Variations in the broad-band spectra of BL Lac objects: millimetre observations of an X-ray-selected sample     

J. A. Stevens  W. K. Gear 《Monthly notices of the Royal Astronomical Society》1999,307(2):403-412

Observations at millimetre wavelengths are presented for a representative sample of 22 X-ray-selected BL Lac objects (XBLs). This sample comprises 19 high-energy cut-off BL Lac objects (HBLs), 1 low-energy cut-off BL Lac object (LBL) and 2 'intermediate' sources. Data for LBLs, which are mostly radio-selected BL Lac objects (RBLs), are taken from the literature. It is shown that the radio–millimetre spectral indices of HBLs     are slightly steeper than those of the LBLs     . A correlation exists between α _5–230 and 230 GHz luminosity. While this correlation could be an artefact of comparing two populations of BL Lac objects with intrinsically different radio properties, it is also consistent with the predictions of existing unified schemes that relate BL Lac objects to Fanaroff–Riley class I radio galaxies.
The HBLs have significantly flatter submillimetre–X-ray spectral indices     than the LBLs     although the two intermediate sources also have intermediate values of α _230–X∼−0.9. It is argued that this difference cannot be explained entirely by the viewing-angle hypothesis and requires a difference in physical-source parameters. The α _230–X values for the HBLs are close to the canonical value found for large samples of radio sources and thus suggest that synchrotron radiation is the mechanism that produces the X-ray emission. As suggested by Padovani & Giommi, the inverse-Compton mechanism is likely to dominate in the LBLs requiring the synchrotron spectra of these sources to steepen or cut off at lower frequencies than those of the HBLs.  相似文献   

太阳的背面     

Bjorn Carey  胡德良 《海洋地质前沿》2006,22(11):40-40

在太阳上,看不到的那一面足以把你烤焦。太阳自转很慢,大约一个月才能转一周,这就意味着太阳另一面的每次活动会隐藏长达两周的时间。如果另一面正在形成一场巨大的磁暴,当太阳最终转到朝向地球时,磁暴就会携带大量的辐射袭击地球。这种情况在2003年发生过。当时意想不到的冲击  相似文献   

Phosphorus solubility mechanisms in haplogranitic aluminosilicate glass and melt: effect of temperature and aluminum content   总被引：1，自引：0，他引：1  

Bjorn O. Mysen 《Contributions to Mineralogy and Petrology》1998,133(1-2):38-50

The solubility behavior of phosphorus in glasses and melts in the system Na₂O-Al₂O₃-SiO₂-P₂O₅ has been examined as a function of temperature and Al₂O₃ content with microRaman spectroscopy. The Al₂O₃ was added (2, 4, 5, 6, and 8 mol% Al₂O₃) to melts with 80 mol% SiO₂ and ∼2 mol% P₂O₅. The compositions range from peralkaline, via meta-aluminous to peraluminous. Raman spectra were obtained of both the phosphorus-free and phosphorous-bearing glasses and melts between 25 and 1218 °C. The Raman spectrum of Al-free, P-bearing glass exhibits a characteristic strong band near 940 cm⁻¹ assigned to P=O stretching in orthophosphate complexes together with a weaker band near 1000 cm⁻¹ assigned P₂O₇ complexes. With increasing Al content, the proportion of P₂O₇ initially increases relative to PO₄ and is joined by AlPO₄ complexes which exhibit a characteristic P-O stretch mode slightly above 1100 cm⁻¹. The latter complex appears to dominate in meta-aluminosilicate glass and is the only phosphate complex in peraluminous glasses. When P-bearing peralkaline silicate and aluminosilicate glasses are transformed to supercooled melts, there is a rapid decrease in PO₄/P₂O₇ so that in the molten state, PO₄ units are barely discernible. The P₂O₇/AlPO₄ abundance ratio in peralkaline compositions increases with increasing temperature. This decrease in PO₄/P₂O₇ with increasing temperature results in depolymerization of the silicate melts. Dissolved P₂O₅ in peraluminous glass and melts forms AlPO₄ complexes only. This solution mechanism has no discernible influence on the aluminosilicate melt structure. There is no effect of temperature on this solution mechanism. Received: 7 October 1997 / Accepted: 11 May 1998  相似文献   

