Studies of relativistic effects with radioastron space mission

Abstract

In the review we discuss possible studies of GR phenomena such as gravitational microlensing and shadow analysis with the forthcoming RadioAstron space mission. It is well-known that gravitational lensing is a powerful tool in the investigation of the distribution of matter, including that of dark matter (DM). Typical angular distances between images and typical time scales depend on the gravitational lens masses. For the microlensing, angular distances between images or typical astrometric shifts are about 10 −5 − 10 −6 as 1 . Such an angular resolution will be reached with the space-ground VLBI interferometer, Radioastron. The basic targets for microlensing searches should be bright point-like radio sources at cosmological distances. In this case, an analysis of their variability and a reliable determination of microlensing could lead to an estimation of their cosmological mass density. Moreover, one could not exclude the possibility that non-baryonic dark matter could also form microlenses if the corresponding optical depth were high enough. It is known that in gravitationally lensed systems, the probability (the optical depth) to observe microlensing is relatively high; therefore, for example, such gravitationally lensed objects, like CLASS gravitational lens B1600+434, appear the most suitable to detect astrometric microlensing, since features of photometric microlensing have been detected in these objects. However, to directly resolve these images and to directly detect the apparent motion of the knots, the Radioastron sensitivity would have to be improved, since the estimated flux density is below the sensitivity threshold, alternatively, they may be observed by increasing the integration time, assuming that a radio source has a typical core -jet structure and microlensing phenomena are caused by the superluminal apparent motions of knots. In the case of a confirmation (or a disproval) of claims about microlensing in gravitational lens systems, one can speculate about the microlens contribution to the gravitational lens mass. Astrometric microlensing due to Galactic Macho's action is not very important because of low optical depths and long typical time scales. Therefore, the launch of the space interferometer Radioastron will give excellent new facilities to investigate microlensing in the radio band, allowing the possibility not only to resolve microimages but also to observe astrometric microlensing. Shadows around supermassive black holes can be detected with the RadioAstron space interferometer.

Constraining the electric charges of some astronomical bodies in Reissner–Nordström spacetimes and generic r −2-type power-law potentials from orbital motions

“…[57]; see also Refs. [70,71]. There is nowadays wide consensus that the compact object with M • = 4 × 10 6 M ⊙ hosted by the Galactic Center in Sgr A * [72] is a supermassive black hole.…”

“…According to the author of Ref. [70], for Sgr A * , a charged black hole could be distinguished from a Schwarzschild black hole with the space radio telescope Radioastron, at least if its charge is close to the extremal value…”

“…For a Reissner-Nordström black hole, its charge changes the size of the shadows up to 30% in the extreme charge case. Therefore, the charge of the black hole can be measured by observing the shadow size, if the other black hole parameters are known with sufficient precision [70]. The emission and absorption properties of charged black holes were studied in Refs.…”

Observations of 44 extragalactic radio sources with the VLBA at 92cm: A List of Potential Calibrators and Targets for LOFAR and RadioAstron

“…In particular, very little is known about the morphology of radio sources at these low frequencies and high resolutions. A very similar situation exists with the space VLBI mission RadioAstron, due to be launched in October 2009 (Zakharov 2007). With a maximum baseline length of 350,000 km, RadioAstron will have a fringe spacing of 0.540 milliarcseconds at 327 MHz (see the RadioAstron booklet for more details 1 ).…”

Observations of 44 extragalactic radio sources with the VLBA at 92 cm

“…In particular, very little is known about the morphology of radio sources at these low frequencies and high resolutions. A very similar situation exists with the space VLBI mission RadioAstron, due to be launched in October 2009 (Zakharov 2007). With a maximum baseline length of 350 000 km, RadioAstron will have a fringe spacing of 0.540 milliarcsec at 327 MHz (see the RadioAstron booklet for more details 1 ).…”