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1.
The nearby (d=5.0 pc) brown dwarf LP 944-20 was observed with the XMM-Newton satellite on 07 January 2001. The target was detected with the Optical Monitor (V=16.736±0.081), but it was not detected during the ≈48 ks observation with the X-ray telescopes. We determine a 3σ upper limit for the X-ray emission from this object of LX<3.1×1023 ergs·s−1, equivalent to a luminosity ratio upper limit of log(LX/Lbol)≤−6.28. This measurement improves by a factor of three the previous Chandra limit on the quiescent X-ray flux. This is the most sensitive limit ever obtained on the quiescent X-ray emission of a brown dwarf. Combining the XMM-Newton data with previous ROSAT and Chandra data, we derive flare duty cycles as a function of their luminosities. We find that very strong flares [Log(LX/Lbol)>−2.5] are very rare (less than 0.7% of the time). Flares like the one detected by Chandra [Log(LX/Lbol)=−4.1] have a duty cycle of about 6%, which is lower than the radio flare duty cycle (13%). When compared with other M dwarfs, LP 944-20 appears to be rather inactive in X-rays despite of its relative youth, fast rotation and its moderately strong activity at radio wavelengths.  相似文献   

2.
N. Hiotelis   《New Astronomy》2002,7(8):531-539
We present density profiles, that are solutions of the spherical Jeans equation, derived under the following two assumptions: (i) the coarse grained phase-density follows a power-law of radius, ρ/σ3 ∝ r−, and (ii) the velocity anisotropy parameter is given by the relation βa(r)=β1+2β2 (r/r*)/[1+(r/r*)2] where β1, β2 are parameters and r* equals twice the virial radius, rvir, of the system. These assumptions are well motivated by the results of N-body simulations. Density profiles have increasing logarithmic slopes γ, defined by γ=−d ln ρ/d ln r. The values of γ at r=10−2.5rvir, a distance where the systems could be resolved by large N-body simulations, lie in the range 1.0–1.6. These inner values of γ increase for increasing β1 and for increasing concentration of the system. On the other hand, slopes at r=rvir lie in the range 2.42–3.82. A model density profile that fits well the results at radial distances between 10−3rvir and rvir and connects kinematic and structural characteristics of spherical systems is described.  相似文献   

3.
The defining property of Soft Gamma Repeaters is the emission of short, bright bursts of X-rays and soft γ-rays. Here we present the continuum and line spectral properties of a large sample of bursts from SGR 1806-20, observed with the Proportional Counter Array (PCA) onboard the Rossi X-ray Timing Explorer (RXTE). Using 10 trail spectral models (5 single and 5 two component models), we find that the burst continua are best fitted by the single component models: cutoff power-law, optically thin bremsstrahlung, and simple power-law. Time resolved spectroscopy show that there are two absorption lines at ∼5 keV and 20 keV in some bursts. The lines are relatively narrow with 90% upper limit on the line widths of 0.5–1.5 keV for the 5 keV feature and 1–3 keV for the 20 keV feature. Both lines have considerable equivalent width of 330–850 eV for the 5 keV feature and 780–2590 eV for the 20 keV feature. We examined whether theses spectral lines are dependent upon the choice of a particular continuum model and find no such dependence. Besides, we find that the 5 keV feature is pronounced with high confidence in the cumulative joint spectrum of the entire burst sample, both in the individual detectors of the PCA and in the co-added detectors spectrum. We confront the features against possible instrumental effects and find that none can account for the observed line properties. The two features do not seem to be connected to the same physical mechanism because (1) they do not always occur simultaneously, (2) while the 5 keV feature occurs at about the same energy, the 20 keV line centroid varies significantly from burst to burst over the range 18–22 keV, and (3) the centroid of the lines shows anti-correlated red/blue shifts. The transient appearance of the features in the individual bursts and in portions of the same burst, together with the spectral evolution seen in some bursts point to a complex emission mechanism that requires further investigation.   相似文献   

