Tag Archives: Fermi Gamma-ray telescope

April 9, 2016

Gamma Rays from Dark Matter at the Center of the Galaxy: Stronger Evidence

Evidence has been growing for the detection of dark matter more directly – at the center of the Milky Way Galaxy. Normally, we detect dark matter through its gravitational effects only, although there have been many attempts to detect it more directly, both through laboratory experiments here on Earth and from astronomical measurements. The Earthbound experiments are inconclusive at best, with some claims of detection being contradicted by other experiments.

But the evidence for astronomical detection of dark matter is growing. Expected sources include dwarf galaxies https://darkmatterdarkenergy.com/tag/dwarf-galaxies/ that are found near our Milky Way. The low luminosity of dwarf galaxies due to stars and supernovae can make it easier to extract evidence of dark matter due to its self annihilation.

Our own Milky Way Galaxy has a higher concentration of normal matter at the center, and is expected to have a higher concentration of dark matter as well. For the past 5 years or so, there has been evidence for possible dark matter annihilation at the Galactic Center. See http://www.sciencedirect.com/science/article/pii/S0370269311001742.

The mechanism is dark matter self-annihilation, resulting in the creation of decay products of ordinary matter and gamma rays (highly energetic photons). See one of my prior blogs at: https://darkmatterdarkenergy.com/tag/dark-matter-annihilation/.

The leading dark matter candidate is some sort of WIMP (weakly interacting massive particle). WIMPs interact only via gravity and perhaps the weak nuclear force. WIMP self-annihilations can produce quarks, neutrinos, gamma rays and other ordinary matter particles.

There is a known gamma ray signal in the Galactic Center (the center of our Milky Way) that extends to 5 degrees away from the center, corresponding to roughly a kiloparsec in extent (a kiloparsec is 3260 light-years, and our Sun is 8 kiloparsecs from the Center). The major alternatives for this signal appear to be dark matter annihilation, cosmic ray interactions with interstellar gas, or emission from rapidly rotating neutron stars (millisecond pulsars).

A recent paper from T. Daylan and co-authors from Harvard, MIT, Princeton, the University of Chicago and the Fermi Laboratory is titled “The Characterization of the Gamma-Ray Signal from the Central Milky Way: A Compelling Case for Annihilating Dark Matter”. They have reanalyzed observations from the Fermi Gamma Ray Space Telescope and confirmed that the distribution of gamma rays in the Galactic Center (GC) is largely spherically symmetric and extended. This spatial distribution likely rules out neutron stars as the source, since these are preferentially found in the galactic disk.

arxiv1402.6703v2.fig10.1to3GeV

1-3 GeV residual gamma ray image. From Fig. 10 of T. Daylan et al., this is corrected for galactic diffuse emission and has point sources subtracted. The image extends over a 10◦ by 10◦ region.

Dark matter, on the other hand, would be expected to have a roughly spherical distribution around the GC. Interstellar gas is also largely confined to the galactic disk, so this explanation is disfavored. Their study also confirms that the emission extends beyond the GC to what is known as the Inner Galaxy, further ruling out the two alternatives other than dark matter annihilation. The emission falls off in intensity away from the GC, in a manner consistent with a spherically symmetric dark matter density distribution that is in accordance with a Navarro-Frenk-White profile often used successfully in modeling dark matter halos. No evidence is found for any significant deviation from spherical symmetry for the GC and Inner Galaxy components, the latter extending out to around 2 kiloparsecs.

There are various possible annihilation channels for dark matter and the authors’ analysis appears to favor a dominant channel to primarily b quarks (and b antiquarks). In this scenario the WIMP mass appears to lie in the range of 36 to 51 GeV (by comparison a proton or neutron mass is about .94 GeV). Recall that there are 6 types of quarks, and protons and neutrons are composed of u and d (up and down) quarks. The others are b, t, c, s (bottom, top, charm and strange). The quarks other than u and d are unstable and will decay to u and d.

