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1.
In many astrophysical black hole systems, episodic jets of plasma blobs have been observed, which are much faster and more powerful than continuous jets. A magnetohydrodynamical model was proposed by Yuan et al. to study the formation of episodic jets in Sgr A*. By taking Sgr A* and a stellar mass black hole as examples, we modify the model of Yuan et al. by including the effects of relativity, and further study the relativistic motion and expansion of episodic jets of plasma blobs. Then we study the collision between two consecutive ejections in the modified model, and calculate the magnetic energy released in the collision. Our results show two consecutive blobs can collide with each other, and the released magnetic energy is more than 1050 erg,which supports the idea that a gamma-ray burst is powered by the collision of episodic jets, as suggested by Yuan & Zhang.  相似文献   

2.
We report two new sets of tidal debris nearby the Sagittarius(Sgr) tidal stream in the north Galactic cap(NGC) identified from the M giant stars in LAMOST DR2. The M giant stars located in the sky area of 210?Λ 290?, and having a distance of 10–20 kpc and [Fe/H]-0.75 show clear bimodality in their velocity distribution. We denote the two peaks as Vel-3+83 for the one within a mean velocity of–3 km s~(-1) with respect to that of the well observed Sgr leading tail at the same Λ and Vel+162+26 for the other one with a mean velocity of 162 km s~(-1) with respect to the Sgr leading tail. Although the projected Λ–V_(gsr) relation of Vel-3+83 is very similar to the Sgr leading tail, the opposite trend in the Λ–distance relation as compared to the Sgr leading tail suggests Vel-3+83 has a different 3D direction of motion with any branch of the simulated Sgr tidal stream from Law Majewski. Therefore, we propose it is new tidal debris not related to the Sgr stream. Similarly, the other substructure Vel+162+26, which is the same one as the NGC group discovered by Chou et al., also moves toward a different direction with respect to the Sgr stream, implying that it may have a different origin than the Sgr tidal stream.  相似文献   

3.
Blazars are characterized by large intensity and spectral variations across the electromagnetic spectrum It is believed that jets emerging from them are almost aligned with the line-of-sight. The majority of identified extragalactic sources in γ-ray catalogs of EGRET and Fermi are blazars. Observationally,blazars can be divided into two classes: flat spectrum radio quasars(FSRQs) and BL Lacs. BL Lacs usually exhibit lower γ-ray luminosity and harder power law spectra at γ-ray energies than FSRQs. We attempt to explain the high energy properties of FSRQs and BL Lacs from Fermi γ-ray space telescope observations. It was argued previously that the difference in accretion rates is mainly responsible for the large mismatch in observed luminosity in γ-ray. However, when intrinsic luminosities are derived by correcting for beaming effects, this difference in γ-ray luminosity between the two classes is significantly reduced. In order to explain this difference in intrinsic luminosities, we propose that spin plays an important role in the luminosity distribution dichotomy of BL Lacs and FSRQs. As the outflow power of a blazar increases with increasing spin of a central black hole, we suggest that the spin plays a crucial role in making BL Lac sources low luminous and slow rotators compared to FSRQ sources.  相似文献   

4.
We suggest a short-lived supermassive magnetar model to account for the X-ray flares following shortγ-ray bursts. In this model the central engine of the shortγ-ray bursts is a supermassive millisecond magnetar, formed in coalescence of double neutron stars. The X-ray flares are powered by the dipole radiation of the magnetar. When the magnetar has lost a significant part of its angular momentum, it collapses to a black hole and the X-ray flares cease abruptly.  相似文献   

5.
The mechanism of formation for double-peaked optical outbursts observed in blazar OJ 287 is studied. It is shown that they could be explained in terms of a lighthouse effect for superluminal optical knots ejected from the center of the galaxy that move along helical magnetic fields. It is assumed that the orbital motion of the secondary black hole in the supermassive binary black hole system induces the 12-year quasi-periodicity in major optical outbursts by the interaction with the disk around the primary black hole. This interaction between the secondary black hole and the disk of the primary black hole(e.g. tidal effects or magnetic coupling) excites or injects plasmons(or relativistic plasmas plus magnetic field) into the jet which form superluminal knots. These knots are assumed to move along helical magnetic field lines to produce the optical double-peaked outbursts by the lighthouse effect. The four double-peaked outbursts observed in 1972, 1983, 1995 and 2005 are simulated using this model. It is shown that such lighthouse models are quite plausible and feasible for fitting the double-flaring behavior of the outbursts. The main requirement may be that in OJ 287 there exists a rather long(～40–60 pc) highly collimated zone, where the lighthouse effect occurs.  相似文献   

