1,721,080 research outputs found
Mind the gaps: improved methods for the detection of periodicities in unevenly-sampled data
No full textThe detection of periodic signals in irregularly-sampled time series is a problem commonly encountered in astronomy. Traditional tools used for periodic searches, such as the periodogram, have poorly defined statistical properties under irregular sampling, which complicate inferring the underlying aperiodic variability used for hypothesis testing. The problem is exacerbated in the presence of stochastic variability, which can be easily mistaken by genuine periodic behaviour, particularly in the case of poorly sampled lightcurves. Here we present a method based on Gaussian Processes (GPs) modelling for period searches and characterization, specifically developed to overcome these problems. We argue that in cases of irregularly-sampled time series, GPs offer an appealing alternative to traditional periodograms, because the known distribution of the data (correlated Gaussian) allows a well-defined likelihood to be constructed. We exploit this property and draw from existing statistical methods to perform traditional likelihood ratio tests for an additional, (quasi-)periodic component, using the aperiodic variability inferred from the data as the null hypothesis. Inferring the noise from the data allows the method to be fully generalizable, with the only condition that the data can be described as a Gaussian process. We demonstrate the method by applying it to a variety of objects showing varying levels of noise and data quality. Limitations of the method are discussed and a package implementing the proposed methodology is made publicly available
Searching for energy-resolved quasi-periodic oscillations in AGN
No full textX-ray quasi-periodic oscillations (QPOs) in active galactic nucleus (AGN) allow us to probe and understand the nature of accretion in highly curved space–time, yet the most robust form of detection (i.e. repeat detections over multiple observations) has been limited to a single source to-date, with only tentative claims of single observation detections in several others. The association of those established AGN QPOs with a specific spectral component has motivated us to search the XMM–Newton archive and analyse the energy-resolved light curves of 38 bright AGNs. We apply a conservative false alarm testing routine folding in the uncertainty and covariance of the underlying broad-band noise. We also explore the impact of red-noise leak and the assumption of various different forms (power-law, broken power-law, and Lorentzians) for the underlying broad-band noise. In this initial study, we report QPO candidates in six AGNs (seven including one tentative detection in MRK 766) from our sample of 38, which tend to be found at characteristic energies and, in four cases, at the same frequency across at least two observations, indicating they are highly unlikely to be spurious in nature
Long-term X-ray/UV variability in ULXs
Full text linkThe focus of NASA's Swift telescope has been transients and
target-of-opportunity observing, resulting in many observations of
ultraluminous X-ray sources (ULXs) over the last ~20 years. For the vast
majority of these observations, simultaneous data has been obtained using both
the X-ray telescope (XRT) and the ultraviolet and optical telescope (UVOT),
providing a unique opportunity to study coupled variability between these
bands. Using a sample of ~40 ULXs with numerous repeat observations, we extract
stacked images to characterise the spatial extent of the UV-Optical emission
and extract long-term light curves to search for first-order linear
correlations between the UV and X-ray emission. We find that a small subset may
show weakly correlated joint variability, while other sources appear to display
non-linear relationships between the bands. We discuss these observations in
the context of several theoretical models: precession, irradiation of the outer
accretion disc and irradiation of the companion star. We conclude that more
complicated analysis or higher quality data may be required to accurately
constrain the nature of the joint X-ray and UV/optical emission in these
sources.Comment: 14 pages, 13 Figure
Resolved atomic lines reveal outflows in two ultraluminous X-ray sources
Full text linkUltraluminous X-ray sources are extragalactic, off-nucleus, point sources in galaxies, and have X-ray luminosities in excess of 3 × 1039 ergs per second. They are thought to be powered by accretion onto a compact object. Possible explanations include accretion onto neutron stars with strong magnetic fields, onto stellar-mass black holes (of up to 20 solar masses) at or in excess of the classical Eddington limit, or onto intermediate-mass black holes (103-105 solar masses). The lack of sufficient energy resolution in previous analyses has prevented an unambiguous identification of any emission or absorption lines in the X-ray band, thereby precluding a detailed analysis of the accretion flow. Here we report the presence of X-ray emission lines arising from highly ionized iron, oxygen and neon with a cumulative significance in excess of five standard deviations, together with blueshifted (about 0.2 times light velocity) absorption lines of similar significance, in the high-resolution X-ray spectra of the ultraluminous X-ray sources NGC 1313 X-1 and NGC 5408 X-1. The blueshifted absorption lines must occur in a fast-outflowing gas, whereas the emission lines originate in slow-moving gas around the source. We conclude that the compact object in each source is surrounded by powerful winds with an outflow velocity of about 0.2 times that of light, as predicted by models of accreting supermassive black holes and hyper-accreting stellar-mass black holes
X-ray eclipse mapping constrains the binary inclination and mass ratio of swift J1858.6-0814
No full textX-ray eclipse mapping is a promising modelling technique, capable of constraining the mass and/or radius of neutron stars (NSs) or blackholes (BHs) in eclipsing binaries and probing any structure surrounding the companion star. In eclipsing systems, the binary inclination, , and mass ratio, relate via the duration of totality, . The degeneracy between and can then be broken through detailed modelling of the eclipse profile. Here we model the eclipses of the NS low-mass X-ray binary Swift J1858.60814 utilising archival NICER observations taken while the source was in outburst. Analogous to EXO0748676, we find evidence for irradiation driven ablation of the companion's surface by requiring a layer of stellar material to surround the companion star in our modelling. This material layer extends km from the companion's surface and is likely the cause of the extended, energy-dependent and asymmetric ingress and egress that we observe. Our fits return an inclination of and a mass ratio . Using Kepler's law to relate the mass and radius of the companion star via the orbital period ( 21.3 hrs), we subsequently determine the companion to have a low mass in the range and a large radius in the range . Our results, combined with future radial velocity amplitudes measured from stellar absorption/emission lines, can place precise constraints on the component masses in this system
