by neufer » Thu Nov 18, 2021 2:33 pm
VictorBorun wrote: ↑Thu Nov 18, 2021 6:07 am
neufer wrote: ↑Wed Nov 17, 2021 3:35 pm
https://en.wikipedia.org/wiki/NGC_3314 wrote:
<<NGC 3314 is a pair of overlapping spiral galaxies between 117 (NGC 3314a) and 140 million (NGC 3314b) light-years away in the constellation Hydra. This unique alignment gives astronomers the opportunity to measure the properties of interstellar dust in the face-on foreground galaxy (NGC 3314a). The dust appears dark against the background galaxy (NGC 3314b).
In a March 2000 observation of the galaxies, a prominent green star-like object was seen in one of the arms. Astronomers theorized that it could have been a supernova, but the unique filtering properties of the foreground galaxy made it difficult to decide definitively.>>
rays of light from a point 140 Mly away (NGC 3314b) that pass by a solar mass object 117 Mly away (NGC 3314a) at a radial distance of 220 AU would converge onto us {Note: Focal length rule: [1/19.22] = [1/(140-117) + 1/117]}
So was that prominent green star actually due to a ~440 AU wide stellar mass gravitational lens [/b]
A SN event takes a day to brighten up and a year to fade.
A gravi-lensing event where a foreground star has a light day zone of focusing and a relative velocity of
с/365 would take a year to brighten up and then a year to fade, and the brightness against time would be symmetric bell-shape curve, would not it?
The "zone of focusing" in this case would have to be
much less than ~440 AU wide.
Relative velocities might be on the order of
с/1000 (= ~1.2 AU/week).
https://en.wikipedia.org/wiki/Gravitational_microlensing wrote:
<<Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light (stars) or large objects that block background light (clouds of gas and dust). These objects make up only a minor portion of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light.
A typical microlensing event like OGLE-2005-BLG-006 one has a very simple shape, and only one physical parameter can be extracted: the time scale, which is related to the lens mass, distance, and velocity. There are several effects, however, that contribute to the shape of more atypical lensing events:
- Lens mass distribution. If the lens mass is not concentrated in a single point, the light curve can be dramatically different, particularly with caustic-crossing events, which may exhibit strong spikes in the light curve. In microlensing, this can be seen when the lens is a binary star or a planetary system.
Finite source size. In extremely bright or quickly-changing microlensing events, like caustic-crossing events, the source star cannot be treated as an infinitesimally small point of light: the size of the star's disk and even limb darkening can modify extreme features.
Parallax. For events lasting for months, the motion of the Earth around the Sun can cause the alignment to change slightly, affecting the light curve.
Most focus is currently on the more unusual microlensing events, especially those that might lead to the discovery of extrasolar planets. Another way to get more information from microlensing events involves measuring the astrometric shifts in the source position during the course of the event and even resolving the separate images with interferometry. The first successful resolution of microlensing images was achieved with the GRAVITY instrument on the Very Large Telescope Interferometer
In practice, because the alignment needed is so precise and difficult to predict, microlensing is very rare. Events, therefore, are generally found with surveys, which photometrically monitor tens of millions of potential source stars, every few days for several years. Dense background fields suitable for such surveys are nearby galaxies, such as the Magellanic Clouds and the Andromeda galaxy, and the Milky Way bulge. In each case, the lens population studied comprises the objects between Earth and the source field: for the bulge, the lens population is the Milky Way disk stars, and for external galaxies, the lens population is the Milky Way halo, as well as objects in the other galaxy itself. The density, mass, and location of the objects in these lens populations determines the frequency of microlensing along that line of sight, which is characterized by a value known as the optical depth due to microlensing. (This is not to be confused with the more common meaning of optical depth, although it shares some properties.) The optical depth is, roughly speaking, the average fraction of source stars undergoing microlensing at a given time, or equivalently the probability that a given source star is undergoing lensing at a given time. The MACHO project found the optical depth toward the LMC to be 1.2×10
−7, and the optical depth toward the bulge to be 2.43×10
−6 or about 1 in 400,000.