6.
The supermassive black hole(SMBH) with a mass of 4 million M inside the radio source Sgr A* in our Galactic center is the nearest SMBH. Once S stars with a shorter period are observed, relativistic precessions especially the Lense-Thirring effect can be measured by astronomical observations at the 10 μas level in the future.An interesting but so far unaddressed problem is that the SMBH not only has spin but also spin precession like similar objects. We study the effect of such spin precession on the orbital precessions of orbiting stars. Our results show that the spin precession can produce a periodic oscillation in the precession of the star's orbital plane, but has no obvious effect on the periapse shift. For stars with an orbital period of O(0.1) yr or less, such visible oscillations occur when the SMBH's spin-precession period ranges from about a few tens of years to hundreds of years. The period of oscillation is the same as the one of the spin precession. In principle, the precession of this oscillating orbital plane can be observed and then the spin and spin precession of the nearest SMBH can be determined.  相似文献   

7.
By employing an adiabatic invariant and implementing the BohrSommerfield quantization rule,I study the quantization of a regular black hole inspired by noncommutative geometry in AdS3spacetime.The entropy spectrum as well as the horizon area spectrum of the black hole is obtained.It is shown that the spectra are discrete,and the spacing of the entropy spectrum is equidistant;in the limit r2h4θ1,the area spectrum depends on the noncommutative parameter and the cosmological constant,but the spacing of the area spectrum is equidistant up to leading order√θe 2Ml2θinθ,and is independent of the noncommutative parameter and the cosmological constant.  相似文献   

8.
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9.
Estimates of AGN Black Hole Mass and Minimum Variability Timescale   总被引：4，自引：0，他引：4  
Black hole mass is one of the fundamental physical parameters of active galactic nuclei (AGNs), for which many methods of estimation have been proposed. One set of methods assumes that the broad-line region (BLR) is gravitationally bound by the central black hole potential, so the black hole mass can be estimated from the orbital radius and the Doppler velocity. Another set of methods assumes the observed variability timescale is determined by the orbital timescale near the innermost stable orbit around the Schwarzschild black hole or the Kerr black hole, or by the characteristic timescale of the accretion disk. We collect a sample of 21 AGNs, for which the minimum variability timescales have been obtained and their black hole masses (Mσ) have been well estimated from the stellar velocity dispersion or the BLR size-luminosity relation. Using the minimum variability timescales we estimated the black hole masses for 21 objects by the three different methods, the results are denoted by Ms, Mk and Md, respectively. We compared each of them with Mσindividually and found that: (1) using the minimum variability timescale with the Kerr black hole theory leads to small differences between Mσand Mk, none exceeding one order of magnitude, and the mean difference between them is about 0.53 dex; (2) using the minimum variability timescale with the Schwarzschild black hole theory leads to somewhat larger difference between Mσand Ms: larger than one order of magnitude for 6 of the 21 sources, and the mean difference is 0.74 dex; (3) using the minimum variability timescale with the accretion disk theory leads to much larger differences between Mσand Md, for 13 of the 21 sources the differences are larger than two orders of magnitude; and the mean difference is as high as about 2.01 dex.  相似文献   

10.
Recent analysis of the long term radio light curve of the extremely variable BL Lacertae object AO 0235+16 by Raiteri et al. have revealed the presence of recurrent outbursts with a period of ～ 5.7±0.5 yr. Periodicity analysis of the optical light curve also shows evidence for a shorter period. Here we discuss whether such a behavior can be explained by a binary black hole model where the accretion disk of one of the supermassive black holes is precessing due to the tidal effects of the companion. We estimate the mass of the accreting hole and analyze constraints on the secondary mass and the orbital parameters of the system. It is possible to provide a viable interpretation of the available multiwavelength data.  相似文献   