The spectral (energy) distribution peaks at gamma ray energies of around 1 to 3 GeV and is a good fit to the predictions for annihilation to a b quark pair (b and anti-b). In addition, the cross-section for annihilation calculated from the gamma ray intensity is consistent with that expected from the required rate of thermal production of dark matter particles in the early universe, of order 10^-26 cm^3/sec (actually a value of the cross-section multiplied by the average velocity). The observed dark matter abundance freezes out from thermal equilibrium in the early universe as it expanded and cooled, and implies a cross section of that order.

There is also the possibility for other decay channels, including decays to u, d, c, s and t quarks and to tau lepton particles. The spectral shapes disfavor decays to tau leptons and u, d quarks in particular. After decays to b quarks, the c (charm) and s (strange) quark channels are the most likely. Either a c or s quark channel implies somewhat lower WIMP masses, around the 20 to 40 GeV range. Annihilations to other fermions appear less likely.

In summary, quoting from their paper:

“This signal consists of a very large number of events, and has been detected with overwhelming statistical significance. The the excess consists of ∼ 10,000 gamma rays per square meter, per year above 1 GeV (from within 10◦ of the Galactic Center). Not only does this large number of events enable us to conclude with confidence that the signal is present, but it also allows us to determine its spectrum and morphology in some detail. And as shown, the measured spectrum, angular distribution, and normalization of this emission does indeed match well with that expected from annihilating dark matter particles.”

“There is no reason to expect that any diffuse astrophysical emission processes would exhibit either the spectrum or the morphology of the observed signal. In particular, the spherical symmetry of the observed emission with respect to the Galactic Center does not trace any combination of astrophysical components (i.e. radiation, gas, dust, star formation, etc.), but does follow the square of the anticipated dark matter density.”

There are also possible detections, marginally significant, of gamma ray emission due to dark matter in nearby dwarf galaxies, and in the direction of the Virgo cluster. We look forward to additional observations and theoretical work to confirm dark matter annihilation signals in our own galaxy and nearby galaxies.

NEW BOOK just released:

S. Perrenod, 2016, 72 Beautiful Galaxies (especially designed for iPad, iOS; ages 12 and up)

Andromeda_galaxy_Galex

Leave a comment | tags: Dark matter, dark matter annihilation, dark matter halos, Fermi Gamma-ray telescope, galactic center, gamma rays, inner galaxy, Milky Way, quarks, WIMPs | posted in Dark Matter, Uncategorized

July 2, 2015

Black Holes Destroy Dark Matter (and Emit Gamma Rays)

By darkmatterdarkenergy

Black holes can cause dark matter to annihilate in their vicinity by concentrating the dark matter and enhancing the collision rate between dark matter particles. The best observational candidates are supermassive black holes, such as the 4 million solar mass black hole found at the center of our Milky Way galaxy. Some galaxies have much larger supermassive black holes, reaching as high as several billion or even tens of billions of solar masses. Most massive galaxies appear to have supermassive black holes in their centers.

Artist’s conception of a supermassive black hole (public domain; courtesy NASA JPL)

We infer the existence of supermassive black holes through their effect on nearby stellar or molecular cloud orbits. And we more directly detect supermassive black holes (SMBHs) by the radiation emitted from ordinary matter that is near the black hole (BH), but has not yet fallen into the BH’s event horizon (from which nothing, not even light, can escape). Such matter will often form a hot accretion disk around the SMBH. The disk or other infalling matter can be heated to millions of degrees by the strong gravitational potential of the BH as the kinetic energy of infall is converted to thermal energy by frictional processes. Ordinary matter (OM) heated to such high temperatures will give off X-rays.

Now if OM is being pulled into a SMBH, so is dark matter, which pervades every galaxy. Dark matter (DM) responds to the same gravitational potential from the SMBH. The difference is that OM is collisional since it easily interacts with other OM via the electromagnetic force, whereas DM is generally collisionless, since it does not interact via electromagnetism.

Nevertheless DM – DM collisions can occur, rarely, via a ‘direct hit’ (as if two bullets hit each other in mid-air) and this leads to annihilation. Two DM particles meet directly and their entire energy content, from their rest mass as well as their kinetic energy of motion, is converted into other particles. The cross-section strength is not known, but it must be small due to observational limits, yet is expected to be non-zero. The most likely candidates for decay products are expected to be photons, neutrinos, and electrons.

