Talk:Gravitational microlensing

From Wikipedia, the free encyclopedia

	Physics Portal This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
Start	This article has been rated as Start-Class on the assessment scale.
High	This article is on a subject of High importance within physics.
Help with this template This article has been rated but has no comments. If appropriate, please review the article and leave comments here to identify the strengths and weaknesses of the article and what work it will need.

Well, the article is not very well descibed, but it is my first Wiki article. The previous article (stating that a planet magnifies the light of its host star) was just good-old wrong, since it is the planet around the lens causing a deviation in the lightcurve!

There are plenty of potential issues to add: Microlensing is a tool producing many papers every month, so there is plenty of potential subjects to cover.

--Christianvinter 14:56, 12 July 2005 (UTC)

Contents 1 Illustration of microlensing 2 Fresh Start 3 Detecting Planets 4 2nd paragraph

[edit] Illustration of microlensing

The NASA figure illustrating the effect of microlensing is technically wrong: The gravitational field of the lens star does not focus the light of the observed source star like a convex lens. Instead, light rays closer to the lens star experience a stronger bending (rather than a weaker one). Can this figure be replaced? I have contacted NASA JPL on this issue.

Martin Dominik, 21 February 2006

There are plenty of good illustrations about microlensing floating around on the web, but I don't know how to get around the copyright restrictions. David s graff 13:13, 20 March 2006 (UTC)

[edit] Fresh Start

I felt like this page needed a new start, so I did a major overhall. Rather than adding my own work to some of the previous work, I included nearly all the pre-existing links and ideas to my own description. I've introduced many sections, currently blank, to be filled in later, but done some major overhall on the links to collaborations. My apologies to the previous contributors if I've left something out.

I don't know why, but it seems to me that Astronomy articles lag behind other technical fields in Wikipedia. Perhaps Astronomers are less likely to use Wikipedia than computer scientists. Perhaps its just because as an Astronomer, my expectations are higher. Or perhaps the neglect of this article reflects how the once red-hot field of microlensing has passed its glory. David s graff 10:35, 21 March 2006 (UTC)

[edit] Detecting Planets

Something that is not clear in the article is how different things are detected. As far as I can tell (from reading), microlensing occurs when a body passes in front of a distant light source. But that sounds no different from gravitational lensing.

From the attached article (Low-Mass Exoplanet), it sounds more like this: Gravitational microlensing is when a body passes in front of a gravitational lens. So a distant star, a closer star, and the closer star's planet all align. We detect the planet because of the change in the lens effect as the planet joins the alignment. (The planet passes in and out of alignment while the other stay aligned.) Does that sounds like an accurate description? --Daev 16:11, 17 April 2006 (UTC)

Microlensing is gravitational lensing - it's just on a smaller scale. The detection of planets using it is when the lensing star has a planet which also does some of the lensing. Mike Peel 16:55, 17 April 2006 (UTC)

Well, yes. But it's a subset isn't it? Like red light is a subset of visible light. There's no point in a separate article if the subset equals the superset. Since the article doesn't seem to mention anything that differentiates the two, I'm confused.

Unless you mean that gravitational lensing is the theory and microlensing is the practice, or something like that.

Saw your edits and modified. I assume you meant transit method? Or perhaps, astrometry. I'm not so sure that it should be in the Exotic Microlensing section (is this really an exotic use for it?). --Daev 17:27, 17 April 2006 (UTC)

A better example would be visible light being a subset of the EM spectrum. Gravitational lensing is generally split into Strong (usually just called Gravitational Lensing), Weak and Microlensing.

This article is very much a work in progress atm; when I get the time, I'll add a lot more to it.

You were correct with transit, not transient. I've been working a lot with transient events recently, hence the slip. Thanks for correcting it. I put it in the 'exotic' section, as it's not a common thing to detect in microlensing events - of the order of 1 in 100 microlensing events - hence it's 'exotic', i.e. different from the norm. Mike Peel 18:14, 17 April 2006 (UTC)

