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1.
Dubrovich  V. K.  Grachev  S. I. 《Astronomy Letters》2015,41(10):537-548

The propagation of an instantaneous burst of isotropic radiation from the time of its onset at some redshift z 0 to the time of its detection at the present epoch (at z = 0) is considered within the framework of a flat Universe. Thomson scattering by free electrons and scattering in primordial hydrogen lines (Hα, Hβ, Pα, and Pβ) are believed to be the sources of opacity, with the single-scattering albedo in the lines being calculated by taking into account the deexcitation of the upper levels of the transitions being considered under the action of background blackbody radiation. The profiles of these lines in the burst spectrum at the present epoch have been constructed for various z0 at various distances from the burst center. To a first approximation, these profiles do not depend on the burst radiation spectrum and intensity. It is shown that the lines are purely absorption ones at a sufficiently large distance, but an emission component can appear with decreasing distance, which strengthens as the distance decreases, while the absorption component weakens. The absorption depth in the combined profile can reach 2 ×10?4 for the Hα and Hβ lines and 7 × 10?6 for the Pα and Pβ lines. In this case, the relative amplitude of the temperature fluctuations lies within the range 10?7?10?9. The calculations have been performed for bursts with different characteristic initial sizes. At the same z 0, the hydrogen line profiles essentially coincide for sizes smaller than some value, and the contrast of the lines decreases with increasing burst size for greater ones.

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2.
We have performed deep, wide‐field imaging on a ∼0.4 deg2 field in the Pleiades (Melotte 22). The selected field was not yet target of a deep search for low mass stars and brown dwarfs. Our limiting magnitudes are R ∼ 22 mag and I ∼ 20 mag, sufficient to detect brown dwarf candidates down to 40 MJ. We found 197 objects, whose location in the (I, R – I) color magnitude diagram is consistent with the age and the distance of the Pleiades. Using CTK R and I as well as JHK photometry from our data and the 2MASS survey we were able to identify 7 new brown dwarf candidates. We present our data reduction technique, which enables us to resample, calibrate, and co‐add many images by just two steps. We estimate the interstellar extinction and the spectral type from our optical and the NIR data using a two‐dimensional χ2 fitting (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
CATSAT is a small, fast and cheap space mission currently funded for Phase A studies under the Student Explorer Demonstration Initiative (STEDI) by University Space Research Association. Its prime scientific objective is to determine burst distances by measuring their spectra at energies from < 500 eV to several MeV. Soft X-ray spectral measurements will be made with 2 cm2 Si Avalanche Photodiodes (APD). The spectrometer will consist of seven collimated arrays, each containing 14 APDs and covering 1 steradians. CATSAT also contains three other context instruments. The Directional Gamma Spectrometer is a NaI-PMT array which will provide burst triggering as well as spectra and directional information from > 200 keV observations. The Hard X-Ray spectrometer consists of CaF2(Eu)-PMT detectors which are optimized in the cyclotron absorption energy band. The X-ray Albedo Polarimeter consists of nine collimated NaI-PMT detectors observing the earth reflected emission. Results from the XAP will be used to determine the burst direction and to place constraints on X-ray polarization. CATSAT was designed at three universities to be built with student help in two years for a cost of $3.5M.  相似文献   

4.

The results of observations of the gravitational-wave (GW) event S190425z recorded by the LIGO/Virgo detectors with the anti-coincidence shield (ACS) of the SPI gamma-ray spectrometer onboard the INTEGRAL observatory are presented. With a high probability (>99%) it was associated with a neutron star (NS) merger in a close binary system. This is only the second event of such a type in the history of gravitational-wave observations (after GW170817). A weak gamma-ray burst, GRB190425, consisting of two pulses ～0.5 and ～5.9 s after the NS merger in the event S190425z with an a priori significance of 3.5 and 4.4σ (taken together 5.5σ) was detected by SPI-ACS. Analysis of the SPI-ACS count rate history recorded on these days (a total of ～125 ks of observations) has shown that the rate of random occurrence of two close spikes with the characteristics of GRB190425 does not exceed 6.4 × 10?5 s?1 (i.e., such events occur by chance, on average, every ～4.3 hours). Note that the time profile of GRB190425 has much in common with the profile of GRB170817A accompanying the event GW170817, that both NS mergers were the nearest (≤150 Mpc) of all the events recorded by the LIGO/Virgo detectors, and that no significant excesses of the gamma-ray flux above the background were detected in any of ～30 black hole merger events recorded to date by these detectors. No bursts of hard radiation were detected in the field of view of the SPI and IBIS/ISGRI gamma-ray telescopes onboard INTEGRAL. This, along with the absence of detection of gamma-ray emission from GRB190425 by the GBM gamma-ray burst monitor of the Fermi observatory suggesting its occultation by the Earth, allows the localization region for the source of this GWevent to be reduced significantly. The parameters Eiso and Ep for GRB190425 are estimated and compared with those for GRB170817A.