4.
X-ray transition radiation can be used to measure the Lorentz factor of relativistic particles. At energies approaching γ=E/mc2=105, transition radiation detectors can be optimized by using thick (5–10 mil) foils with large (5–10 mm) spacings. This implies X-ray energies 100 keV and the use of scintillators as the X-ray detectors. Compton scattering of the X-rays out of the particle beam then becomes an important effect. We discuss the design of very high energy detectors, the use of metal radiator foils rather than the standard plastic foils, inorganic scintillators for detecting Compton scattered transition radiation, and the application to the ACCESS cosmic ray experiment.  相似文献   

5.
M. Pedani   《New Astronomy》2003,8(8):805-815
In the last three decades, the Ultra Steep Spectrum technique has been exploited by many groups since it was demonstrated that radio sources with very steep spectra (<−1.0; S ∝ ν) are good tracers of high-z radio galaxies (HzRGs; z>2). Though more than 150 HzRGs have been discovered up to now with this technique, little is known about its real effectiveness, as most of the ongoing searches still have incomplete follow-up programs. By selecting a new appropriate sample of USS sources from the MRC survey, the true searching efficiency of the USS technique has been quantitatively demonstrated for the first time in this paper. Moreover, it was compared with that of an optical search of HzRGs based on a simple cut of the galaxies r-band magnitude distribution. When no bias other than the radio-spectrum steepness is applied, the USS technique may be up to four times more efficient in selecting HzRGs with respect to an optical search. Nevertheless, when the search is limited to objects fainter than the POSS-II plates (r21), the USS technique is still 2.5 times more efficient (εUSS=0.52 vs. εOPT=0.19). For an optical search to reach a comparable efficiency it is necessary to select objects fainter than r=23, but this implies that about half of the HzRGs are lost because of the imposed magnitude bias. The advantage of the USS technique is that a 0.5 search efficiency is already reached at the POSS-II plates limit, where all the optical identification work is done without telescopes. However, this technique has the drawback that up to 40% of the HzRGs of the sample are lost simply because of the applied spectral index bias. Interestingly, the introduction of a strong angular-size bias such as θ<15″ can double the searching efficiency irrespectively of the adopted technique, but only in the case that no optical bias has been introduced first.  相似文献   

6.
We present a detailed classification of the X-ray states of Cyg X-3 based on the spectral shape and a new classification of the radio states based on the long-term correlated behaviour of the radio and soft X-ray light curves. We find a sequence of correlations, starting with a positive correlation between the radio and soft X-ray fluxes in the hard spectral state, changing to a negative one at the transition to soft spectral states. The temporal evolution can be in either direction on that sequence, unless the source goes into a very weak radio state, from which it can return only following a major radio flare. The flare decline is via relatively bright radio states, which results in a hysteresis loop on the flux–flux diagram. We also study the hard X-ray light curve, and find its overall anticorrelation with the soft X-rays. During major radio flares, the radio flux responds exponentially to the level of a hard X-ray high-energy tail. We also specify the detailed correspondence between the radio states and the X-ray spectral states. We compare our results to those of black hole and neutron star binaries. Except for the effect of strong absorption and the energy of the high-energy break in the hard state, the X-ray spectral states of Cyg X-3 closely correspond to the canonical X-ray states of black hole binaries. Also, the radio/X-ray correlation closely corresponds to that found in black hole binaries, but it significantly differs from that in neutron star binaries. Overall, our results strongly support the presence of a black hole in Cyg X-3.  相似文献   

7.
Using extensive N-body simulations we estimate redshift space power spectra of clusters of galaxies for different cosmological models (SCDM, TCDM, CHDM, ΛCDM, OCDM, BSI, τCDM) and compare the results with observational data for Abell–ACO clusters. Our mock samples of galaxy clusters have the same geometry and selection functions as the observational sample which contains 417 clusters of galaxies in a double cone of galactic latitude |b|>30° up to a depth of 240 h−1 Mpc. The power spectrum has been estimated for wave numbers k in the range 0.03k0.2 h Mpc−1. For k>kmax0.05 h Mpc−1 the power spectrum of the Abell–ACO clusters has a power-law shape, P(k)∝kn, with n≈−1.9, while it changes sharply to a positive slope at k<kmax. By comparison with the mock catalogues SCDM, TCDM (n=0.9), and also OCDM with Ω0=0.35 are rejected. Better agreement with observation can be found for the ΛCDM model with Ω0=0.35 and h=0.7 and the CHDM model with two degenerate neutrinos and ΩHDM=0.2 as well as for a CDM model with broken scale invariance (BSI) and the τCDM model. As for the peak in the Abell–ACO cluster power spectrum, we find that it does not represent a very unusual finding within the set of mock samples extracted from our simulations.  相似文献   