11.
We have studied the simultaneous spectral energy distributions(SEDs) of the 2009 December flare and those of the quiescent state of blazar 3C 454.3 by constructing a multi-component model. We find that all six SEDs can be explained by a one-zone leptonic model involving synchrotron self-Compton(SSC) plus external Compton emission from an accretion disk(ECD) and that from a broad-line region(ECC). X-ray emission is dominated by the SSC mechanism, and the γ-ray spectrum is well represented by a combination of ECD and ECC processes. Our results indicate that the energy density of the magnetic field and electrons decrease with distance from the central engine, and the Doppler factor increases with the blob moving outward in the development of the 2009 December flare. The increase in the observed flux density is possibly due to the increase in the Doppler factor of the blob. The relation between the Doppler factor δb and the distance from the central black hole suggests the magnetically driven jets span a sub-pc scale, and the relation between the magnetic field B′ and the dimension of the emission region R′b is in good agreement with what is required by conservation of magnetic flux. The weak "harder-when-brighter" behavior of the γ-ray spectrum could be a result of the increase in Doppler factor during the outward motion of the blob. The parameters during the quiescent state obviously deviate from those during the flare state. We propose that the flare was likely caused by the ejection of a new blob. The gamma-ray emissions in different states are associated with the evolution of the blob.  相似文献   

12.
Carbon stars are excellent kinematic tracers of galaxies and can serve as a viable standard candle,so it is worthwhile to automatically search for them in a large amount of spectra.In this paper,we apply the efficient manifold ranking algorithm to search for carbon stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST) pilot survey,whose performance and robustness are verified comprehensively with four test experiments.Using this algorithm,we find a total of183 carbon stars,and 158 of them are new findings.According to different spectral features,our carbon stars are classified as 58 C-H stars,11 C-H star candidates,56C-R stars,ten C-R star candidates,30 C-N stars,three C-N star candidates,and four C-J stars.There are also ten objects which have no spectral type because of low spectral quality,and a composite spectrum consisting of a white dwarf and a carbon star.Applying the support vector machine algorithm,we obtain the linear optimum classification plane in the J — H versus H — K_s color diagram which can be used to distinguish C-H from C-N stars with their J — H and H — K_s colors.In addition,we identify 18 dwarf carbon stars with their relatively high proper motions,and find three carbon stars with FUV detections likely have optical invisible companions by cross matching with data from the Galaxy Evolution Explorer.In the end,we detect four variable carbon stars with the Northern Sky Variability Survey,the Catalina Sky Survey and the LINEAR variability databases.According to their periods and amplitudes derived by fitting light curves with a sinusoidal function,three of them are likely semiregular variable stars and one is likely a Mira variable star.  相似文献   

13.
Using Parikh’s tunneling method,the Hawking radiation on the apparent horizon of a Vaidya-Bonner black hole is calculated.When the back-reaction of particles is neglected,the thermal spectrum can be precisely obtained.Then,the black hole thermodynamics can be calculated successfully on the apparent horizon.When a relativistic perturbation is applied to the apparent horizon,a similar calculation can also lead to a purely thermal spectrum.The first law of thermodynamics can also be derived successfully at the...  相似文献   

14.
LAMOST Data Release 5,covering~17 000 deg~2from-10?to 80?in declination,contains 9million co-added low-resolution spectra of celestial objects,each spectrum combined from repeat exposure of two to tens of times during Oct 2011 to Jun 2017.In this paper,we present the spectra of individual exposures for all the objects in LAMOST Data Release 5.For each spectrum,the equivalent width of 60lines from 11 different elements are calculated with a new method combining the actual line core and fitted line wings.For stars earlier than F type,the Balmer lines are fitted with both emission and absorption profiles once two components are detected.Radial velocity of each individual exposure is measured by minimizingχ~2between the spectrum and its best template.The database for equivalent widths of spectral lines and radial velocities of individual spectra are available online.Radial velocity uncertainties with different stellar type and signal-to-noise ratio are quantified by comparing different exposure of the same objects.We notice that the radial velocity uncertainty depends on the time lag between observations.For stars observed in the same day and with signal-to-noise ratio higher than 20,the radial velocity uncertainty is below 5 km s~(-1),and increases to 10 km s~(-1)for stars observed in different nights.  相似文献   

15.
This paper presents a hadronic dominated jet model to investigate multiwavelength emission from the microquasar LS I + 61 303.In this scenario,we take into account evolutions of the primary particles and secondary e± pairs;these pairs are produced by the collisional interactions of the accelerated protons with the cold jet protons and the stellar wind ions.In this model,the non-thermal photons are produced by π0 decay emission,synchrotron and inverse Compton scattering processes from the primary electrons a...  相似文献   