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5.
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   

6.
Lin  R. P. 《Solar physics》1987,113(1-2):217-220

We present observations of an intense solar flare hard X-ray burst on 1980 June 27, made with a balloon-borne array of liquid nitrogen-cooled germanium detectors which provided unprecedented spectral resolution (≲1 keV FWHM). The hard X-ray spectra throughout the impulsive phase burst fitted well to a double power-law form, and emission from an isothermal 108–109K plasma can be specifically excluded. The temporal variations of the spectrum indicate that the hard X-ray burst is made up of two superposed components: individual spikes lasting ∼3–15 s, whch have a hard spectrum and a break energy of 30–65 keV; and a slowly varying component characterized by a soft spectrum with a constant low-energy slope and a break energy which increases from 25 keV to ≳100 keV through the event. The double power-law shape indicates that acceleration by DC electric fields parallel to the magnetic field, similar to that occurring in the Earth's auroral zone, may be the source of the energetic electrons which produce the hard X-ray emission. The total potential drop required for flares is typically ∼102 kV compared to ∼10 kV for auroral substorms.

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7.
A total of 185 luminous blue variable star (LBV) candidates with V<18·m 5 are selected based on the results of aperture photometry. The primary selection criterion is that the prospective candidate should be a blue star with Hα emission. In order not to overlook appreciably reddened LBV candidates, we compose an additional list of 25 red (0·m35< B − V < 1·m2, V < 18·m5) emission star candidates. A comparison with the list of known variables in the M33 galaxy showed 29% of our selected candidates to be photometrically variable. We also find our list to agree well with the lists of emission-line objects obtained in earlier papers using different methods.  相似文献   

8.
We present results of an optical and near‐infrared (IR) 1.8 deg2 survey in the Pleiades open cluster to search for substellar objects. From optical I ‐band images from the CFHT and J ‐band images from the 3.5m CAHA Telescope, we identify 18 faint and very red L brown dwarf candidates, with I > 20.9 and I – J > 3.2. The follow‐up observations of nine objects in the H ‐ and K s‐bands confirm that eight belong to the IR sequence of the cluster and the proper motion measurements of seven candidates confirm that they are Pleiades members. A preliminary estimation of the substellar mass spectrum dN/ dM in the form of a power law M c–α provides α = +0.57 ± 0.14. We extrapolate this function to estimate the number of planetary mass objects that could be present in the cluster down to 1 MJup. Sensitive searches combining far red and near‐IR observations may unveal these objects in a near future. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

9.
We present results from the spectroscopic follow‐up observation of additional QSO candidates in the variability and proper motion (VPM) survey field around M92. In the previous studies, the VPMsurvey has shown to be an efficient way to select QSOs; the QSO sample was confined to the magnitude range B ≲ 19.8 with a completeness estimated to ∼ 90% for B ≲ 19.5. Here we study the faint end (B > 19.8) of the candidate list and check the completeness of the brighter (B < 19.7) QSO subsample. We first study an additional 34 brighter QSO candidates selected by FOCA UV brightness, radio detection, or optical colours. Only one new QSO was found. We show that a two‐colour search making use of FOCA 2 000 magnitudes efficiently selects QSOs, Seyfert galaxies, and NELGs. However, there is no QSO found from this method which would not have been selected by the VPM method as well. The main aim of this paper is to identify the VPM QSO candidates with 19.8 < B < 20.5. We select 53 candidates of which 50 were observed; 26 QSOs, 2 Sey1, and 2 NELGs were found. The fraction of QSOs and Sey1s among the faint high‐priority candidates amounts to 71%. The whole AGN sample comprises now 123 QSOs and 18 Sey1s. We find that the extended VPM QSO sample does not significantly differ from other optically selected samples with regard to their low‐resolution spectra and optical broad‐band colours.  相似文献   

10.
Work at Goddard is preséntly being carried out in three major areas of gamma-ray burst research: (1) A pair of simultaneously operating 0.8-m2 burst detectors were successfully balloon-borne at locations 800 miles apart on 9 May, 1975, each to atmospheric depths of 3 to 4 g cm–2, for a 20-h period of coincident data coverage. This experiment investigates the size spectrum of bursts in the 10–7 to 10–6 erg cm–2 size region where dozens of events per day are expected on a –1.5 index integral power-law extrapolation. Considerable separation in latitude was used to avoid possible atmospheric and auroral secondary effects. Its results are not yet available. This experiment is the sequel to a single balloon flight in May 1974, in which candidate events were found to fit the –1.5 spectral extrapolation, indicating the need for positive event identification. (2) A deep-space burst detector, the first spacecraft instrument built specifically for gamma-ray burst studies, was recently successfully integrated into the Helios-B space probe. Its use at distances of up to 2 AU will make possible the first high-resolution directional study of gamma-ray burst source locations. Similar modifications to several other space vehicles are also being prepared. (3) Our gamma-ray instrument on the IMP-7 satellite is presently the most sensitive burst detector still operating in orbit. Its results have shown that all measured event-average energy spectra are consistent with being alike. Using this characteristic spectrum to select IMP-7 candidate events of smaller size than those detected using other spacecraft in coincidence, a size spectrum is constructed which fits the –1.5 index power law down to 2.5×10–5 erg cm–2 per event, at an occurrence rate of about once per month.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.  相似文献   