The black hole spin in GRS 1915+105, revisited
Full text linkWe estimate the black hole spin parameter in GRS 1915+105 using the continuum-fitting method with revised mass and inclination constraints based on the very long baseline interferometric parallax measurement of the distance to this source. We fit Rossi X-ray Timing Explorer observations selected to be accretion disk-dominated spectral states as described in McClintock et al. (2006) and Middleton et al. (2006), which previously gave discrepant spin estimates with this method. We find that, using the new system parameters, the spin in both datasets increased, providing a best-fit spin of for the Middleton et al. data and a poor fit for the McClintock et al. dataset, which becomes pegged at the BHSPEC model limit of . We explore the impact of the uncertainties in the system parameters, showing that the best-fit spin ranges from to 0.99 for the Middleton et al. dataset and allows reasonable fits to the McClintock et al. dataset with near maximal spin for system distances greater than kpc. We discuss the uncertainties and implications of these estimates.14 pages, 5 figures, Accepted for publication in the Astrophysical Journa
Angle-dependent hardening of the reprocessed spectra in ULXs powered by accretion on to neutron stars
Full text linkIt is anticipated that mass accretion rates exceeding approximately in X-ray pulsars lead to radiation-driven outflows from supercritical accretion discs. The outflows launched from the disc influence the angular distribution of X-ray radiation, resulting in geometrical beaming. The beaming, in turn, impacts the apparent luminosity of the X-ray pulsar, detectability of pulsations, and the spectral composition of the X-ray flux. We employ a straightforward geometrical model of the outflows, perform Monte Carlo simulations, and model the spectra of radiation, reprocessed by the walls of the accretion cavity formed by the outflows. We consider the reprocessed emission only; direct pulsar emission is not included in our modelling. Our results demonstrate that the spectra of reprocessed radiation depend on the actual luminosity of the central engine, the geometry of the outflows, and the viewing angle – most notably on the latter, through changing visibility of the hotter wall regions near the disc plane. The high-energy part of the reprocessed spectrum depends strongly on viewing angle (harder at lower inclinations), while the soft flux varies comparatively little with inclination. In our model, this contrast is a prediction: variable ultraluminous X-ray sources are expected to exhibit strong high-energy angle sensitivity together with comparatively modest soft-band variation, naturally arising if precession modulates the effective inclination
The high energy X-ray probe (HEX-P): studying extreme accretion with ultraluminous X-ray sources
No full textIntroduction: ultraluminous X-ray sources (ULXs) represent an extreme class of accreting compact objects: from the identification of some of the accretors as neutron stars to the detection of powerful winds travelling at 0.1–0.2 c, the increasing evidence points towards ULXs harbouring stellar-mass compact objects undergoing highly super-Eddington accretion. Measuring their intrinsic properties, such as the accretion rate onto the compact object, the outflow rate, the masses of accretor/companion-hence their progenitors, lifetimes, and future evolution-is challenging due to ULXs being mostly extragalactic and in crowded fields. Yet ULXs represent our best opportunity to understand super-Eddington accretion physics and the paths through binary evolution to eventual double compact object binaries and gravitational-wave sources.Methods: through a combination of end-to-end and single-source simulations, we investigate the ability of HEX-P to study ULXs in the context of their host galaxies and compare it to XMM-Newton and NuSTAR, the current instruments with the most similar capabilities.Results: HEX-P’s higher sensitivity, which is driven by its narrow point-spread function and low background, allows it to detect pulsations and broad spectral features from ULXs better than XMM-Newton and NuSTAR.Discussion: we describe the value of HEX-P in understanding ULXs and their associated key physics, through a combination of broadband sensitivity, timing resolution, and angular resolution, which make the mission ideal for pulsation detection and low-background, broadband spectral studies
The broadband spectral variability of Holmberg IX X-1
No full textWe present results from four new broadband X-ray observations of the extreme ultraluminous X-ray source Holmberg IX X-1 ( erg s−1), performed by Suzaku and NuSTAR in coordination. Combined with the archival data, we now have broadband observations of this remarkable source from six separate epochs. Two of these new observations probe lower fluxes than seen previously, allowing us to extend our knowledge of the broadband spectral variability exhibited. The spectra are well fit by two thermal blackbody components that dominate the emission below 10 keV, as well as a steep () power-law tail that dominates above ~15 keV. Remarkably, while the 0.3–10.0 keV flux varies by a factor of ~3 between all these epochs, the 15–40 keV flux varies by only ~20%. Although the spectral variability is strongest in the ~1–10 keV band, both of the thermal components are required to vary when all epochs are considered. We also revisit the search for iron absorption features by leveraging the high-energy NuSTAR data to improve our sensitivity to extreme velocity outflows in light of the ultra-fast outflow recently detected in NGC 1313 X-1. Iron absorption from a similar outflow along our line of sight can be ruled out in this case. We discuss these results in the context of super-Eddington accretion models that invoke a funnel-like geometry for the inner flow, and propose a scenario in which we have an almost face-on view of a funnel that expands to larger radii with increasing flux, resulting in an increasing degree of geometrical collimation for the emission from intermediate-temperature regions
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