a dedicated vacuum chamber with a specific mechanical and thermal controlled system. Once dark current, dark count rate and PDE were measured, we made irradiation tests on two selected detectors to understand the susceptibility of SiPM to radiation damage. Finally, we aim to perform thermal cycling and mechanical tests on detectors and study their coupling to scintillators, in parallel with their space qualification.
On the basis of frequentist analyses of experimental constraints from electroweak precision data, (
g
−2)
μ
,
B
-physics and cosmological data, we investigate the parameters of the constrained MSSM ...(CMSSM) with universal soft supersymmetry-breaking mass parameters, and a model with common non-universal Higgs masses (NUHM1). We present
χ
2
likelihood functions for the masses of supersymmetric particles and Higgs bosons, as well as BR(
b
→
s
γ
), BR(
B
s
→
μ
+
μ
−
) and the spin-independent dark-matter scattering cross section,
σ
p
SI
. In the CMSSM we find preferences for sparticle masses that are relatively light. In the NUHM1 the best-fit values for many sparticle masses are even slightly smaller, but with greater uncertainties. The likelihood functions for most sparticle masses are cut off sharply at small masses, in particular by the LEP Higgs mass constraint. Both in the CMSSM and the NUHM1, the coannihilation region is favored over the focus-point region at about the 3-
σ
level, largely but not exclusively because of (
g
−2)
μ
. Many sparticle masses are highly correlated in both the CMSSM and NUHM1, and most of the regions preferred at the 95% C.L. are accessible to early LHC running, though high-luminosity running would be needed to cover the regions allowed at the 3-
σ
levels. Some slepton and chargino/neutralino masses should be in reach at the ILC. The masses of the heavier Higgs bosons should be accessible at the LHC and the ILC in portions of the preferred regions in the (
M
A
,tan
β
) plane. In the CMSSM, the likelihood function for BR(
B
s
→
μ
+
μ
−
) is peaked close to the Standard Model value, but much larger values are possible in the NUHM1. We find that values of
σ
p
SI
>10
−10
pb are preferred in both the CMSSM and the NUHM1. We study the effects of dropping the (
g
−2)
μ
, BR(
b
→
s
γ
),
Ω
χ
h
2
and
M
h
constraints, demonstrating that they are not in tension with the other constraints.
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