The leading candidate for DM is some sort of weakly interacting massive particle with a mass of perhaps 5 to 300 GeV; this is the range where DM searches from satellites and on Earth are focused. (The proton mass is a little less than 1 GeV = billion electron Volts.) So if two DM particles mutually annihilate, there is of order 10 GeV to 600 GeV of available rest mass energy to produce highly energetic gamma rays.

The likelihood of a direct hit is proportional to the square of the density of the DM. A SMBH’s gravitational potential acts as a concentrator for DM, allowing the density to be high enough that there could be a significant number of annihilation events, resulting in a detectable flux of escaping photons reaching Earth. Relativistic effects work to further increase the annihilation rate. And it is possible that the annihilation signal could scale as M³ (mass of the SMBH cubed), and thus the most massive SMBHs would be very strong gamma ray emitters. These would be highly energetic gamma rays with well over 1 GeV of energy.

Movie from NASA Goddard showing Dr. Jeremy Schnittman’s simulation

Dr. Jeremy Schnittman of the NASA Goddard Space Flight Center has investigated possible annihilation rates and the nature of the observable gamma ray spectrum for some simple dark matter models. He used a compute cluster to simulate hundreds of millions of DM particles moving in the general direction of a SMBH. One of his remarkable findings is that much higher gamma ray energies can be produced than previously believed, in the case of SMBHs which are rapidly spinning.

This is a result of something known as the Penrose process, which allows energy to be extracted from a rotating BH. There is a region called the ergosphere outside of the event horizon and when two DM particles annihilate in this region and produce two gamma rays, one gamma ray photon would fall into the event horizon (into the BH), and the other photon would escape to infinity, possibly in the direction of Earth. Dr. Schnittman’s simulation indicates that the energy boost can be as high as 6 times or more. The faster the SMBH is spinning, the greater the potential energy boost.

He also has looked at DM particles on bound orbits, which are likely to form into a (donut-shaped) corotating torus around the SMBH, aligned with its spin vector. The bound DM particle annihilations lead to lower energy gamma ray production, as compared to the unbound particles.

One of the important considerations is that the influence radius of the BH is very large. The size of the BH itself (event horizon or Schwarzschild radius) is small, even for SMBHs. The radius is proportional to the mass, via the relation 2GM/R = c² (G is the gravitational constant, c the speed of light and M and R are the BH mass and radius, respectively). A SMBH with a mass of 10 million solar masses will have a radius of only around 30 million kilometers, or about 1/5 of the Earth-Sun distance (an AU, or astronomical unit).

But the gravitational influence is much greater, since DM particles are typically expected to be moving at around only a couple of hundred kilometers per second far away from the SMBH. Thus DM particles that are 1 million times further away than the SMBH will have their orbits in their galaxy perturbed by the SMBH. And the scale of influence is thus parsecs (1 parsec = 3.26 light-years) or tens of parsecs or even hundreds of parsecs, depending on the SMBH mass.

The most energetic gamma rays can be produced by unbound DM particles. These are on orbits which can approach near to the SMBH after falling from far away (a “swan dive” toward the SMBH) and these DM particles would then typically head out away from the SMBH in the opposite direction. But before they are able to, they have a direct hit with another DM particle and annihilate into gamma rays or some other decay products.

The search for gamma rays from annihilating DM around SMBHs is already underway. There is in fact a possible detection by the Fermi telescope at 130 GeV in our Milky Way galaxy, from the direction of the Sagittarius A* SMBH. Future more sensitive gamma ray surveys may lead to many detections, helping us to better understand both dark matter and black holes.

References

J.D. Schnittman, 2015. “The Distribution and Annihilattion of Dark Matter around Black Holes”, http://arxiv.org/abs/1506.06728

J.D. Schnittman, 2014. Phys. Rev. Letters 113, 261102, “Revised Upper Limit to Energy Extraction from a Kerr Black Hole”

1 Comment | tags: black holes, Dark matter, dark matter annihilation, Fermi Gamma-ray telescope, gamma rays, Milky Way galaxy, supermassive black holes, WIMPs | posted in Dark Matter

April 11, 2012

Fermi Gamma-ray Telescope Search for Dark Matter

By darkmatterdarkenergy

Dwarf Galaxy in Fornax, Credit: ESO/Digital Sky Survey 2

All of our evidence for dark matter is indirect, that is, we deduce the existence of substantial amounts of dark matter – exceeding the amount of ‘ordinary matter’ by 5 times – from its gravitational effects at large distances, on the scale of galaxies, clusters of galaxies, and even across regions of a billion light-years in size.

