scholarly journals Measuring the density structure of an accretion hot spot

Nature ◽  
2021 ◽  
Vol 597 (7874) ◽  
pp. 41-44
Author(s):  
C. C. Espaillat ◽  
C. E. Robinson ◽  
M. M. Romanova ◽  
T. Thanathibodee ◽  
J. Wendeborn ◽  
...  
Keyword(s):  
Hot Spots ◽  
Hot Spot ◽  
Time Lag ◽  
Optical Emission ◽  
Stellar Surface ◽  
Radial Density ◽  
Coronal Emission ◽  
X Ray ◽  
Emit Radiation ◽  
Magnetospheric Accretion

AbstractMagnetospheric accretion models predict that matter from protoplanetary disks accretes onto stars via funnel flows, which follow stellar magnetic field lines and shock on the stellar surfaces1–3, leaving hot spots with density gradients4–6. Previous work has provided observational evidence of varying density in hot spots7, but these observations were not sensitive to the radial density distribution. Attempts have been made to measure this distribution using X-ray observations8–10; however, X-ray emission traces only a fraction of the hot spot11,12 and also coronal emission13,14. Here we report periodic ultraviolet and optical light curves of the accreting star GM Aurigae, which have a time lag of about one day between their peaks. The periodicity arises because the source of the ultraviolet and optical emission moves into and out of view as it rotates along with the star. The time lag indicates a difference in the spatial distribution of ultraviolet and optical brightness over the stellar surface. Within the framework of a magnetospheric accretion model, this finding indicates the presence of a radial density gradient in a hot spot on the stellar surface, because regions of the hot spot with different densities have different temperatures and therefore emit radiation at different wavelengths.

scholarly journals Chandra Observations of the Guitar Nebula

2003 ◽  
Vol 214 ◽  
pp. 135-136 ◽  
Author(s):  
Diane S. Wong ◽  
James M. Cordes ◽  
Shami Chatterjee ◽  
Ellen G. Zweibel ◽  
John P. Finley ◽  
...  
Keyword(s):  
Proper Motion ◽  
Hot Spot ◽  
Optical Emission ◽  
Position Angle ◽  
Bow Shock ◽  
Radio Pulsar ◽  
X Ray ◽  
Multi Wavelength ◽  
Pulsar Wind

As part of a multi-wavelength study, we report on a 50 ks Chandra/ACIS observation of the Guitar Nebula, a bow shock nebula associated with the radio pulsar B2224+65. We see a “hot spot” at the tip of the bow shock. We also notice a “jet” of X-ray emission at position angle (PA) −69°. However, the proper motion of the pulsar and the axis of optical emission is at PA 52°.1. We discuss the resulting interpretations of the relativistic pulsar wind and the surrounding ISM.

scholarly journals The frequency of extreme X-ray variability for radio-quiet quasars

2020 ◽  
Vol 498 (3) ◽  
pp. 4033-4050
Author(s):  
John D Timlin ◽  
W N Brandt ◽  
S Zhu ◽  
H Liu ◽  
B Luo ◽  
...  
Keyword(s):  
Time Scales ◽  
Time Scale ◽  
Physical Mechanism ◽  
Optical Emission ◽  
Occurrence Rate ◽  
High Luminosity ◽  
Coronal Emission ◽  
Similar Time ◽  
X Ray ◽  
Random Fluctuations

ABSTRACT We analyse 1598 serendipitous  Chandra X-ray observations of 462 radio-quiet quasars to constrain the frequency of extreme amplitude X-ray variability that is intrinsic to the quasar corona and innermost accretion flow. The quasars in this investigation are all spectroscopically confirmed, optically bright (mi ≤ 20.2), and contain no identifiable broad absorption lines in their optical/ultraviolet spectra. This sample includes quasars spanning z ≈ 0.1–4 and probes X-ray variability on time-scales of up to ≈12 rest-frame years. Variability amplitudes are computed between every epoch of observation for each quasar and are analysed as a function of time-scale and luminosity. The tail-heavy distributions of variability amplitudes at all time-scales indicate that extreme X-ray variations are driven by an additional physical mechanism and not just typical random fluctuations of the coronal emission. Similarly, extreme X-ray variations of low-luminosity quasars seem to be driven by an additional physical mechanism, whereas high-luminosity quasars seem more consistent with random fluctuations. The amplitude at which an X-ray variability event can be considered extreme is quantified for different time-scales and luminosities. Extreme X-ray variations occur more frequently at long time-scales (Δt ≳ 300 d) than at shorter time-scales and in low-luminosity quasars compared to high-luminosity quasars over a similar time-scale. A binomial analysis indicates that extreme intrinsic X-ray variations are rare, with a maximum occurrence rate of $\lt 2.4{{\ \rm per\ cent}}$ of observations. Finally, we present X-ray variability and basic optical emission-line properties of three archival quasars that have been newly discovered to exhibit extreme X-ray variability.

