Diagnosing the accretion flow in ultraluminous X-ray sources using soft X-ray atomic features

The lack of unambiguous detections of atomic features in the X-ray spectra of ultraluminous X-ray sources (ULXs) has proven a hindrance in diagnosing the nature of the accretion flow. The possible association of spectral residuals at soft energies with atomic features seen in absorption and/or emission and potentially broadened by velocity dispersion could therefore hold the key to understanding much about these enigmatic sources. Here we show for the first time that such residuals are seen in several sources and appear extremely similar in shape, implying a common origin. Via simple arguments we assert that emission from extreme colliding winds, absorption in a shell of material associated with the ULX nebula and thermal plasma emission associated with star formation are all highly unlikely to provide an origin. Whilst CCD spectra lack the energy resolution necessary to directly determine the nature of the features (i.e. formed of a complex of narrow lines or intrinsically broad lines), studying the evolution of the residuals with underlying spectral shape allows for an important, indirect test for their origin. The ULX NGC 1313 X-1 provides the best opportunity to perform such a test due to the dynamic range in spectral hardness provided by archival observations. We show through highly simplified spectral modelling that the strength of the features (in either absorption or emission) appears to anticorrelate with spectral hardness, which would rule out an origin via reflection of a primary continuum and instead supports a picture of atomic transitions in a wind or nearby material associated with such an outflow

Pathways for recent Cerrado soybean expansion : extending the soy moratorium and implementing integrated crop livestock systems with soybeans.

The Brazilian Soy Moratorium has effectively reduced forest conversion for soybeans in Amazonia. This has come at the expense of the region?s pasturelands, which have increasingly ceded space for compliant soy expansion. The question of extending the policy to the Cerrado, where recent soy expansion has come at the cost of ecologically valuable vegetation, plugs into a wider discussion on how to reconcile competing commodities on finite amounts of cleared area. Innovative management strategies that allow different land uses to coexist are urgently needed. Integrated crop-livestock systems with soybeans(ICLS)rotates beef and soy on the same area, and shows promise as a means to improve production, farmer benefit, and environmental impacts. Here we reconstruct historical land use maps to estimate Cerrado Soy Moratorium outcomes with benchmark years in 2008 and 2014, we then estimate additional production afforded by ICLS implementation between 2008 and 2014. We find that if a 2008 Cerrado Soy Moratorium were in place, 0.7 Mha of 2014 Cerrado soy area would currently be in violation of the policy. Roughly 96% of this acreage is found in Matopiba (82%) and Mato Grosso (14%)states, suggesting that adoption may have slowed recent production in these rapidly transforming soy centers, in contrast to central and southwestern Cerrado where there is more concentrated eligible expansion area. Changing the benchmark to 2014 could have added 0.7 Mha of eligible expansion area, though over 80% of these additions would be in states with the most 2008 eligible area (Distrito Federal, Mato Grosso, Maranh?o, Minas Gerais, Mato Grosso do Sul). Meanwhile, ICLS adoption could have added between 4.0 and 32 Mha of new soy land to the study area without additional clearing between 2008 and 2014, though this would depend on rigorous accompanying land zoning policy to guide implementation. The roughly 5 Mha of Cerrado soybean expansion that actually occurred between 2008 and 2014 could have been accommodated on 2008 suitable pasture area given an ICLS rotation frequency of every 6 years or less. Conservation estimates presented here represent the upper limit of what is possible, as our scenario modeling does not account for variables such as leakage, laundering, or rebound effects

Differential modulation of the N2 and P3 event-related potentials by response conflict and inhibition

Background: Developing reliable and specific neural markers of cognitive processes is essential to improve understanding of healthy and atypical brain function. Despite extensive research there remains uncertainty as to whether two electrophysiological markers of cognitive control, the N2 and P3, are better conceptualised as markers of response inhibition or response conflict. The present study aimed to directly compare the effects of response inhibition and response conflict on the N2 and P3 event-related potentials, within-subjects. Method: A novel hybrid go/no-go flanker task was performed by 19 healthy adults aged 18 to 25 years while EEG data were collected. The response congruence of a central target stimulus and 4 flanking stimuli was manipulated between trials to vary the degree of response conflict. Response inhibition was required on a proportion of trials. N2 amplitude was measured at two frontal electrode sites; P3 amplitude was measured at 4 midline electrode sites. Results: N2 amplitude was greater on incongruent than congruent trials but was not enhanced by response inhibition when the stimulus array was congruent. P3 amplitude was greater on trials requiring response inhibition; this effect was more pronounced at frontal electrodes. P3 amplitude was also enhanced on incongruent compared with congruent trials. Discussion: The findings support a role for N2 amplitude as a marker of response conflict and for the frontal shift of the P3 as a marker of response inhibition. This paradigm could be applied to clinical groups to help clarify the precise nature of impaired action control in disorders such as attention deficit/hyperactivity disorders (ADHD)