11.
While analyzing the archival data of the INTEGRAL observatory, we detected and localized a cosmic gamma-ray burst recorded on April 28, 2006, by the IBIS/ISGRI and SPI telescopes in their fields of view. Since the burst was not revealed by the INTEGRAL burst alert system (IBAS), information about its coordinates was not distributed in time and no search for its afterglow was conducted. The burst was recorded by the KONUS/WIND and RHES SI satellites. Its 20–200-keV fluence was 2.3 × 10?6 erg cm?2, the peak flux was 3.6 × 10?7 erg cm?2 s?1 (3.9 phot. cm?2 s?1). The burst had a complex multipeaked profile and stood out among typical bursts by an increase in its hardness with time. At the flux peak, the spectrum was characterized by a photon index α ? ?1.5 and a peak energy E p ? 95 keV. The burst lasted for ～12 s, after which its afterglow decaying as a power law with an index γ ～ ?4.5 was observed at energies 15–45 keV. The spectral hardness decreased noticeably during the afterglow.  相似文献   

12.
The detection of GRB 070912 recorded in the field of view of the SPI, IBIS/ISGRI, and JEMX telescope on September 12, 2007, at 07h32m19s (UT) when analyzing the INTEGRAL archival data is reported. The burst is one of the well-localized events closest to the direction toward the Galactic center (less than from the source Sgr A*) over the entire history of burst observations. Since it was not promptly revealed by the INTEGRAL Burst Alert System (IBAS), no information about its coordinates was disseminated and no search for optical and soft X-ray afterglows was conducted. The 3–200 keV fluence was 2.8 × 10?6 erg cm?2 and the peak flux was 1.8 × 10?7 erg cm?2 s?1 (1.9 ph cm?2 s?1). The burst was also observed in the KONUS/WIND experiment in the background mode, although it was not included in the list of recorded bursts. GRB 070912 is among a limited number of events for which a broadband (3 keV-2 MeV) spectrum of X-ray and gamma-ray emission has been obtained and their evolution from the first instants to complete decay has been traced. It shows how the fast evolution of its spectrum gives rise to absorption features at energies of ～100 keV.Within the first seconds after the onset of the burst, its spectrum was a power law with a photon index of ～0.8, but it exhibited a noticeable deficit of photons at energies below 20 keV. Such an initial deficit (a delay in appearance) of X-ray photons can be explained by their “high-latitude” origin relative to the line of sight. The spectrum rapidly softened and at the decay phase was well described by a blackbody (or Wien) law. This allows the distance (redshift) to the burst source to be estimated.  相似文献   

13.
We present an RI photometric survey covering an area of 430 arcmin2 around the multiple star σ Orionis. The observations were conducted with the 0.8 m IAC‐80 Telescope at the Teide Observatory. The survey limiting R and I magnitudes are 22.5 and 21, and completeness magnitudes 21 and 20, respectively. We have selected 53 candidates from the I vs. R–I colour‐magnitude diagram (I = 14–20) that follow the previously known photometric sequence of the cluster. Adopting an age of 2–4 Myr for the cluster, we find that these objects span a mass range from 0.35 M⊙ to 0.015 M⊙. We have performed J‐band photometry of 52 candidates and Ks photometry for 12 of them, with the result that 50 follow the expected infrared sequence for the cluster, thus confirming with great confidence that the majority of the candidates are bona fide members. JHKs photometry from the Two Micron All Sky Survey (2MASS) is available for 50 of the candidates and are in good agreement with our data. Out of 48 candidates, which have photometric accuracies better than 0.1 mag in all bands, only three appear to show near‐infrared excesses. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

14.
We have applied detailed theories of gyro-synchrotron emission and absorption in a magnetoactive plasma, X-ray production by the bremsstrahlung of non-thermal electrons on ambient hydrogen, and electron relaxation in a partially ionized and magnetized gas to the solar flare burst phenomenon. The hard X-ray and microwave bursts are shown to be consistent with a single source of non-thermal electrons, where both emissions arise from electrons with energies < mc 2. Further-more, the experimental X-ray and microwave data allow us to deduce the properties of the electron distribution, and the values of the ambient magnetic field, the hydrogen density, and the size of the emitting region. The proposed model, although derived mostly from observations of the 7 July 1966 flare, is shown to be representative of this type of event.NAS-NRC Resident Research Associate.  相似文献   

