Ulugh Beg Astronomical Institute

We study the shadows cast by the different types of rotating regular black holes viz. Ay\'on-Beato-Garc\'{\i}a (ABG), Hayward, and Bardeen. These black holes have in addition to the total mass ($M$) and rotation parameter ($a$), different parameters as electric charge ($Q$), deviation parameter ($g$), and magnetic charge (${g}_{*}$). Interestingly, the size of the shadow is affected by these parameters in addition to the rotation parameter. We found that the radius of the shadow in each case decreases monotonically, and the distortion parameter increases when the values of these parameters increase. A comparison with the standard Kerr case is also investigated. We have also studied the influence of the plasma environment around regular black holes to discuss its shadow. The presence of the plasma affects the apparent size of the regular black hole's shadow to be increased due to two effects: (i) gravitational redshift of the photons and (ii) radial dependence of plasma density.

We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to 2009 September, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A, and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host-galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z = 1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, are weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) is very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at 1 day after the GRB in the z = 1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without, reveals no indication that the former are statistically significantly more luminous. Furthermore, we propose the existence of an upper ceiling on afterglow luminosities and study the luminosity distribution at early times, which was not accessible before the advent of the Swift satellite. Most GRBs feature afterglows that are dominated by the forward shock from early times on. Finally, we present the first indications of a class of long GRBs, which form a bridge between the typical high-luminosity, high-redshift events and nearby low-luminosity events (which are also associated with spectroscopic supernovae) in terms of energetics and observed redshift distribution, indicating a continuous distribution overall.

The shadow of a rotating non-Kerr black hole has been studied, and it was shown that in addition to the specific angular momentum $a$, the deformation parameter of non-Kerr spacetime essentially deforms the shape of the black hole shadow. For a given value of the black hole spin parameter $a$, the presence of a deformation parameter $ϵ$ reduces the shadow and enlarges its deformation with respect to the one in the Kerr spacetime. Optical features of the rotating non-Kerr black hole in terms of rotation of the polarization vector along null congruences have been studied. A comparison of the obtained theoretical results on the polarization angle with the observational data on Faraday rotation measurements provides the upper limit for the dimensionless deformation parameter as $ϵ\ensuremath{\le}19$.

Context.Accretion and ejection are complex and related processes that vary on various timescales in young stars.

A black hole casts a shadow as an optical appearance because of its strong gravitational field. We study the shadow cast by the five-dimensional Myers-Perry black hole with equal rotation parameters. We demonstrate that the null geodesic equation can be integrated, which provides us an opportunity to investigate the shadow cast by a black hole. The shadow of a black hole is found to be a dark zone covered by a deformed circle. Interestingly, the shapes of the black hole shadow are more distorted and the size decreases for larger black hole spins. Interestingly, it turns out that, for fixed values of the rotation parameter, the shadow is slightly smaller and less deformed than for its four-dimensional Kerr black hole counterpart. Further, the shadow of the five-dimensional Kerr black hole is concentric deformed circles. The effect of the rotation parameter on the shape and size of a naked singularity shadow is also analyzed.

Based on the Newman-Janis algorithm, the Ay\'on-Beato-Garc\'{\i}a spacetime metric [Phys. Rev. Lett. 80, 5056 (1998)] of the regular spherically symmetric, static, and charged black hole has been converted into rotational form. It is shown that the derived solution for rotating a regular black hole is regular and the critical value of the electric charge for which two horizons merge into one sufficiently decreases in the presence of the nonvanishing rotation parameter $a$ of the black hole.

We have studied photon motion around axially symmetric rotating Kerr black holes in the presence of a plasma with radial power-law density. It is shown that in the presence of a plasma, the observed shape and size of the shadow changes depending on the (i) plasma parameters, (ii) black hole spin, and (iii) inclination angle between the observer plane and the axis of rotation of the black hole. In order to extract the pure effect of the plasma influence on the black hole image, the particular case of the Schwarzschild black hole has also been investigated and it has been shown that the photon sphere around the spherically symmetric black hole is left unchanged under the plasma influence; however, the Schwarzschild black hole shadow size in the plasma is reduced due to the refraction of the electromagnetic radiation in the plasma environment of the black hole. The study of the energy emission from the black hole in plasma environment shows that in the presence of the plasma the maximal energy emission rate from the black hole decreases.