Predicting X-ray emission from wind-blown bubbles — limitations of fits to ROSAT spectra     

David K. Strickland  & Ian R. Stevens 《Monthly notices of the Royal Astronomical Society》1998,297(3):747-759

  相似文献   

Millimetre and submillimetre continuum observations of the core and hotspots of Cygnus A     

E. I. Robson    L. L. Leeuw    J. A. Stevens  & W. S. Holland 《Monthly notices of the Royal Astronomical Society》1998,301(4):935-940

We present millimetre photometry and submillimetre imaging of the central core and two hotspots in the radio lobes of the galaxy Cygnus A. For both hotspots and the central core, the synchrotron spectrum continues smoothly from the radio to a frequency of 677 GHz. The spectral index of the hotspots is constant over our frequency range, with a spectral index of α ≈ −1.0 ( S _ν ∝ ν^α), which is steeper than at lower frequencies and represents the emission from an aged population of electrons. The core is significantly flatter, with α = −0.6 ± 0.1, suggestive of an injected spectrum with no ageing, but some evidence for steepening exists at our highest observing frequency. Although IRAS data suggest the presence of dust in Cygnus A, our 450-μm data show no evidence of cold dust, therefore the dust component must have a temperature lying between 85 and 37 K, corresponding to dust masses of 1.4 × 10⁶ and 1.0 × 10⁸ M_⊙ respectively.  相似文献   

A two-component model of impulsive microwave burst emission consistent with soft and hard X-rays     

A. Böhme  F. Fürstenberg  J. Hildebrandt  O. Saal  A. Krüger  P. Hoyng  G. A. Stevens 《Solar physics》1977,53(1):139-155

A two-component (core-halo) emission model has been applied reconciling hard and soft X-ray burst emissions with the microwave burst radiation. The core region is represented by a nonthermal energy distribution (Maxwellian+power law tail) and assumed to be surrounded by a thermal halo. Parameters characterizing the energy distribution and emission measures have been derived numerically from soft and hard X-ray measurements. Using an artificial magnetic field model the microwave flux spectrum has been calculated on the basis of gyro-synchrotron emission and absorption by solving the equation of radiation transfer along the ray trajectories. Open parameters were used to adapt the spectrum to the radio measurements.Thus probable informations about the most appropriate magnetic field parameters as well as about the time- and frequency- dependent source diameters (yielding growth velocities of the core region during the impulsive phase) are deduced for the burst of 1972 May 18 as an example. A fit of the observed spectrum at the burst maximum is consistent with a magnetic field of 150O G at the core centre decreasing up to about 40 G at the top of the halo at a height of 50 000 km above the centre, a core density of 10¹⁰ cm^–3 decreasing to 10⁹ cm^–3 at the outer halo boundary, and a core diameter of 15 000 km (]20).Due to the simple geometry and emission process adopted,- the model refers primarily to special impulsive bursts. For the representation of broad band microwave bursts, e.g. type IV , events, a more complex source geometry and/or other variants of the emission mechanism must be invoked.  相似文献   

A test of independence for stratigraphic sequences with respect to embedded Markov chains     

Sture Holm  Irene Isaksson  Rodney Stevens 《Mathematical Geology》1986,18(6):551-561

Identification of cyclic sequences gives valuable insight into depositional associations of stratigraphic facies. An embedded Markov chain is a reasonable general model for facies transitions. But a model with independent random occurrences of facies is not an appropriate null hypothesis to be tested to show the presence of cycles because of definitional restriction in transition observations to only those between different facies. This is a common stratigraphic situation and the problem has been raised recently by several authors. We present here a test statistic for null hypothesis derived from the concept of partial independence and inherent to the model of embedded Markov processes.  相似文献   