The generally favored hypothesis is that dark matter is composed of some sort of new weakly interacting massive particle (WIMP). Such a WIMP would not interact via the electromagnetic force or the strong nuclear force, and thus is very hard to detect directly. There are a number of experiments underway attempting to directly detect the elusive particle. The majority of these are earthbound experiments wherein ordinary matter in crystalline form is used as the detector. Three of these experiments are in fact claiming statistically significant detection rates, the DAMA/Libra experiment in Italy, the CRESST experiment based in Germany, and the COGENT experiment in the U.S.

In this class of experiments one is looking to detect a collision of a dark matter particle with an ordinary matter particle, and if detection is successful, to determine the mass of the dark matter particle, as well as its cross-section for collision with ordinary matter. The cross-section is a way of measuring how close the ordinary matter particle and dark matter particle have to approach each other to have a collision event. A collision event produces decay products (additional particles) that the experiments are then able to detect. The results of the 3 experiments named above are still highly controversial, as a number of other similar experiments are not confirming detection, but collectively they may indicate detection of WIMPs with mass in a range around 5 to 40 times the mass of a proton.

Another approach to more directly detecting dark matter (i.e., not through its gravitational effects) is to look for dark matter particles colliding with one another. The number of such events occurring at the Earth’s surface is expected to be quite low, so one must look into the cosmos. Recently some scientists calculated that about one dark matter particle a month on average strikes an atom inside a human on Earth (but not to worry, other background radiation to which we are exposed is much more significant). But we are in a region of over-concentration of ordinary matter; dark matter is spread out on the largest scales. We need to examine much bigger regions to observe dark matter particles striking one another. But look for what, and how?

NASA’s orbiting Fermi Gamma-ray Telescope provides one way. When two dark matter particles collide, depending on the nature of the dark matter particle (is it its own anti-particle), they can possibly mutually annhilate and produce very energetic gamma-ray photons. Gamma rays are at the most energetic end of the electromagnetic spectrum, which includes X-rays, visible light, and radio waves.

Recently the LAT, or Large Area Telescope, which is the main instrument on board the Fermi Gamma-ray Telescope (in orbit) reported on results based on two years of searching for gamma ray production from dark matter annihilation. The method used was to monitor 10 dwarf galaxies that are gravitationally bound to our Milky Way galaxy.

Take a look at this video from NASA Goddard: No WIMPS in Space?

Dwarf galaxies are thought to be good candidates for this type of dark matter search, as they are the remnants of so-called dark matter halos that may have been the first large-scale gravitationally bound objects to form. Larger galaxies in turn grew from multiple such halos coming together, but today’s remaining dwarf galaxies did not get caught up in significant merger activity. They also have mature stellar populations, so there is not a lot of ongoing activity with respect to supernovae or black holes that would produce gamma rays from these other causes.

The international team using the Fermi telescope looked for gamma rays with energies up to 100 billion electron volts (about 100 times the rest mass energy equivalent of a proton), but did not find any that could be clearly attributed to dark matter. The search will continue, gathering more statistics over time and adding additional target dwarf galaxies to their measurements, in the hopes of either finding dark matter through this method, or putting more constraints on its properties.

References:

http://www.cresst.de/darkmatter.php – CRESST experiment

http://cogent.pnnl.gov/ – COGENT experiment

http://people.roma2.infn.it/~dama/web/home.html – DAMA/LIBRA experiment

http://en.wikipedia.org/wiki/Fermi_Gamma-ray_Space_Telescope

http://en.wikipedia.org/wiki/Dark_matter#Direct_detection_experiments

http://www.nasa.gov/mission_pages/GLAST/news/dark-matter-insights.html – Fermi Gamma-ray telescope

Leave a comment | tags: Dark matter, dark matter annhilation, dwarf galaxies, Fermi Gamma-ray telescope, gamma rays | posted in Dark Matter