A large international effort is under way to assess the presence of a shadow in the radio emission from the compact source at the centre of our Galaxy, Sagittarius A * (Sgr A * ). If detected, this shadow would provide the first direct evidence of the existence of black holes and that Sgr A * is a supermassive black hole. In addition, the shape of the shadow could be used to learn about extreme gravity near the event horizon and to determine which theory of gravity better describes the observations. The mathematical description of the shadow has so far used a number of simplifying assumptions that are unlikely to be met by the real observational data. We here provide a general formalism to describe the shadow as an arbitrary polar curve expressed in terms of a Legendre expansion. Our formalism does not presume any knowledge of the properties of the shadow, e.g. the location of its centre, and offers a number of routes to characterize the distortions of the curve with respect to reference circles. These distortions can be implemented in a coordinate-independent manner by different teams analysing the same data. We show that the new formalism provides an accurate and robust description of noisy observational data, with smaller error variances when compared to previous approaches for the measurement of the distortion.

We present UBVRI light curves of BL Lacertae from May 2000 to January 2001, obtained by 24 telescopes in 11 countries. More than 15 000 observations were performed in that period, which was the extension of the Whole Earth Blazar Telescope (WEBT) campaign originally planned for July–August 2000. The exceptional sampling reached allows one to follow the flux behaviour in fine detail. Two different phases can be distinguished in the light curves: a first, relatively low-brightness phase is followed by an outburst phase, after a more than brightening in a few weeks. Both the time duration (about ) and the variation amplitude (roughly ) are similar in the two phases. Rapid flux oscillations are present all the time, involving variations up to a few tenths of mag on hour time scales, and witnessing an intense intraday activity of this source. In particular, a half-mag brightness decrease in about was detected on August 8–9, 2000, immediately followed by a ~ brightening in . Colour indexes have been derived by coupling the highest precision B and R data taken by the same instrument within and after subtracting the host galaxy contribution from the fluxes. The 620 indexes obtained show that the optical spectrum is weakly sensitive to the long-term trend, while it strictly follows the short-term flux behaviour, becoming bluer when the brightness increases. Thus, spectral changes are not related to the host galaxy contribution, but they are an intrinsic feature of fast flares. We suggest that the achromatic mechanism causing the long-term flux base-level modulation can be envisaged in a variation of the relativistic Doppler beaming factor, and that this variation is likely due to a change of the viewing angle. Discrete correlation function (DCF) analysis reveals the existence of a characteristic time scale of variability of ~ in the light curve of the core WEBT campaign, while no measurable time delay between variations in the B and R bands is found.

Context.The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting more than 1 year and culminating in spring 2005. The maximum brightness detected was , which represents the most luminous quasar state thus far observed ().

The Schwarzschild–de Sitter space–time describes the gravitational field of a spherically symmetric mass in a universe with cosmological constant Λ. Based on this space–time we calculate Solar system effects like gravitational redshift, light deflection, gravitational time delay, perihelion shift, geodetic or de Sitter precession, as well as the influence of Λ on a Doppler measurement, used to determine the velocity of the Pioneer 10 and 11 spacecraft. For Λ=Λ0∼10−52m−2 the cosmological constant plays no role for all of these effects, while a value of Λ∼−10−37m−2, if hypothetically held responsible for the Pioneer anomaly, is not compatible with the perihelion shift.

We study the shadow of the rotating black hole with quintessential energy (i) in vacuum, (ii) in the presence of plasma with radial power-law density. For the vacuum case, the quintessential field parameter of the rotating black hole significantly changes the shape of the shadow. With increasing quintessential field parameter, the radius of the shadow also increases. With the increase of the radius of the shadow of the rotating black hole, the quintessential field parameter causes decrease of the distortion of the shadow shape: in the presence of the quintessential field parameter, the shadow of the fast rotating black hole becomes too close to the circle. We assume the distant observer of the black hole shadow to be located near the so-called static radius where the gravitational attraction of the black hole is just balanced by the cosmic repulsion. The shape and size of the shadow of quintessential rotating black hole surrounded by plasma depends on (i) plasma parameters, (ii) black hole spin and (iii) quintessential field parameter. With the increase of the plasma refraction index, the apparent radius of the shadow increases. However, for the large values of the quintessential field parameter, the change of the black hole shadow shape due to the presence of plasma is not significant, i.e. the effect of the quintessential field parameter dominates over the plasma effect.

