A shared accretion instability for black holes and neutron stars.

Accretion disks around compact objects are expected to enter an unstable phase at high luminosity1. One instability may occur when the radiation pressure generated by accretion modifies the disk viscosity, resulting in the cyclic depletion and refilling of the inner disk on short timescales2. Such a scenario, however, has only been quantitatively verified for a single stellar-mass black hole3-5. Although there are hints of these cycles in a few isolated cases6-10, their apparent absence in the variable emission of most bright accreting neutron stars and black holes has been a continuing puzzle11. Here we report the presence of the same multiwavelength instability around an accreting neutron star. Moreover, we show that the variability across the electromagnetic spectrum-from radio to X-ray-of both black holes and neutron stars at high accretion rates can be explained consistently if the accretion disks are unstable, producing relativistic ejections during transitions that deplete or refill the inner disk. Such a new association allows us to identify the main physical components responsible for the fast multiwavelength variability of highly accreting compact objects.

The corona contracts in a black-hole transient.

The geometry of the accretion flow around stellar-mass black holes can change on timescales of days to months1-3. When a black hole emerges from quiescence (that is, it 'turns on' after accreting material from its companion) it has a very hard (high-energy) X-ray spectrum produced by a hot corona4,5 positioned above its accretion disk, and then transitions to a soft (lower-energy) spectrum dominated by emission from the geometrically thin accretion disk, which extends to the innermost stable circular orbit6,7. Much debate persists over how this transition occurs and whether it is driven largely by a reduction in the truncation radius of the disk8,9 or by a reduction in the spatial extent of the corona10,11. Observations of X-ray reverberation lags in supermassive black-hole systems12,13 suggest that the corona is compact and that the disk extends nearly to the central black hole14,15. Observations of stellar-mass black holes, however, reveal equivalent (mass-scaled) reverberation lags that are much larger16, leading to the suggestion that the accretion disk in the hard-X-ray state of stellar-mass black holes is truncated at a few hundreds of gravitational radii from the black hole17,18. Here we report X-ray observations of the black-hole transient MAXI J1820+07019,20. We find that the reverberation time lags between the continuum-emitting corona and the irradiated accretion disk are 6 to 20 times shorter than previously seen. The timescale of the reverberation lags shortens by an order of magnitude over a period of weeks, whereas the shape of the broadened iron K emission line remains remarkably constant. This suggests a reduction in the spatial extent of the corona, rather than a change in the inner edge of the accretion disk.

Simultaneous X-Ray and Infrared Observations of Sagittarius A*'s Variability.

Emission from Saggitarius A* is highly variable at both X-ray and infrared (IR) wavelengths. Observations over the last ~20 yr have revealed X-ray flares that rise above a quiescent thermal background about once per day, while faint X-ray flares from Sgr A* are undetectable below the constant thermal emission. In contrast, the IR emission of Sgr A* is observed to be continuously variable. Recently, simultaneous observations have indicated a rise in IR flux density around the same time as every distinct X-ray flare, while the opposite is not always true (peaks in the IR emission may not be coincident with an X-ray flare). Characterizing the behavior of these simultaneous X-ray/IR events and measuring any time lag between them can constrain models of Sgr A*'s accretion flow and the flare emission mechanism. Using 100+ hours of data from a coordinated campaign between the Spitzer Space Telescope and the Chandra X-ray Observatory, we present results of the longest simultaneous IR and X-ray observations of Sgr A* taken to date. The cross-correlation between the IR and X-ray light curves in this unprecedented data set, which includes four modest X-ray/IR flares, indicates that flaring in the X-ray may lead the IR by approximately 10-20 min with 68% confidence. However, the 99.7% confidence interval on the time-lag also includes zero, i.e., the flaring remains statistically consistent with simultaneity. Long-duration and simultaneous multi-wavelength observations of additional bright flares will improve our ability to constrain the flare timing characteristics and emission mechanisms, and must be a priority for Galactic Center observing campaigns.

The influence of a novel simulated learning environment upon student clinical subjective refraction performance: A pilot study.

BACKGROUND: Optometry students are taught the process of subjective refraction through lectures and laboratory-based practicals before progressing to supervised clinical practice. Simulated leaning environments (SLEs) form part of an emerging technology used in a range of health disciplines; however, there is limited evidence regarding the effectiveness of clinical simulators as educational tools. METHODS: Forty optometry students (20 fourth year and 20 fifth year) were assessed twice by a qualified optometrist (two examinations separated by four to eight weeks), while completing a monocular non-cycloplegic subjective refraction on the same patient with an unknown refractive error, simulated using contact lenses. Half of the students were granted access to an online simulated learning environment, The Brien Holden Vision Institute (BHVI) Virtual Refractor, and the remaining students formed a control group. The primary outcome measures at each visit were; accuracy of the clinical refraction compared to a qualified optometrist and relative to the Optometry Council of Australia and New Zealand (OCANZ) subjective refraction examination criteria. Secondary measures of interest included descriptors of student SLE engagement, student self-reported confidence levels and correlations between performance in the simulated and real-world clinical environment. RESULTS: Eighty per cent of students in the intervention group interacted with the simulated learning environment (for an average of 100 minutes); however, there was no correlation between measures of student engagement with the BHVI Virtual Refractor and speed or accuracy of clinical subjective refractions. Fifth year students were typically more confident and refracted more accurately and more quickly than fourth year students. A year group by experimental group interaction (p = 0.03) was observed for accuracy of the spherical component of refraction and post hoc analysis revealed that less experienced students exhibited greater gains in clinical accuracy following exposure to the SLE intervention. CONCLUSIONS: Short-term exposure to a SLE can positively influence clinical subjective refraction outcomes for less experienced optometry students and may be of benefit in increasing the skills of novice refractionists to levels appropriate for commencing supervised clinical interactions.