scholarly journals Multiwavelength power-spectrum analysis of NGC 5548

2020 ◽  
Vol 499 (2) ◽  
pp. 1998-2006
Author(s):  
C Panagiotou ◽  
I E Papadakis ◽  
E S Kammoun ◽  
M Dovčiak
Keyword(s):  
Power Spectrum ◽  
Time Lag ◽  
Optical Emission ◽  
X Rays ◽  
Short Term ◽  
X Ray ◽  
Uv Emission ◽  
Accretion Rates ◽  
Measured Time ◽  
Ngc 5548

ABSTRACT NGC 5548 was recently monitored intensively from NIR to X-rays as part of the STORM campaign. Its disc emission was found to lag behind the observed X-rays, while the measured time lag was increasing with wavelength. These results are consistent with the assumption that short-term variability in AGN emission is driven by the X-ray illumination of the accretion disc. In this work, we studied the power spectrum of UV/optical and X-ray emission of NGC 5548, using the data of the STORM campaign as well as previous Swift data, in order to investigate the relation between the UV/optical and X-ray variability and to examine its consistency with the above picture. We demonstrate that even the power spectrum results are compatible with a standard disc being illuminated by X-rays, with low accretion rates, but the details are not entirely consistent with the results from the modelling of the ‘τ versus λ’ relation. The differences indicate that the inner disc might be covered by a ‘warm corona’ which does not allow the detection of UV/optical emission from the inner disc. Finally, we found strong evidence that the UV emission of NGC 5548 is not stationary.

scholarly journals Experimental Observations of Thermal Spikes in Microwave Processing of Ceramic Oxide Fibers

1994 ◽  
Vol 347 ◽  
Author(s):  
Gerald J. Vogt ◽  
Wesley P. Unruh ◽  
J. R. Thomas
Keyword(s):  
Microwave Heating ◽  
Hot Spots ◽  
Hot Spot ◽  
Single Mode ◽  
Microwave Cavity ◽  
X Ray Diffraction ◽  
Thermal Spike ◽  
X Ray ◽  
Heating Behavior ◽  
Fiber Processing

ABSTRACTMicrowave heating of alumina/silica fiber tows in a single-mode microwave cavity at 2.45 GHz has produced a surprising thermal spike behavior on the fiber bundles. During a thermal spike, a “hot spot” on the tow brightens rapidly, persists for a few seconds, and then rapidly extinguishes. A hot spot can encompass the entire tow in the cavity or just a localized portion of the tow. Some local hot spots propagate along the fiber. Thermal spikes are triggered by relatively small (<15%) increases in power, thus having obvious implications for the development of practical microwave fiber processing systems. A tow can be heated through several successive thermal spikes, after which the tow is left substantially cooler than it was originally, although the applied microwave electric field is much larger. X-ray diffraction studies show that after each temperature spike there is a partial phase transformation of the tow material into mullite. After several excursions the tow has been largely transformed to the new, less lossy phase and is more difficult to heat. Heating experiments with Nextel 550 tows are examined for a plausible explanation of this microwave heating behavior.

X‐Ray and Optical Emission from Radio Hot Spots of Powerful Quasars

2005 ◽  
Vol 630 (2) ◽  
pp. 721-728 ◽  
Author(s):  
F. Tavecchio ◽  
R. Cerutti ◽  
L. Maraschi ◽  
R. M. Sambruna ◽  
J. K. Gambill ◽  
...  
Keyword(s):  
Hot Spots ◽  
Optical Emission ◽  
X Ray

New Localization Technique for Breast Cancer Biopsy: Mammotome Guidance with Imaging Probe

2002 ◽  
Vol 88 (3) ◽  
pp. S37-S39 ◽  
Author(s):  
A Soluri ◽  
R Scafè ◽  
F Falcini ◽  
R Sala ◽  
N Burgio ◽  
...  
Keyword(s):  
High Resolution ◽  
Hot Spots ◽  
Hot Spot ◽  
Radioguided Surgery ◽  
Exact Matching ◽  
Heterogeneous Distribution ◽  
X Ray ◽  
Imaging Probe ◽  
Stereotactic Core Biopsy ◽  
Cancer Biopsy

Aims and Background The “Imaging probe” (IP) is a small, portable, high-resolution gamma camera to be used in radioguided surgery. The present work discusses a special prototype designed for guiding biopsies. The IP was mounted to a Fischer digital X-ray stereotactic core biopsy system in such a way that biopsy could be guided simultaneously by X-ray stereotaxis and 99mTc-Sestamibi (MIBI) images from IP. Methods The IP field of view was 22.8 × 22.8 mm2, with a spatial resolution of approx. 2.5 mm. We used off-line software for image fusion on a dedicated Pentium III portable PC. It was matched with a Fischer digital X-ray stereotactic biopsy system dedicated to direct the mammotome towards breast opacities. The operator was allowed to slightly correct the direction of the mammotome needle taking into account stereotactic X-ray, scintigraphic and fused images. Biopsy samples were counted by IP before they were sent to the pathologist. Results High-resolution IP scintigraphy showed substantial, though not exact, matching between MIBI hot spots and X-ray opacities. More than one hot spot was detected even in the smallest (0.6 cm) lesion. Post-biopsy scintigraphy showed absence of significant hot spots in two patients, whereas in the third patient one of the three hot spots was still partially present. All lesions showed cancer on histological examination. Conclusions Measurement of radioactivity in biopsy specimens confirmed the heterogeneous distribution of radioactivity within cancers that IP had detected before biopsy.

scholarly journals CXOU J160103.1–513353: another central compact object with a carbon atmosphere?