Analytical solutions of Maxwell equations in background spacetime of a black hole in a braneworld immersed in an external uniform magnetic field have been found. The influence of both magnetic and brane parameters on the effective potential of the radial motion of a charged test particle around a slowly rotating black hole in a braneworld immersed in a uniform magnetic field has been investigated by using the Hamilton-Jacobi method. An exact analytical solution for dependence of the radius of the innermost stable circular orbits (ISCO) ${r}_{\mathrm{ISCO}}$ from the brane parameter for the motion of a test particle around a nonrotating isolated black hole in a braneworld has been derived. It has been shown that the radius ${r}_{\mathrm{ISCO}}$ is monotonically growing with the increase of the module of the brane tidal charge. A comparison of the predictions on ${r}_{\mathrm{ISCO}}$ of the braneworld model and of the observational results of ISCO from relativistic accretion disks around black holes provided the upper limit for the brane tidal charge $\ensuremath{\lesssim}{10}^{9}\text{ }\text{ }{\mathrm{cm}}^{2}$.

We construct regular rotating black hole and no-horizon spacetimes based on the recently introduced spherically symmetric generic regular black hole spacetimes related to electric or magnetic charge under nonlinear electrodynamics coupled to general relativity that for special values of the spacetime parameters reduce to the Bardeen and Hayward spacetimes. We show that the weak and strong energy conditions are violated inside the Cauchy horizons of these generic rotating black holes. We give the boundary between the rotating black hole and no-horizon spacetimes and determine the black hole horizons and the boundary of the ergosphere. We introduce the separated Carter equations for the geodesic motion in these rotating spacetimes. For the most interesting new class of the regular spacetimes, corresponding for magnetic charges to the Maxwell field in the weak field limit of the nonlinear electrodynamics, we determine the structure of the circular geodesics and discuss their properties. We study the epicyclic motion of a neutral particle moving along the stable circular orbits around the ``Maxwellian'' rotating regular black holes. We show that epicyclic frequencies measured by the distant observers and related to the oscillatory motion of the neutral test particle along the stable circular orbits around the rotating singular and regular Maxwellian black holes are always smaller than ones in the Kerr spacetime.

BL Lacertae has been the target of four observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration. In this paper we present light curves obtained by the WEBT from 1994 to 2002, including the last, extended BL Lac 2001 campaign. A total of about 7500 optical observations performed by 31 telescopes from Japan to Mexico have been collected, to be added to the ~ observations of the BL Lac Campaign 2000. All these data allow one to follow the source optical emission behaviour with unprecedented detail. The analysis of the colour indices reveals that the flux variability can be interpreted in terms of two components: longer-term variations occurring on a few-day time scale appear as mildly-chromatic events, while a strong bluer-when-brighter chromatism characterizes very fast (intraday) flares. By decoupling the two components, we quantify the degree of chromatism inferring that longer-term flux changes imply moving along a ~ bluer-when-brighter slope in the versus R plane; a steeper slope of ~ would distinguish the shorter-term variations. This means that, when considering the long-term trend, the B-band flux level is related to the R-band one according to a power law of index ~. Doppler factor variations on a “convex" spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism.

Can the observation of the ``shadow'' allow us to distinguish a black hole from a more exotic compact object? We study the motion of photons in a class of vacuum static axially symmetric space-times that is continuously linked to the Schwarzschild metric through the value of one parameter that can be interpreted as a measure of the deformation of the source. We investigate the lensing effect and shadow produced by the source with the aim of comparing the expected image with the shadow of a Schwarzschild black hole. In the context of astrophysical black holes, we found that it may not be possible to distinguish an exotic source with small deformation parameter from a black hole. However, as the deformation increases, noticeable effects arise. Therefore, the future more precise measurement of the shadow of astrophysical black hole candidates would in principle allow one to put constraints on the deviation of the object from spherical symmetry.

Context. The quasar 3C 279 is among the most extreme blazars in terms of luminosity and variability of flux at all wavebands. Its variations in flux and polarization are quite complex and therefore require intensive monitoring observations at multiple wavebands to characterise and interpret the observed changes.