Dissecting x-ray-emitting gas around the center of our galaxy.

Most supermassive black holes (SMBHs) are accreting at very low levels and are difficult to distinguish from the galaxy centers where they reside. Our own Galaxy's SMBH provides an instructive exception, and we present a close-up view of its quiescent x-ray emission based on 3 megaseconds of Chandra observations. Although the x-ray emission is elongated and aligns well with a surrounding disk of massive stars, we can rule out a concentration of low-mass coronally active stars as the origin of the emission on the basis of the lack of predicted iron (Fe) Kα emission. The extremely weak hydrogen (H)-like Fe Kα line further suggests the presence of an outflow from the accretion flow onto the SMBH. These results provide important constraints for models of the prevalent radiatively inefficient accretion state.

Is optimal vision required for the successful execution of an interceptive task?

The importance of optimal visual function in demanding interceptive tasks is far from established. The aim of the study was to determine whether induced myopic blur and hence sub-optimal visual function would give rise to a detrimental effect on performance in the execution of an interceptive task. The batting performance of grade level cricket players was assessed facing a bowling machine whilst wearing contact lenses of four different refractive conditions (plano (nil), +1.00, +2.00 and +3.00D over-refraction), inducing increasing amounts of myopic blur. Performance for each condition was assessed based both on the shot quality against each delivery judged by a qualified cricket coach blind to each condition, along with an evaluation of the quality of ball-bat contact for each delivery. No significant change was found in batting performance with the introduction of +1.00 and +2.00D of induced myopic blur. A +3.00D over-correction was required before any significant decrease in batting performance was detected, demonstrating that batters needed to be essentially legally blind (as simulated through the use of the +3.00D over-refraction) before there was any significant measurable decrement in batting performance. We concluded that optimal visual correction is not necessarily required for optimal performance in a demanding interceptive task, and that the human perceptual-motor system is capable of compensating for marked alterations in input.

Screening for psychiatric morbidity. The pattern of psychological illness and premorbid characteristics in four chronic pain populations.

Three hundred and seventy-eight patients from 4 chronic pain populations have been examined by self-assessment questionnaire methods to estimate the amount of psychiatric morbidity present. Using the General Health Questionnaire-28 scaled version (GHQ-28) the findings for probable psychiatric illness by clinic were: for anaesthetists' pain clinics serving a mixed urban and rural population--37%; for an oral medicine facial pain clinic--30%; for a rural hospital pain clinic--37%; and for a psychiatrists' pain assessment and treatment service--51%. These findings demonstrate the effects of selection upon the psychiatric characteristics of different pain populations. On the subscales of the General Health Questionnaire the psychiatric clinic patients were significantly more depressed than those in the other 3 groups (P less than 0.001) and also showed more social dysfunction (P less than 0.001). On the Irritability/Depression and Anxiety Questionnaire (IDA), depression and inward irritability were higher in the psychiatric clinic patients (P less than 0.001) but the amount of anxiety did not differ by clinic or by diagnosis (P greater than 0.05). These findings are taken to indicate that the extent of somatic complaints and anxiety does not differentiate the majority of pain patients in pain services with psychological illness from others attending for treatment. However, in the patients who have definite psychological symptoms, depression, social dysfunction and irritability provide a characteristic pattern. The psychiatric clinic patients were demonstrably more introverted or obessional on the Hysteroid/Obsessoid Questionnaire (HOQ) than those in other clinics. Childhood experience as seen by the patients did not differ by clinic or diagnosis and did not correlate significantly with personality as measured by the HOQ. It did correlate very significantly with measures of the current mood represented by the IDA. This effect was relatively weak, permitting the inference that the major portion of those psychological abnormalities which were found to be present was related to other factors such as the occurrence of painful lesions.

Immunological studies in acquired immunodeficiency syndrome. Functional studies of lymphocyte subpopulations.

The lymphocyte transformation response in vitro to mitogens (phytohaemagglutinin, concanavalin A, and pokeweed mitogen) and antigens (purified protein derivative and tetanus) was studied in three patients with acquired immunodeficiency syndrome (AIDS), three patients with pre-AIDS, and six healthy controls before and after depletion of T4- or T8-positive cells. In controls, T8-depleted lymphocytes responded as well as peripheral blood mononuclear cells (PBMC) when monocytes were added, whereas T4-depleted cells gave about 50% of this response to mitogens and no response at all to antigens. No evidence of suppression was seen when various mixtures of T4- and T8-depleted cells were made. In particular, there was a virtually linear relationship between the percentage of T8-depleted cells and the response to antigens. The PBMC of all AIDS and pre-AIDS patients had very low or absent responses to mitogens and antigens, and except in one case, this response did not increase after depletion of T8-positive cells (and addition of monocytes), indicating that these patient cells also lack suppressor activity in this assay. However, a significantly increased response to mitogens was seen when the T8-depleted suspensions were adjusted to contain 20,000 T4-positive cells per well, but the response was still significantly lower than that of similar suspensions from controls. Thus, not only are the poor responses of PBMC from AIDS and pre-AIDS patients due to a low concentration of T4-positive cells, but the responsiveness of these cells also seems deficient. Furthermore, T8-positive patient cells also have an impaired responsiveness. Our experiments do not exclude the possibility that the low response is due to a T8-negative suppressor cell, but it seems more likely that both the T4- and the T8-positive cells are deficient and/or that there is a deficiency in accessory cells. These possibilities are currently under study.