8.
We present high spatial resolution X-ray Chandra HRC and HST WFPC2 H α observations of the prototypical infrared-luminous galaxy NGC 6240. The central region of this system shows a remarkably complex morphology, with filaments and loops observed in the optical and X-rays. The total X-ray luminosity is dominated by the extended emission. Both nuclei are clearly detected in the HRC image and both appear to be extended. The energetics of the nuclei imply that the southern nucleus is the more plausible counterpart to the obscured active galactic nucleus. The overall spectral energy distribution of the galaxy is in good agreement with a blend of starburst and AGN components that have similar bolometric luminosities,   L bol∼5×1045 erg s-1  , with the starburst dominating the observed continuum in the near-infrared ( K band), optical and soft X-ray bands.  相似文献   

9.
From a combination of high-quality X-ray observations from the NASA Rossi X-ray Timing Explorer ( RXTE ) and IR observations from the UK Infrared Telescope (UKIRT) we show that the medium-energy X-ray (3–20 keV) and near-IR fluxes in the quasar 3C 273 are highly correlated. It is widely believed that the X-ray emission in quasars like 3C 273 arises from Compton scattering of low-energy seed photons, and our observations provide the first reliable detection of correlated variations in 3C 273 between the X-ray band and any lower energy band. For a realistic electron distribution we demonstrate that it is probable that each decade of the seed-photon distribution from the mm to IR wavebands contributes roughly equally to the medium-energy X-ray flux. However, the expected mm variations are too small to be detected above the noise, probably explaining the lack of success of previous searches for a correlation between X-ray and mm variations. In addition, we show that the IR leads the X-rays by  0.75±0.25 d  . These observations rule out the 'External Compton' emission process for the production of the X-rays.  相似文献   

10.
We review recent observational and theoretical results concerning the presence of actinide nuclei on the surfaces of old halo stars and their use as an age determinant. We present model calculations which show that the observed universality of abundances for 56<Z<75 elements in these stars does not necessarily imply a unique astrophysical site for the r-process. Neither does it imply a universality of abundances of nuclei outside of this range. In particular, we show that a variety of astrophysical r-process models can be constructed which reproduce the same observed universal r-process curve for 56<Z<75 nuclei, yet have vastly different abundances for Z≥75 and possibly Z<56 as well. This introduces an uncertainty into the use of the Th/Eu chronometer as a means to estimate the ages of the metal deficient stars. We do find, however, that the U/Th ratio is a robust chronometer. This is because the initial production ratio of U to Th is almost independent of the astrophysical nucleosynthesis environment. The largest remaining uncertainties in the U/Th initial production ratio are due to the input nuclear physics models.  相似文献   

11.
The prompt optical emission of GRB 990123 was uncorrelated to the γ-ray light curve and exhibited temporal properties similar to those of the steeply decaying, early X-ray emission observed by Swift at the end of many bursts. These facts suggest that the optical counterpart of GRB 990123 was the large-angle emission released during (the second pulse of) the burst. If the optical and γ-ray emissions of GRB 990123 have, indeed, the same origin then their properties require that (i) the optical counterpart was synchrotron emission and γ-rays arose from inverse-Compton scatterings (the 'synchrotron self-Compton model'), (ii) the peak energy of the optical-synchrotron component was at ∼20 eV and (iii) the burst emission was produced by a relativistic outflow moving at Lorentz factor  ≳450  and at a radius  ≳1015  cm, which is comparable to the outflow deceleration radius. Because the spectrum of GRB 990123 was optically thin above 2 keV, the magnetic field behind the shock must have decayed on a length-scale of  ≲1  per cent  of the thickness of the shocked gas, which corresponds to  106–107  plasma skin depths. Consistency of the optical counterpart decay rate and its spectral slope (or that of the burst, if they represent different spectral components) with the expectations for the large-angle burst emission represents the most direct test of the unifying picture proposed here for GRB 990123.  相似文献   