Amplitudes of X-ray Variability in Accreting Black Holes

The properties of X-ray variability from accreting black holes reveal much about conditions close to the event horizon. Observing common timing signals in many objects, suggests similarities within their accretion flows. To further this aim this thesis presents a systematic survey of the short term variability properties in 19 observations of 16 Ultraluminous X-ray Sources (ULXs) taken with XMM-Newton using the power spectra. Significant short term variability is detected in 8 observations, but 4 of those remaining have upper limits on levels of variability below those observed in Galactic Black Hole Binaries (BHBs). Suggested causes for this suppression include large scale optically thick outflows destroying correlated variability from the source, or that the variability concentrated over shorter timescales than those studied here. Tests for a positive linear correlation between the amplitude of variability (rms) and flux within an observation are presented for archival observations of 9 BHBs. Revealing that this relation is ubiquitous in the broad-band noise for all long, bright observations with sufficient variability to measure the rms. Interestingly, comparisons between the properties of the rms-flux relations over the course of many outbursts, reveal that the x-axis offsets become strongly positive as the source moves into the hard intermediate state. The presence of a linear rms-flux correlation is also found in the light curve from a ULX (NGC 5408 X-1) and in some observations of the type C QP0 from the 1998 outburst of XTE 11550-564. In the latter case the rms-flux relation is found to be dependent on the frequency of the QPO, becoming constant or even negative once the QPO moves above ~5 Hz. A possible time lag between soft and hard emission is also identified from the ULX

Amplitudes of X-ray variability in accreting black holes

The properties of X-ray variability from accreting black holes reveal much about conditions close to the event horizon. Observing common timing signals in many objects, suggests similarities within their accretion flows. To further this aim this thesis presents a systematic survey of the short term variability properties in 19 observations of 16 Ultraluminous X-ray Sources (ULXs) taken with XMM-Newton using the power spectra. Significant short term variability is detected in 8 observations, but 4 of those remaining have upper limits on levels of variability below those observed in Galactic Black Hole Binaries (BHBs). Suggested causes for this suppression include large scale optically thick outflows destroying correlated variability from the source, or that the variability concentrated over shorter timescales than those studied here. Tests for a positive linear correlation between the amplitude of variability (rms) and flux within an observation are presented for archival observations of 9 BHBs. Revealing that this relation is ubiquitous in the broad-band noise for all long, bright observations with sufficient variability to measure the rms. Interestingly, comparisons between the properties of the rms-flux relations over the course of many outbursts, reveal that the x-axis offsets become strongly positive as the source moves into the hard intermediate state. The presence of a linear rms-flux correlation is also found in the light curve from a ULX (NGC 5408 X-1) and in some observations of the type C QP0 from the 1998 outburst of XTE 11550-564. In the latter case the rms-flux relation is found to be dependent on the frequency of the QPO, becoming constant or even negative once the QPO moves above ~5 Hz. A possible time lag between soft and hard emission is also identified from the ULX.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Towards practical autonomous deep-space navigation using X-Ray pulsar timing

We investigate the feasibility of deep-space navigation using the highly stable periodic signals from X-ray pulsars in combination with dedicated instrumentation on the spacecraft: a technique often referred to as ‘XNAV’. The results presented are based on the outputs from a study undertaken for the European Space Agency. The potential advantages of this technique include increased spacecraft autonomy and lower mission operating costs. Estimations of navigation uncertainties have been obtained using simulations of different pulsar combinations and navigation strategies. We find that the pulsar PSR B1937 + 21 has potential to allow spacecraft positioning uncertainties of ~2 and ~5 km in the direction of the pulsar after observation times of 10 and 1 h respectively, for ranges up to 30 AU. This could be achieved autonomously on the spacecraft using a focussing X-ray instrument of effective area ~50 cm2 together with a high performance atomic clock. The Mercury Imaging X-ray Spectrometer (MIXS) instrument, due to be launched on the ESA/JAXA BepiColombo mission to Mercury in 2018, is an example of an instrument that may be further developed as a practical telescope for XNAV. For a manned mission to Mars, where an XNAV system could provide valuable redundancy, observations of the three pulsars PSR B1937 + 21, B1821-24 and J0437-4715 would enable a three-dimensional positioning uncertainty of ~30 km for up to 3 months without the need to contact Earth-based systems. A lower uncertainty may be achieved, for example, by use of extended observations or, if feasible, by use of a larger instrument. X-ray instrumentation suitable for use in an operational XNAV subsystem must be designed to require only modest resources, especially in terms of size, mass and power. A system with a focussing optic is required in order to reduce the sky and particle background against which the source must be measured. We examine possible options for future developments in terms of simpler, lower-cost Kirkpatrick-Baez optics. We also discuss the principal design and development challenges that must be addressed in order to realise an operational XNAV system

A variable ULX and possible IMBH candidate in M51a

Ultraluminous X-ray source (ULX)-7, in the northern spiral arm of M51, demonstrates unusual behaviour for an ULX, with a hard X-ray spectrum but very high short-term variability. This suggests that it is not in a typical ultraluminous state. We analyse the source using archival data from XMM–Newton, Chandra and NuSTAR, and by examining optical and radio data from HST and Very Large Array. Our X-ray spectral analysis shows that the source has a hard power-law spectral shape with a photon index Γ ∼ 1.5, which persists despite the source's X-ray luminosity varying by over an order of magnitude. The power spectrum of the source features a break at 6.5+0.5−1.16.5−1.1+0.5 × 10−3 Hz, from a low-frequency spectral index of α1=−0.1+0.5−0.2α1=−0.1−0.2+0.5 to a high-frequency spectral index of α2=6.5+0.05−0.14α2=6.5−0.14+0.05, making it analogous to the low-frequency break found in the power spectra of low/hard state black holes (BHs). We can take a lower frequency limit for a corresponding high-frequency break to calculate a BH mass upper limit of 1.6 × 103 M⊙. Using the X-ray/radio Fundamental Plane, we calculate another upper limit to the BH mass of 3.5 × 104 M⊙ for a BH in the low/hard state. The hard spectrum, high rms variability and mass limits are consistent with ULX-7 being an intermediate-mass BH; however we cannot exclude other interpretations of this source's interesting behaviour, most notably a neutron star with an extreme accretion rate