ASCA observations of colliding stellar winds in gamma velorum     

I. R. Stevens  S. L. Skinner  F. Nagase  M. F. Corcoran  A. J. Willis  A. M. T. Pollock  K. Koyama 《Astrophysics and Space Science》1995,224(1-2):569-570

We present new high spectral resolution X-ray observations of the colliding wind binary Vel taken with the ASCA satellite. We find two spectral components, one of which is post-shock emission from the colliding winds. Spectral variability is also seen, consistent with current notions of colliding wind phenomena.  相似文献   

Changes in otolith microstructure at metamorphosis of plaice, Pleuronectes platessa L.     

Modin  Johan; Fagerholm  Bjorn; Gunnarsson  Bjorn; Pihl  Leif 《ICES Journal of Marine Science》1996,53(4):745-748

  相似文献   

Petrogenesis of contact-style PGE mineralization in the northern lobe of the Bushveld Complex: comparison of data from the farms Rooipoort,Townlands, Drenthe and Nonnenwerth   总被引：1，自引：0，他引：1  

W. D. Maier  L. de Klerk  J. Blaine  T. Manyeruke  S.-J. Barnes  M. V. A. Stevens  J. A. Mavrogenes 《Mineralium Deposita》2008,43(3):255-280

In the present study, we document the nature of contact-style platinum-group element (PGE) mineralization along >100 km of strike in the northern lobe of the Bushveld Complex. New data from the farm Rooipoort are compared to existing data from the farms Townlands, Drenthe, and Nonnenwerth. The data indicate that the nature of the contact-style mineralization shows considerable variation along strike. In the southernmost portion of the northern Bushveld, on Rooipoort and adjoining farms, the mineralized sequence reaches a thickness of 700 m. Varied-textured gabbronorites are the most common rock type. Anorthosites and pyroxenites are less common. Chromitite stringers and xenoliths of calcsilicate and shale are largely confined to the lower part of the sequence. Layering is locally prominent and shows considerable lateral continuity. Disseminated sulfides may reach ca. 3 modal % and tend to be concentrated in chromitites and melanorites. Geochemistry indicates that the rocks can be correlated with the Upper Critical Zone. This model is supported by the fact that, in a down-dip direction, the mineralized rocks transform into the UG2-Merensky Reef interval. Between Townlands and Drenthe, the contact-mineralized sequence is thinner (up to ca. 400 m) than in the South. Chromitite stringers occur only sporadically, but ultramafic rocks (pyroxenites, serpentinites, and peridotites) are common. Xenoliths of calcsilicate, shale, and iron formation are abundant indicating significant assimilation of the floor rocks. Sulfides may locally form decimeter- to meter-sized massive lenses. PGE grades tend to be higher than elsewhere in the northern Bushveld. The compositions of the rocks show both Upper Critical Zone and Main Zone characteristics. At Nonnenwerth, the mineralized interval is up to ca. 400 m thick. It consists largely of varied-textured gabbronorites, with minor amounts of igneous ultramafic rocks and locally abundant and large xenoliths of calcsilicate. Layering is mostly weakly defined and discontinuous. Disseminated sulfides (<ca. 3 modal %) occur throughout much of the sequence. Geochemistry indicates that the rocks crystallized mainly from tholeiitic magma and thus have a Main Zone signature. The implication of our findings is that contact-style PGE mineralization in the northern lobe of the Bushveld Complex cannot be correlated with specific stratigraphic units or magma types, but that it formed in response to several different processes. At all localities, the magmas were contaminated with the floor rocks. Contamination with shale led to the addition of external sulfur to the magma, whereas contamination with dolomite may have oxidized the magma and lowered its sulfur solubility. In addition to contamination, some of the magmas, notably those of Upper Critical Zone lineage present at the south-central localities, contained entrained sulfides, which precipitated during cooling and crystallization.  相似文献