12.
GRB 990123 was a long, complex gamma-ray burst accompanied by an extremely bright optical flash. We find different constraints on the bulk Lorentz of this burst to be consistent with the speculation that the optical light is emission from the reverse shock component of the external shock. Motivated by this currently favoured idea, we compute the prompt reverse shock emission to be expected for bursts in which multiwavelength observations allow the physical parameters to be constrained. We find that for reasonable assumptions about the velocity of source expansion, a strong optical flash  mV≈9  was expected from the reverse shocks, which were usually found to be mildly relativistic. The best observational prospects for detecting these prompt flashes are highlighted, along with the possible reasons for the absence of optical prompt detections in ongoing observations.  相似文献   

13.
The recent detection of a transient absorption feature in the X-ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma-ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor.  相似文献   

14.
We calculate the high-energy (sub-GeV to TeV) prompt and afterglow emission of GRB 080319B that was distinguished by a naked-eye optical flash and by an unusual strong early X-ray afterglow. There are three possible sources for high-energy emission: the prompt optical and γ-ray photons IC scattered by the accelerated electrons, the prompt photons IC scattered by the early external reverse-forward shock electrons, and the higher band of the synchrotron and the synchrotron self-Compton emission of the external shock. There should have been in total hundreds of high-energy photons detectable for the Large Area Telescope onboard the Fermi satellite, and tens of photons of those with energy >10 GeV. The >10 GeV emission had a duration about twice that of the soft γ-rays. Astro-rivelatore Gamma a Immagini Leggero (AGILE) could have observed these energetic signals if it was not occulted by the Earth at that moment. The physical origins of the high-energy emission detected in GRB 080514B, GRB 080916C and GRB 081024B are also discussed. These observations seem to be consistent with the current high-energy emission models.  相似文献   

15.
Diffuse γ-rays probe the highest-energy processes at the largest scales. Here we derive model-independent constraints on the hadronic contribution to the Galactic and extragalactic γ-ray spectra at in the energy range 50 MeVEγ10 GeV. The hadronic component is dominated by emission from neutral pions, with a characteristic spectrum symmetric about mπ0/2. We exploit the well-defined properties of the pion decay spectrum to quantify the maximum pionic fraction of the observed γ-ray intensity. We find that the Galactic spectrum above 30 MeV can be atmost about 50% pionic. The maximum pionic contribution to the extragalactic spectrum is energy dependent; it also depends on the redshift range over which the sources are distributed, ranging from as low as about 20% for pions generated very recently, to as much as 90% if the pions are generated around redshift 10. The implications of these constraints for models of γ-ray and neutrino emission are briefly discussed.  相似文献   

16.
The near-infrared data of the quasar 3C273 are collected from available literature and are used to analyze its variability properties. The largest amplitudes of variations are ΔJ = 1.0, ΔH = 0.9, and ΔK = 0.86. The color indices are J − H = 0.82, J − K = 1.96, and H − K = 1.13. Analysis with the discrete correlation function (DCF) method indicates that the variability between any two infrared bands is correlated without any time lag. The relation between the color index and brightness is also investigated and the result indicates that the spectrum steepens when the source first dims, while it flattens after the source has dimmed to a some extent, suggesting that the emission of 3C 273 at near-IR consists of at least two components. Some discussions are presented.  相似文献   