15.
No absorption lines between 15 and 100 keV have been detected in burst spectra accumulated by BATSE. Almost 250 bursts have been inspected visually, and a computerized search has begun. Our simulations show that BATSE could indeed detect lines similar to those observed byGinga in strong bursts, and our tests of the detectors' actual capabilities demonstrate they function as understood. The apparent discrepancy between BATSE andGinga is not yet compelling, and therefore the BATSE nondetections do not invalidate theGinga detections. Nonetheless, the absence of BATSE line detections indicates a low line-occurrence frequency.  相似文献   

16.
We examine the recent report by Prochter et al. that gamma-ray burst (GRB) sight lines have a much higher incidence of strong Mg II absorption than quasar sight lines. We propose that the discrepancy is due to the different beam sizes of GRBs and quasars, and that the intervening Mg II systems are clumpy with the dense part of each cloudlet of a similar size as the quasars, i.e. ≲1016 cm, but bigger than GRBs. We also discuss observational predictions of our proposed model. Most notably, in some cases the intervening Mg II absorbers in GRB spectra should be seen varying, and quasars with smaller sizes should show an increased rate of strong Mg II absorbers. In fact, our prediction of variable Mg II lines in the GRB spectra has been now confirmed by Hao et al., who observed intervening Fe II and Mg II lines at z=1.48 to be strongly variable in the multi-epoch spectra of z=4.05 GRB 060206.  相似文献   

17.
The occurrence rate of type III solar bursts in the frequency range 4.9 MHz to 30 kHz is analyzed as a function of burst intensity and burst arrival direction. We find that (a) the occurrence rate of bursts falls off with increasing flux, S, according to the power law S –1.5, and (b) the distribution of burst arrival directions at each frequency shows a significantly larger number of bursts observed west of the Earth-Sun line than east of it. This western excess in occurrence rate appears to be correlated with the direction of the average interplanetary magnetic field, and is interpreted as beaming of the observed burst radiation along the magnetic field direction.Presently at the University of Maryland, College Park, Maryland.  相似文献   

18.
A small air shower array operating over many years has been used to search for ultra-high energy (UHE) gamma radiation ( 50 TeV) associated with gamma-ray bursts (GRBs) detected by the BATSE instrument on the Compton Gamma-Ray Observatory (CGRO). Upper limits for a one minute interval after each burst are presented for seven GRBs located with zenith angles < 20°. A 4.3 excess over background was observed between 10 and 20 minutes following the onset of a GRB on 11 May 1991. The confidence level that this is due to a real effect and not a background fluctuation is 99.8%. If this effect is real then cosmological models are excluded for this burst because of absorption of UHE gamma rays by the intergalactic radiation fields.  相似文献   

19.
The interpretation of the observed-ray burstV/V max statistic in terms of spatial distributions is model-dependent. Detection of-ray bursts requires the counting rate in one or more detectors to exceed a thresholdC lim determined from a time-dependent background rateB(t). The sampling depth of the burst detector is thus time-dependent, and, if burst sources are non-uniform in space, the observedV/V max distribution will be affected byB(t). We demonstrate this effect with a simple geometric distribution of standard candles and argue thatV/V max statistic without information on threshold variations is insufficient for rigorous data analysis. Peak count rates and threshold values must be given separately for all events in order to facilitate a meaningful comparison of observations with theoretical distribution models.  相似文献   

20.
P. A. Robinson 《Solar physics》1991,134(2):299-314
A new model is developed for electron-cyclotron maser emission from flaring loops, which incorporates competition between driving of the instability and maser-induced relaxation, together with interactions between small neighboring regions of unstable plasma. This results in a picture in which radiation is emitted in bursts from regions whose length scale is determined self-consistently by previous bursts, while the unstable plasma fluctuates about the point, close to marginal stability, at which driving of the instability is balanced by relaxation due to maser-induced electron diffusion. Under the conditions applicable to flaring loops, time scales of fundamental x-mode (x1) driving and saturation are approximately equal at 1 ms, resolving a (104–106)-fold discrepancy in previous models and agreeing with the observed time scales of microwave spike bursts. Saturation effects are found to be especially effective in suppressing amplification of the most strongly growing modes. This suppression enables fundamental o-mode (o1) and second-harmonic x-mode (x2) emission to compete more effectively against x1 emission for the available free energy than has previously been estimated. Consideration of mode competition, burst time scales, suppression of growth due to overlap between amplification and absorption bands, and escape of radiation through absorption layers to the observer, implies that the observed radiation probably escapes from the corona principally in the o-mode, either emitted directly as o1 radiation or mode converted from x1 emission.  相似文献   

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