2018 ◽  
Vol 618 ◽  
pp. A76 ◽  
Author(s):  
V. Doroshenko ◽  
V. Suleimanov ◽  
A. Santangelo
Keyword(s):  
Neutron Star ◽  
Hot Spots ◽  
Supernova Remnant ◽  
Compact Object ◽  
Hydrogen Atmosphere ◽  
Stellar Surface ◽  
Entire Sample ◽  
X Ray ◽  
Upper Limits ◽  
Two Component

We report on the analysis of XMM-Newton observations of the central compact object CXOU J160103.1–513353 located in the center of the non-thermally emitting supernova remnant (SNR) G330.2+1.0. The X-ray spectrum of the source is well described with either single-component carbon or two-component hydrogen atmosphere models. In the latter case, the observed spectrum is dominated by the emission from a hot component with a temperature ∼3.9 MK, corresponding to the emission from a hotspot occupying ∼1% of the stellar surface (assuming a neutron star with mass M  =  1.5 M⊙, radius of 12 km, and distance of ∼5 kpc as determined for the SNR). The statistics of the spectra and obtained upper limits on the pulsation amplitude expected for a rotating neutron star with hot spots do not allow us to unambiguously distinguish between these two scenarios. We discuss, however, that while the non-detection of the pulsations can be explained by the unfortunate orientation in CXOU J160103.1–513353, this is not the case when the entire sample of similar objects is considered. We therefore conclude that the carbon atmosphere scenario is more plausible.

scholarly journals Multi-Wavelength Predictions of the DIM

2004 ◽  
Vol 194 ◽  
pp. 257-257
Author(s):  
M. R. Schreiber ◽  
J.-M. Hameury ◽  
J.-P. Lasota
Keyword(s):  
Boundary Layer ◽  
White Dwarf ◽  
Hot Spot ◽  
Optical Emission ◽  
Local Spectrum ◽  
X Ray ◽  
Multi Wavelength ◽  
On Line ◽  
Extreme Uv ◽  
Reasonable Model

Using the disc instability model (DIM) and a simple but reasonable model for the X-ray, extreme UV, UV and optical emission we investigate the multi-wavelength properties of dwarf novae. We discuss the predictions of the model in the context of the observationally best studied systems, i.e. SS Cyg and VW Hyi. We use the version of the DIM described in Buat-Ménard et al. (2001). The local spectrum of the emission from the disc is assumed to be given by Kurucz (1993,VizieR On-line Data Catalog). We also take into account emission from the (irradiated) secondary, the white dwarf, the hot spot, and the boundary layer.

scholarly journals Observations of accretion shocks

2007 ◽  
Vol 3 (S243) ◽  
pp. 103-114
Author(s):  
David R. Ardila
Keyword(s):  
Transition Region ◽  
Hot Spot ◽  
Line Emission ◽  
High Plasma ◽  
Stellar Surface ◽  
Surface Radiation ◽  
Continuum Emission ◽  
X Ray ◽  
Accretion Shock ◽  
Accretion Shocks

AbstractI review our current understanding of accretion shocks in classical T Tauri stars (CTTs), from a UV and X-ray perspective. The region of the accretion shock is a good candidate as a source of UV transition region lines from Li/Na-like ions, which are stronger in CTTs than in naked atmospheres. Disk gas captured by the stellar magnetic field produces a strong radiative shock upon falling on the stellar surface. Radiation from the shock creates a radiative precursor and heats the stellar surface resulting in a hot spot. Stellar and shock models indicate that unless the post-shock column is very large, it will be buried on the stellar photosphere. Models of the continuum emission produced by this configuration can roughly reproduce the observed excess spectra down to 1650 Å. Transition region lines in CTTs are broad, very variable, and present blueshifted, centered, and redshifted centroids. Detailed models of the line emission have so far failed to reproduce the fluxes, line shapes, and line ratios. High resolution X-ray line observations indicate the presence of larger amounts of cool plasma in CTTs with respect to WTTs. Observations of density sensitive line ratios of He-like ions suggest high plasma densities, as expected from lines originating in the accretion shock. For most stars, the interpretation of these ratios in terms of density remains equivocal due to the presence of the strong UV continuum.

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