16.
A Periodicity Analysis of the Light Curve of 3C 454.3   总被引：2，自引：0，他引：2  
We analyzed the radio light curves of 3C 454.3 at frequencies 22 and 37 GHz taken from the database of Metsahovi Radio Observatory, and found evidence of quasi-periodic activity. The light curves show great activity with very complicated non-sinusoidal variations. Two possible periods, a very weak one of 1.57±0.12 yr and a very strong one of 6.15±0.50 yr were consistently identified by two methods, the Jurkevich method and power spectrum estimation. The period of 6.15±0.50 yr is consistent with results previously reported by Ciaramella et al. and Webb et al. Applying the binary black hole model to the central structure we found black hole masses of 1.53×109M⊙and 1.86×108M⊙, and predicted that the next radio outburst is to take place in 2006 March and April.  相似文献   

17.
In this work,we have searched for quasi-periodic oscillations(QPOs)in the 15 GHz light curve of the FSRQ PKS J0805-0111 monitored by the Owens Valley Radio Observatory(OVRO)40 m telescope during the period from 2008 January 9 to 2019 May 9,using the weighted wavelet Z-transform(WWZ)and the Lomb-Scargle Periodogram(LSP)techniques.This is the first time to search for a periodic radio signal in the FSRQ PKS J0805-0111 by these two methods.Both methods consistently reveal a repeating signal with a periodicity of 3.38±0.8 yr(>99.7%confidence level).In order to determine the significance of the periods,the false alarm probability method was applied,and a large number of Monte Carlo simulations were performed.As possible explanations,we discuss a number of scenarios including the thermal instability of thin disks scenario,the spiral jet scenario and the binary supermassive black hole scenario.We expect that the binary black hole scenario,where the QPO is caused by the precession of binary black holes,is the most likely explanation.FSRQ PKS J0805-0111 thus could be a good binary black hole candidate.In the binary black hole scenario,the distance between the primary black hole and the secondary black hole is about 1.71×1016 cm.  相似文献   

18.
Motivated by the Fermi observations of someγ-ray pulsars in which the phases of radio andγ-ray peaks are almost the same,we investigate the outer gap model in a retarded dipole with a current-induced magnetic field and apply it to explain pulsedγ-ray properties of the Crab pulsar.Our results show that the observedγ-ray energy-dependent light curves,which almost align with the radio light curve and phase averaged spectrum for the Crab pulsar,are reproduced well.  相似文献   

19.
The curvature of the γ-ray spectrum in blazars may reflect the intrinsic distribution of emitting electrons, which will further give some information on the possible acceleration and cooling processes in the emitting region. The γ-ray spectra of Fermi blazars are normally fitted either by a single power-law(PL) or a log-normal(call Logarithmic Parabola, LP) form. The possible reason for this difference is not clear. We statistically explore this issue based on the different observational properties of 1419 Fermi blazars in the 3 LAC Clean Sample. We find that the γ-ray flux(100 Me V–100 Ge V) and variability index follow bimodal distributions for PL and LP blazars, where the γ-ray flux and variability index show a positive correlation. However, the distributions of γ-ray luminosity and redshift follow a unimodal distribution. Our results suggest that the bimodal distribution of γ-ray fluxes for LP and PL blazars may not be intrinsic and all blazars may have an intrinsically curved γ-ray spectrum, and the PL spectrum is just caused by the fitting effect due to less photons.  相似文献   

20.
In a previous paper, it was suggested that contamination of the nuclear luminosity by the host galaxy plays an important role in determining the parameters of the standard a disk of AGNs. Using the nuclear absolute B band magnitude instead of the total absolute B band magnitude, we have recalculated the central black hole masses, accretion rates and disk inclinations for 20 Seyfert 1 galaxies and 17 Palomar-Green (PG) quasars. It is found that a small value of a is needed for the Seyfert 1 galaxies than for the PG quasars. This difference in a possibly leads to the different properties of Seyfert 1 galaxies and quasars. Furthermore, we find most of the objects in this sample are not accreting at super-Eddington rates if we adopt the nuclear optical luminosity in our calculation.  相似文献   

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