tag:blogger.com,1999:blog-2846514233477399562.post9212996244536868951..comments2024-01-08T09:37:04.406+01:00Comments on RÉSONAANCES: Dark matter or pulsars? AMS hints it's neither.Jesterhttp://www.blogger.com/profile/08947218566941608850noreply@blogger.comBlogger11125tag:blogger.com,1999:blog-2846514233477399562.post-22894023732641557882014-09-24T22:44:45.556+01:002014-09-24T22:44:45.556+01:00I think most of the electrons are produced from pr...I think most of the electrons are produced from primary sources, therefore propagation effects are different for the two.Jesterhttps://www.blogger.com/profile/08947218566941608850noreply@blogger.comtag:blogger.com,1999:blog-2846514233477399562.post-82443946064682777092014-09-24T19:02:43.890+01:002014-09-24T19:02:43.890+01:00I don't understand this cooling time argument....I don't understand this cooling time argument. I would imagine the processes that "cool" electrons act exactly the same way on positrons. So why would the _fraction_ of positrons change with energy?Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-2846514233477399562.post-86239540059194354552014-09-24T16:21:03.107+01:002014-09-24T16:21:03.107+01:00To clarify: I don't think they *assume* that ...To clarify: I don't think they *assume* that radiative cooling can be neglected. They merely say that the situation with no cooling represents a robust upper limit for the secondary positron flux. I find it intriguing that the positron flux measured by AMS asymptotes to that upper limit. This does not prove the positrons are secondary, but for primary positrons of whatever origin it would have to be a weird coincidence than the (in principle arbitrary) local flux falls close to that value. <br />Now, assuming the positrons are secondary, one has to conclude that cooling is small above 100 GeV. That is indeed challenging to understand in terms of an underlying propagation model. But there's no robust proof either that t_cooling < t_escape in any sensible physical propagation model. Jesterhttps://www.blogger.com/profile/08947218566941608850noreply@blogger.comtag:blogger.com,1999:blog-2846514233477399562.post-54304961953298301422014-09-24T02:49:36.991+01:002014-09-24T02:49:36.991+01:00I can smell an elephant in the room.I can smell an elephant in the room.Robert L. Oldershawhttps://www.blogger.com/profile/15396555790655312393noreply@blogger.comtag:blogger.com,1999:blog-2846514233477399562.post-44279397773112987172014-09-23T09:09:34.588+01:002014-09-23T09:09:34.588+01:00Thank you Jester for this nice summary. However, i...Thank you Jester for this nice summary. However, if you choose to emphasize a work that states "But maybe primary positron sources are not needed at all?", you should be cautious. This work is based on a very questionable assumption: radiative losses are neglected. While this is correct for cosmic-ray nuclei, it is well known for decades that this cannot be the case for e+e-, because of the very efficient synchrotron and inverse Compton processes (see the seminal works of Ginzburg & Syrovatskii in the 60's, or all available textbooks, for instance Berezinskii's, Longair's, or Schlickeiser's). And as far as I understand, electrodynamics has not been ruled out yet ;-).It is not because they are ignorant that the majority of high-energy astrophysiciscts expert in the domain believe that this large positron fraction is a serious hint for primary positrons. Pulsar winds and SNRs are indeed the best candidates, which are demonstrated to exist around there. Now, it is also true that it is hard to make predictions from first principles, but come on, since the effect is local (see for example Shen ApJL 162, L181, 1970, or Aharonian et al 1995, etc.), and since these sources are macroscopic, you can hardly blame the experts for that ... There is no shame in not controlling things as long as one does not look for exotic signals there ... and it is well known that nobody does ;-) (as it is the case for channels strongly polluted with QCD background at the LHC).Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-2846514233477399562.post-29962087561578386842014-09-23T02:45:26.598+01:002014-09-23T02:45:26.598+01:00What, nothing about BICEP? Come on, Jester, gloat ...What, nothing about BICEP? Come on, Jester, gloat a little. You earned it!Rastus Odinga Odingahttps://www.blogger.com/profile/09615544434035028500noreply@blogger.comtag:blogger.com,1999:blog-2846514233477399562.post-61782844543604133772014-09-21T13:41:42.206+01:002014-09-21T13:41:42.206+01:00" This allows the collaboration to conclude ..." This allows the collaboration to conclude that the positron fraction has a maximum and starts to decrease at high energies :] "<br /><br />They haven't shown the data above 500 GeV. <br />But... It doesn't mean they haven't looked at them.<br />Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-2846514233477399562.post-18576861853176915322014-09-20T18:43:37.705+01:002014-09-20T18:43:37.705+01:00So, They dropped the positron cooling (which must ...So, They dropped the positron cooling (which must be there) and they call this simple (wrong) model as "upper-limit". Ok, it is a real upper limit because the cooling will pull down the positron fraction. They also claim it's model-independent but it's not, because propagation effects are still there (btw they are even less understood than the cooling), in fact they use a very crude propagation model with very bad fit to the B/C ratio, to fix alpha. <br />Finally, they claim that whatever is below their curve will support the scenario, no matter the spectral shape. No matter if the AMS data increase with energy, in clear contradiction with any secondary-production scenario. Cool, eh?!Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-2846514233477399562.post-80529756797161483822014-09-20T10:05:20.006+01:002014-09-20T10:05:20.006+01:00Hi there,
to be honest, I don't understand wh...Hi there,<br /><br />to be honest, I don't understand why all this is a hint to a purely secondary origin of the positron fraction. <br />The only conclusion I draw from the paper you mention (and the updated version 1305.1324) is the following: the rise is <b>compatible</b> with purely secondary production, if one doesn't make any assumptions about energy losses.<br /><br />But the mere fact that it is possible to place an upper limit on a quantity compatible with the data does <b>not</b> mean that the data can be explained by the underlying model (here the secondary production). In other words, I think obviously there are a lot of theories out there for which it is possible to find very conservative upper limits compatible with the AMS-02 data.<br /><br />To really support the secondary origin of the positron fraction, I would like to see a concrete, well-motivated model for propagation/energy losses, which can really explain the rise of the fraction, instead of just placing an upper limit compatible with the data.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-2846514233477399562.post-79098754066554633422014-09-20T02:55:13.929+01:002014-09-20T02:55:13.929+01:00Could primordial black holes be a source of excess...Could primordial black holes be a source of excess positrons?Robert L. Oldershawhttps://www.blogger.com/profile/15396555790655312393noreply@blogger.comtag:blogger.com,1999:blog-2846514233477399562.post-81912289112504037812014-09-20T02:48:46.267+01:002014-09-20T02:48:46.267+01:00Cool!
To me it's kind of funny how usually in...Cool!<br /><br />To me it's kind of funny how usually in astronomy, the issue is how the interstellar medium interferes with photons, through their interactions with all the charged particles out there, but for these kinds of experiments, the issue might be how the interstellar medium interferes with charged particles, through their interactions with all the photons out there. It's like the hero and the villain of the story have traded places.<br /><br />A note: you have "the, the" in the last paragraph.Xezlecnoreply@blogger.com