Complicating the search is the fact that for every star undergoing microlensing, there are thousands of stars changing in brightness for other reasons (about 2% of the stars in a typical source field are naturally variable stars) and other transient events (such as novae and supernovae), and these must be weeded out to find true microlensing events. After a microlensing event in progress has been identified, the monitoring program that detects it often alerts the community to its discovery, so that other specialized programs may follow the event more intensively, hoping to find interesting deviations from the typical light curve. This is because these deviations – particularly ones due to exoplanets – require hourly monitoring to be identified, which the survey programs are unable to provide while still searching for new events. The question of how to prioritize events in progress for detailed followup with limited observing resources is very important for microlensing researchers today.>>
[quote=VictorBorun post_id=318361 time=1637215656 user_id=145500]
[quote=neufer post_id=318346 time=1637163317 user_id=124483]
[quote=https://en.wikipedia.org/wiki/NGC_3314]
<<NGC 3314 is a pair of overlapping spiral galaxies between 117 (NGC 3314a) and 140 million (NGC 3314b) light-years away in the constellation Hydra. This unique alignment gives astronomers the opportunity to measure the properties of interstellar dust in the face-on foreground galaxy (NGC 3314a). The dust appears dark against the background galaxy (NGC 3314b).[b][u][color=#00BF00] In a March 2000 observation of the galaxies, a prominent green star-like object was seen in one of the arms. Astronomers theorized that it could have been a supernova, but the unique filtering properties of the foreground galaxy made it difficult to decide definitively.[/color][/u][/b]>>[/quote]
[color=#0000FF]rays of light from a point 140 Mly away (NGC 3314b) that pass by a solar mass object 117 Mly away (NGC 3314a) at a radial distance of 220 AU would converge onto us :!: [/color] {Note: Focal length rule: [1/19.22] = [1/(140-117) + 1/117]}
[u][color=#00BF00]So was that prominent green star actually due to a ~440 AU wide stellar mass gravitational lens :?: [/color][/u][/b]
[/quote]
A SN event takes a day to brighten up and a year to fade.
A gravi-lensing event where a foreground star has a light day zone of focusing and a relative velocity of [i]с[/i]/365 would take a year to brighten up and then a year to fade, and the brightness against time would be symmetric bell-shape curve, would not it?[/quote]
The "zone of focusing" in this case would have to be [u]much less than ~440 AU wide[/u].
Relative velocities might be on the order of [i]с[/i]/1000 (= ~1.2 AU/week).
[quote=https://en.wikipedia.org/wiki/Gravitational_microlensing]
<<Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light (stars) or large objects that block background light (clouds of gas and dust). These objects make up only a minor portion of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light.
[float=right][img3=Typical light curve of gravitational microlensing event (OGLE-2005-BLG-006)]https://upload.wikimedia.org/wikipedia/commons/b/ba/Gravitational.Microlensing.Light.Curve.OGLE-2005-BLG-006.png[/img3][img3=Comparative supernova type light curves]https://upload.wikimedia.org/wikipedia/commons/e/e0/Comparative_supernova_type_light_curves.png[/img3][/float]
A typical microlensing event like OGLE-2005-BLG-006 one has a very simple shape, and only one physical parameter can be extracted: the time scale, which is related to the lens mass, distance, and velocity. There are several effects, however, that contribute to the shape of more atypical lensing events:
[list] Lens mass distribution. If the lens mass is not concentrated in a single point, the light curve can be dramatically different, particularly with caustic-crossing events, which may exhibit strong spikes in the light curve. In microlensing, this can be seen when the lens is a binary star or a planetary system.
Finite source size. In extremely bright or quickly-changing microlensing events, like caustic-crossing events, the source star cannot be treated as an infinitesimally small point of light: the size of the star's disk and even limb darkening can modify extreme features.
Parallax. For events lasting for months, the motion of the Earth around the Sun can cause the alignment to change slightly, affecting the light curve.[/list]
Most focus is currently on the more unusual microlensing events, especially those that might lead to the discovery of extrasolar planets. Another way to get more information from microlensing events involves measuring the astrometric shifts in the source position during the course of the event and even resolving the separate images with interferometry. The first successful resolution of microlensing images was achieved with the GRAVITY instrument on the Very Large Telescope Interferometer
In practice, because the alignment needed is so precise and difficult to predict, microlensing is very rare. Events, therefore, are generally found with surveys, which photometrically monitor tens of millions of potential source stars, every few days for several years. Dense background fields suitable for such surveys are nearby galaxies, such as the Magellanic Clouds and the Andromeda galaxy, and the Milky Way bulge. In each case, the lens population studied comprises the objects between Earth and the source field: for the bulge, the lens population is the Milky Way disk stars, and for external galaxies, the lens population is the Milky Way halo, as well as objects in the other galaxy itself. The density, mass, and location of the objects in these lens populations determines the frequency of microlensing along that line of sight, which is characterized by a value known as the optical depth due to microlensing. (This is not to be confused with the more common meaning of optical depth, although it shares some properties.) The optical depth is, roughly speaking, the average fraction of source stars undergoing microlensing at a given time, or equivalently the probability that a given source star is undergoing lensing at a given time. The MACHO project found the optical depth toward the LMC to be 1.2×10[sup]−7[/sup], and the optical depth toward the bulge to be 2.43×10[sup]−6[/sup] or about 1 in 400,000.
Complicating the search is the fact that for every star undergoing microlensing, there are thousands of stars changing in brightness for other reasons (about 2% of the stars in a typical source field are naturally variable stars) and other transient events (such as novae and supernovae), and these must be weeded out to find true microlensing events. After a microlensing event in progress has been identified, the monitoring program that detects it often alerts the community to its discovery, so that other specialized programs may follow the event more intensively, hoping to find interesting deviations from the typical light curve. This is because these deviations – particularly ones due to exoplanets – require hourly monitoring to be identified, which the survey programs are unable to provide while still searching for new events. The question of how to prioritize events in progress for detailed followup with limited observing resources is very important for microlensing researchers today.>>[/quote]