Tuesday, 25 September 2012

Fermi line cont'd

A look at hep-ph listing tells you that what excites particle theorists these days is the Fermi line. Recall, that an independent analysis of gamma-ray data from the Fermi telescope discovered the monochromatic emission from the center of our galaxy at the energy of approximately 130 GeV. The signal is so strong that it's unlikely a fluctuation, and no known astrophysical processes are expected to produce monochromatic lines. The line may be a weird instrumental effect, or it may be  the signal of dark matter annihilating into a pair photons with the cross section of few*10^-27 cm^3/sec. If the latter is true, it would dwarf the Higgs boson discovery...  

As usual, the most popular game is to fit the signal into every possible model, including those that firmly resist. There's been some interesting developments on this front, but I'll keep that for another post. For now, I'll restrict to the properties of the signal and astrophysical constraints.  

  •  The statistical significance of the line is large, the precise number depending on how the data are cut and cooked. In the original paper the significance was 4.6σ (before taking into account the trial factor), but for example in this paper the numbers 5.0σ or even 5.5σ are bandied around. That paper also claims that a slightly better fit to the data is with 2 lines, one at 129 GeV and another at 111 GeV, and that the center of the emission is off by 1.5 degree from the galactic centre.   The former may be a good news for dark matter, as most models predict 2 separate lines, from annihilation into γγ and into γZ.  The latter doesn't have to be a bad news, in view of the recent  simulations of dark matter distribution.   
  • Two groups  were recently scanning the Fermi data for suspicious features that could suggest hat the line is an instrumental artifact.  They may have found one: a 130 GeV line in the Earth limb sample. Cosmic rays hitting the atmosphere produce gamma-rays that sometimes fall into Fermi's field of view. This provides a sort of calibration sample where no signal is expected. Instead, there seems to be a 3σ line in the Earth limb photons that can be made even more prominent with specific cuts on the photon incidence angle.  Is that an unlucky fluctuation? On the other hand, it's difficult to imagine an instrumental effects or a software bug that could be responsible for both the galactic center and the Earth limb lines.   
  • There are 2 more places in the sky where the presence of the 130 GeV line was claimed. The line was observed in the nearby galaxy clusters, which may be a good news. Also, the line was observed in the unassociated gamma-ray sources, which is probably a bad news given those were later claimed to be AGNs. No line was detected from the dwarf satellite galaxies of the Milky Way, which is probably not a problem, and no line emission was found in the galactic plane, which is good. 
  • In most models of dark matter a gamma-ray line would be accompanied by a 1000 times more intense continuum photon signal, just because dark matter annihilation into other final states (that later emit photons) would be dominant. However, the observed photon  spectrum from the galactic center - the same one that displays the monochromatic signal - puts very strong constraints on the continuum emission. Typically, the cross section for dark matter annihilation into other final states can be at most 10 times larger than the cross section for the annihilation into 2 photons. For example, this paper claims the limits on the annihilation rate of 130 GeV dark matter into most final states is comparable to the thermal cross section  3*10^-26 cm^3/sec (the one that guarantees the correct relic abundance if dark matter is of thermal origin),  and even stronger with less conservative assumptions about the dark matter density profile. This is a severe constraint on theory, such that the models explaining the Fermi line have to be tailor-made to satisfy it. 

In summary, there are 2 main arguments against the Fermi line being a signal of dark matter. One is  the presence of the line in the Earth limb photon sample. The other is that it's good to be true. Based on that, it's probably worth staying excited for a little longer, until there are better reasons to stop the fun. 


Anonymous said...

Wowie, another mystery bump!

Shall we inundate arxiv.org with pseudo-science?

A little excitement is a good motivator, but objectivity makes for healthy science and genuine progress.

Anonymous said...

The claimed Galactic Center continuum limits from arxiv:1209.3015 are overstated, as the analysis fixes the flux and spectrum of all of the point sources except that from the most central source. There are 6 point sources within 2 degrees of the Galactic Center, and not letting their fluxes be free allows for much too strong of a dark mater constraint than that truly allowed by the data.

Anonymous said...

Nice and very enjoyable overview :-)

I almost started to be worried about what happend to Jester seeing no update at this site for such a long time ...


Jester said...

Might be that 1209.3015 is too aggressive, but others obtain similar limits, e.g. 1207.0800.

Dan D. said...

Thanks for the update, and glad to see you posting again!

Disclaimer: I'm not a particle physicist (I'm an atmospheric scientist), but I have an informed layperson's interest in particle physics, and I enjoy your blog and others!

Regarding the two papers discussing the line in the limb data, aren't they arguing that the line there is only "marginally" significant within a certain range of incidence angles, and that when the whole range of incidence angles is taken into account, there's no feature at 130 GeV? In fact, both papers try to argue that such a feature a statistical fluctuation, and the first points out that you can get bumps at different energies for the limb data by selecting different subsets of the data (if I interpreted it correctly; I just skimmed it). But, on the other hand are much more confident that the GC line is not, and that moreover, no plausible systematics could explain it (obviously they could be missing something).

Here's my question: is it possible that the line in the limb data is actually representing photons from deep space that just happen to be near the limb, and thus may actually be due to whatever process is producing it in the GC (assuming its real for the moment)?

Jester said...

Dan, yes, both papers argue the line in the limb is not so bad. Off the record I heard the chances of it happening by fluctuation are about 1%, so it may be just bad luck. The good thing is that we just have to wait and see if it goes away with more data.
As for the 2nd question, I think there's no way the limb photons have anything to do with dark matter or any fundamental physics. The sample is dominated by radiation from electromagnetic cascades in the atmosphere.

Anonymous said...

Note that, if it should indeed turn out that the DM distribution peaks some distance away from the visible Galactic Center, the continuum gamma limits would become stronger because there's less astrophysical background off-center.

Klaus Lange said...

This fermi results gets support from planck satellite, look here: http://www.space.com/17436-dark-matter-radiation-planck-satellite.html

Best regards,

Jester said...

Klaus, that article is about the WMAP haze/Fermi bubbles. It's another phenomenon which, most likely, has no connection with the Fermi monochromatic line and, almost certainly, has nothing to do with dark matter.