'Gravitational lensing' means the bending of radiation due to the gravitational field, which can involve any mass scale of the deflector and any distance scale. 'Microlensing' is a special case with a fuzzy definition (in fact, there are at least four different ones, which partially overlap: lensing with unresolved images, lensing with ~ micro-as deflection, splitting of macro-image into micro-images by substructure in a deflector, or lensing by compact objects). Two observed effects are commonly described as 'gravitational microlensing': the variation of lensed quasar images caused by individual stars in the lensing galaxy, and the brightening of background stars in our own or neighbouring galaxies caused by an intervening foreground lens star (galactic microlensing). Planets around the lens star alter the gravitational field and therefore can affect the observed light curve during a microlensing event, which is not a superposition of the effects by isolated planet and host star. Instead, the tidal field (and higher orders) of the source star at the position of the planet significantly increases its detection probability. The duration of the planetary signal (hours to days) is much shorter than the orbital period, so that the motion of the planet relative to its host star can typically be neglected. MD:astronomer 22:26, 17 April 2006 (UTC)

I've tried to fix the fuzzyness of the definition of microlensing with a new one: Microlensing is the subset of gravitational lensing whose variations in time can be measured. Typically, this means that the lens mass must be small enough that it will cross its own Einstein ring radius in less than the time it takes for a graduate student to finish a PhD thesis. The EROS collaboration is analysing our own and MACHOs data for evidence of very long time scale events of order 100s or 1000s of years (see sec. 5.6 of [1]). Although these may not be confirmed as microlensing events (rather than some other very long time-scale variability), the non-detection of these events would allow a limit on dark matter by 100-1000 Mo MACHOs. Any event where the lens mass is so big and far away that it takes millions of years to cross its einstein ring radius, and thus changes too slowly in time to be studied in the time domain, is a "macrolens".

I also disagree with the four definitions by MDAstronomer. It is not true that any lensing event with unresolved images is microlensing. A galaxy can lens a quasar, but have the images be too close to be resolved. This is not microlensing. Likewise,lensing by a compact object does not describe microlensing. "Any" gravitational lens must be physically smaller or about the same size as its own einstein radius to cause any measurable lensing effect. That is why we do not see any lensing effects from the Moon. It is so close that its Einstein radius is tiny. However, if the moon were a few kiloparsecs away, its einstein radius would be larger than its physical radius and it could be a perfectly ordinary microlens. The supermassive black hole at the center of the milky way is a compact object, but it cannot be a microlens... its einstein radius is too big for any changes in lensing to be monitored in time. It could in principle be a macrolens if there were a quasar right behind it. The microlensing at the edges of gravitationally lensed quasars is called microlensing because it causes time-varying effects in the apparent flux of the images. This has been significant because it interferes with attempts to use these gravitational lenses to measure the Hubble Constant.

The time-varying nature of a microlens is the key to all of its observations. And the need to take over large blocks of telescope time to do microlensing has revolutionized time-domain astronomy in general, in part through a bureaucratic reorganization of Telescope Allocation Committees and the advent of dedicated telescopes. There have been great resulting changes not only in microlensing but in searches for supernovae, asteroids, variable stars.

None of the various types of microlensing observations (photometric brightenning, astrometric shift, interferometric visibility reduction due to image splitting, shifts in color, spectrum, or variability amplitude) are strong enough to determine a microlensing event from a single observation. All of them require detecting some change in time, if only because there are plenty of natural causes that can mimic any one of the shifts for a single measurement. For example, how could one seperate a star which was split into two images from an ordinary binary star without time-domain information?

I disagree with Mike's splitting of lensing into strong, weak and microlensing. A lens is strong if it is within one Einstein radius of the line of sight to the source, and weak otherwise. Nearly all photometric microlenses are strong lenses, but astrometric lensing is much more sensitive to weak lensing than photometric lensing. Eddington's verification of general relativity was classic weak astrometric microlensing, and we will use SIM to study weak astrometric microlensing to measure the masses of other stars [2]. David s graff 22:17, 2 June 2006 (UTC)

[edit] 2nd paragraph

in the second paragraph, it says: "When a nearby object passes in front of a distant star or quasar, the gravity of the nearby object magnifies the distant star. By detecting that magnification, astronomers can study the near star." Don't we want that last word to be "object," not "star"? Because before we were talking about some object that might not be a star. Am I right? If so, could someone change it? Thanks.

You are right. Changed. Christianvinter 15:43, 5 February 2007 (UTC)

In fact, 'nearby object' is technically wrong. The probability for (phometric) microlensing tends to zero as the lens object gets close to the observer. A lens object half-way between source and observer is ideal. I took that opportunity to rephrase some other paragraphs. When I find some time, I should make further edits with respect to the historic developments. Maybe someone could send me a reminder? MD:astronomer 23:17, 10 February 2007 (UTC)