17.
We present Swift observations of the black hole X-ray transient, GRO J1655−40, during the recent outburst. With its multiwavelength capabilities and flexible scheduling, Swift is extremely well suited for monitoring the spectral evolution of such an event. GRO J1655−40 was observed on 20 occasions and data were obtained by all instruments for the majority of epochs. X-ray spectroscopy revealed spectral shapes consistent with the 'canonical' low/hard, high/soft and very high states at various epochs. The soft X-ray source (0.3–10 keV) rose from quiescence and entered the low/hard state, when an iron emission line was detected. The soft X-ray source then softened and decayed, before beginning a slow rebrightening and then spending ∼3 weeks in the very high state. The hard X-rays (14–150 keV) behaved similarly but their peaks preceded those of the soft X-rays by up to a few days; in addition, the average hard X-ray flux remained approximately constant during the slow soft X-ray rebrightening, increasing suddenly as the source entered the very high state. These observations indicate (and confirm previous suggestions) that the low/hard state is key to improving our understanding of the outburst trigger and mechanism. The optical/ultraviolet light curve behaved very differently from that of the X-rays; this might suggest that the soft X-ray light curve is actually a composite of the two known spectral components, one gradually increasing with the optical/ultraviolet emission (accretion disc) and the other following the behaviour of the hard X-rays (jet and/or corona).  相似文献   

18.
Very high energy γ-rays have recently been detected from the microquasar LS I +61 303 using the MAGIC telescope. A phenomenological study on the concomitant neutrinos that would be radiated if the γ-ray emission is hadronic in origin is herein presented. Neutrino oscillations are considered, and the expected number of events in a km-scale detector such as ICECUBE is computed under different assumptions including orbital periodicity and modulation, as well as different precision in the modeling of the detector. We argue that the upper limits already imposed on the neutrino emission of LS I +61 303 using AMANDA-II and the forthcoming measurements by ICECUBE may significantly constrain – in an independent and unbiased way – the γ-ray to neutrino flux ratio, and thus the possibility of a hadronic origin of the γ-rays. The viability of hadronic models based on wind–jet interactions in the LS +61 303 system after MAGIC measurements is discussed.  相似文献   

19.
This paper gives the results of our photometric observations of ζ Aur during its 1979/80 eclipse in 6 narrow wave bands and in the UBV colours. Useful data were obtained on 31 nights covering all phases of the eclipse. Our narrow band observations enable us to determine accurately the times of the second and third contacts, and hence a new formula for the epoch Minl = JD 2444219.643+972.176E. Our observations clearly show that eclipse depth increases with decreasing wavelength. Observations in the 3940 A band agrees with the variation in the spectral K-line of Ca II in showing a high chromosphere activity during this period. Our UBV results agree well with previous measurements.  相似文献   

20.
The transient black-hole binary XTE J1118+480 exhibited dramatic rapid variability at all wavelengths which were suitably observed during its 2000 April–July outburst. We examine time-resolved X-ray, ultraviolet, optical and infrared data spanning the plateau phase of the outburst. We find that both X-ray and infrared bands show large amplitude variability. The ultraviolet and optical variability is more subdued, but clearly correlated with that seen in the X-rays. The ultraviolet, at least, appears to be dominated by the continuum, although the lines are also variable. Using the X-ray variations as a reference point, we find that the ultraviolet (UV) variability at long wavelengths occurs later than that at short wavelengths. Uncertainty in the Hubble Space Telescope timing prohibits a determination of the absolute lag with respect to the X-rays, however. The transfer function is clearly not a delta-function, exhibiting significant repeatable structure. For the main signal we can rule out an origin in reprocessing on the companion star – the lack of variation in the lags is not consistent with this, given a relatively high orbital inclination. Weak reprocessing from the disc and/or companion star may be present, but is not required, and another component must dominate the variability. This could be variable synchrotron emission correlated with X-ray variability, consistent with our earlier interpretation of the infrared (IR) flux as due to synchrotron emission rather than thermal disc emission. In fact, the broad-band energy distribution of the variability from IR to X-rays is consistent with expectations of optically thin synchrotron emission. We also follow the evolution of the low-frequency quasi-periodic oscillation in X-rays, UV, and optical. Its properties at all wavelengths are similar, indicating a common origin.  相似文献   

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