Full list of authors: Raiteri, C. M.; Villata, M.; D'Ammando, F.; Larionov, V. M.; Gurwell, M. A.; Mirzaqulov, D. O.; Smith, P. S.; Acosta-Pulido, J. A.; Agudo, I.; Arévalo, M. J.; Bachev, R.; Benítez, E.; Berdyugin, A.; Blinov, D. A.; Borman, G. A.; Böttcher, M.; Bozhilov, V.; Carnerero, M. I.; Carosati, D.; Casadio, C.; Chen, W. P.; Doroshenko, V. T.; Efimov, Yu. S.; Efimova, N. V.; Ehgamberdiev, Sh. A.; Gómez, J. L.; González-Morales, P. A.; Hiriart, D.; Ibryamov, S.; Jadhav, Y.; Jorstad, S. G.; Joshi, M.; Kadenius, V.; Klimanov, S. A.; Kohli, M.; Konstantinova, T. S.; Kopatskaya, E. N.; Koptelova, E.; Kimeridze, G.; Kurtanidze, O. M.; Larionova, E. G.; Larionova, L. V.; Ligustri, R.; Lindfors, E.; Marscher, A. P.; McBreen, B.; McHardy, I. M.; Metodieva, Y.; Molina, S. N.; Morozova, D. A.; Nazarov, S. V.; Nikolashvili, M. G.; Nilsson, K.; Okhmat, D. N.; Ovcharov, E.; Panwar, N.; Pasanen, M.; Peneva, S.; Phipps, J.; Pulatova, N. G.; Reinthal, R.; Ros, J. A.; Sadun, A. C.; Schwartz, R. D.; Semkov, E.; Sergeev, S. G.; Sigua, L. A.; Sillanpää, A.; Smith, N.; Stoyanov, K.; Strigachev, A.; Takalo, L. O.; Taylor, B.; Thum, C.; Troitsky, I. S.; Valcheva, A.; Wehrle, A. E.; Wiesemeyer, H.

Abstract In this study, we observe that, in the presence of the string cloud parameter a and the quintessence parameter γ , with the equation of state parameter , the radius of the shadow of the Schwarzschild black hole increases as compared with that in the pure Schwarzschild black hole case. The existence of both quintessential dark energy and the cloud of strings increases the shadow size; hence, the strength of the gravitational field around the Schwarzschild black hole increases. Using the data collected by the Event Horizon Telescope (EHT) collaboration for M87* and Sgr A*, we obtain upper bounds on the values of a and γ . Further, we see the effects of a and γ on the rate of emission energy for the Schwarzschild black hole. We notice that the rate of emission energy is higher in the presence of clouds of strings and quintessence. Moreover, we study the weak deflection angle using the Gauss-Bonnet theorem. We show the influence of a and γ on the weak deflection angle. We notice that both a and γ increase the deflection angle α .

The optical afterglow spectrum of GRB 050401 (at z=2.8992+/-0.0004) shows the presence of a large damped Ly-alpha absorber (DLA), with log N(HI)=22.5+/-0.3. This is the highest column density DLA ever observed, and is nearly an order of magnitude larger than any DLA observed in QSO spectra. The X-ray spectrum shows absorption above Galactic of log N(H)=22.21^{+0.06}_{-0.08} (assuming solar abundances and a redshift z=2.8992). The comparison of the X-ray column density measurement, which is dominated by metal absorption, and the optical H(I) column, allows us to derive [X/H]=-0.3+/-0.3. From the optical spectrum, we infer a zinc abundance of [Zn/H] = -0.8+/-0.4. We can then derive [alpha/Zn] = 0.5+/-0.2, suggestive of an overabundance of alpha-elements in the absorber, consistent with alpha-element overabundances observed in DLAs with similar Fe-group metallicities. There is evidence of dust depletion in Fe, Si and Cr and the optical spectrum is very reddened. This can be well reproduced with an SMC extinction curve, with A_V=0.62+/-0.06. However the total extinction can also be constrained from the optical--X-ray SED, to be 0.5>A_V>4.5, independent of assumptions about the shape of the extinction curve. These limits are less than the A_V=9.1^{+1.4}_{-1.5} inferred from the soft X-ray absorption. This discrepancy implies a low dust-to-metals ratio. 'Grey' extinction cannot explain the discrepancy, even with an arbitrarily flat extinction curve (i.e. large absorption with little reddening), but a large alpha-element overabundance can. If such overabundance is common in afterglow absorbers it may explain the high X-ray column densities with little optical reddening observed in afterglow spectra, and would necessitate a cautious approach to dust content based on metallicity arguments in these environments.