Category: WIMPs

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A search for dark matter among Fermi-LAT unidentified sources with systematic features in Machine Learning

Written by Viviana Gammaldi.

Summary of the paper with the same title published in MNRAS.

arXiv: 2207.09307

The recent 4FGL Fermi-LAT catalogue, the result of 8 years of telescope operation, is a collection of sources with associated gamma-ray spectra, containing important information about their nature. As shown in Fig. 1, somehow surprisingly, an important fraction of objects in the Fermi-LAT catalogs, ca. 1/3 of the total, remain as unidentified (unIDs), i.e., objects lacking a clear single association to a known object identified at other wavelengths, or to a well-known spectral type emitting only in gamma rays, e.g. certain pulsars. Indeed, there is the exciting possibility that some of these sources could be a DM signal. Among other prospective sources of gamma rays from DM annihilation events, dark satellites or subhalos in the Milky Way, with no optical counterparts, are the preferred candidates, as they are expected to exist in high number according to standard cosmology and they would not be massive enough to retain gas/stars. Further, main galaxies in local Universe, e.g. dwarf irregular galaxies, may also represent good candidates for unIDs.

Fig. 1: Fermi-LAT detected sources.

We propose a new approach to solve the standard, Machine Learning (ML) binary classification problem of disentangling prospective DM sources (simulated data) from astrophysical sources (observed data) among the unIDs of the 4FGL Fermi-LAT catalogue.

In particular, we are interested in one of the parametrizations of the gamma-ray spectrum used in the 4FGL, known as the Log-Parabola (LP), which allow us to identify different astrophysical sources of gamma rays by means of at least two parameters, the emission peak ,Epeak, and the spectral curvature, beta. Indeed, we introduce the DM sample in the parameter space by fitting the simulated DM gamma-ray spectrum with the same LP functional form (Fig. 2, left panel). Furthermore, we artificially build two systematic features for the DM data which are originally inherent to observed data: the detection significance and the relative uncertainty on the spectral curvature, beta_rel. We do it by sampling from the observed population of unIDs, assuming that the DM distributions would, if any, follow the latter. In Fig. 2 we show the parameter space without the uncertainty on beta (left panel) and by including the uncertainty on beta, created for the DM sample as systematic feature.

Fig. 2: beta-Epeak parameter space. Left panel: Astrophysical (yellow), DM (magenta) and unIDs (red) sources are shown. Right panel: Same as left panel, but including the uncertainty on beta for the training/test set (grey data) and the unIDs sources to be classified (red data point).

Finally, we consider different ML models for the classification task: Logistic Regression, Neural Network (NN), Naive Bayes and Gaussian Process, out of which the best, in terms of classification accuracy, is the NN, achieving around 93% performance. Applying the NN to the unIDs sample, we find that the degeneracy between some astrophysical and DM sources (visible as overlapping region in Fig. 2) can be partially solved within by including systematic features in the classification task (Fig. 3). Nonetheless, due to strong statistical fluctuations, we conclude that there are no DM source candidates among the pool of 4FGL Fermi-LAT unIDs.

Fig. 3: Probability for each unIDs to be a DM source. Left panel: results adopting only two feature beta-Epeak for classification. Right panel: results for the four-features (beta, Epeak, sigma, beta_rel) classification.

Further details can be found in https://doi.org/10.1093/mnras/stad066 .

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Dark Matter search in dwarf irregular galaxies with the Fermi Large Area Telescope

Written by Viviana Gammaldi.

Summary of the paper with the same title published in PRD.

arXiv: 2109.11291

Almost a century ago, first astrophysical hints came to light pointing to the existence of new physics: the gravitational interaction was failed to describe the kinematics of extragalactic objects. The gap between theory and observation could be filled both on astrophysical and cosmological scale, by assuming the existence of a new kind of non-luminous, yet gravitationally-interacting matter, i.e. the Dark Matter (DM). Currently, the abundance of DM has been estimated to be the 27% of the total content of the Universe, although its nature remains still unknown. Among other, the Weakly Interactive Massive Particle (WIMP) represents one plausible candidate beyond the Standard Model (SM) of particle physics. WIMPs particle can annihilate in astrophysical objects producing SM particles, whose decay processes generate secondary fluxes of gamma rays, among others detectable fluxes. Searching for DM hints in secondary fluxes produced in astrophysical sources is what we call indirect searches for DM.


Recently, dwarf irregular galaxies have been proposed as astrophysical targets of interest for indirect searches of DM. In fact, they are DM dominated astrophysical objects, with a negligible astrophysical background in gamma-rays. Nonetheless, dwarf irregular galaxies also represent an interesting laboratory from the point of view of the DM distribution. In fact, although the rotation curves of spiral galaxies have been the first observational evidence of the existence of DM, there is still a lot to learn about the distribution of DM in galaxies. From the benchmark LCDM cosmology and N-body DM-only simulation (N-body DM-only simulation are a good approximation due to the abundance of DM in the Universe – 27% of the total content – with respecto to the visible baryon matter, i.e. stars, gas, etc. – which represent only the 5% of the total content of the Universe), we know that the DM distribution generally follows the well-known Navarro-Frenk-White (NFW) profile, i.e. a profile with a “cusp” in the center of big structures. On the other hand, the experimental data of the rotation curves of smaller objects, point to “core” DM distribution profile. The reason of such a discrepancy is still unknown, and represent the so called “cusp/core problem”, an open question in physics, astrophysics and particle physics. Dwarf irregular galaxies are indeed an example of the objects where the cusp/core problem is observed.


In this work we remain agnostic about the cusp/core problem: we analysed the rotation curves of 7 dwarf irregular galaxies (see e.g. the rotation curve of the IC10 galaxy, Fig. 1).

The Burkert core profile (full blue line) corresponds to the best fit of the rotation curves. Instead, the NFW cusp profile has been theoretically constructed, and the corresponding rotation curve has been created and compared with the data. With these two different DM distributions, we have constructed the DM modelling, including the enhancement in the expected gamma-ray flux due to the existence of substructures. Substructures in DM halo are over density, sub clumps in the DM distribution profiles, relics of the bottom-up formation history of the galaxy. Their effect is to enhance the DM annihilation rate, i.e. the gamma-ray production, due to the enhance of the DM density locally. We have created the two-dimensional spacial templete for our targets. In Fig. 2 the spatial template of the IC10 galaxy is shown for four different DM density distribution profiles. Indeed, the MIN model corresponds to a Burket core profile without any substructure, the MED model is the Burkert profile with a medium contribution from substructures. The maximum contribution from substructures has been calculated for both the Burkert core and the cusp NFW profile (respectively, Bur-MAX and NFW-MAX).

Finally, we have analysed the data of the Fermi Large Area Telescope. We didn’t find any gamma-ray signal from these objects, indeed we can exclude some region of the DM mass and annihilation cross section parameter space. The exclusion limit for the DM mass and annihilation cross-section are presented in Fig. 3. The blue line corresponds to the results of this work for the combined analysis of all the 7 targets with a MED model, i.e. considered as extended sources. These results are compared with the thermally averaged annihilation cross section (dotted black line), the result of the previous proof-of-concepts paper (yellow dashed line), the results of 100 simulation with a null signal from DM (yellow band) and the exclusion limit from well known dwarf spheroidal galaxy obtained with Fermi-LAT (green dashed line). The results of our study are dominated by the constraints obtained by IC10 and NGC6822, and dimly depend on the considered DM profile.

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Sensitivity of the Cherenkov Telescope Array to dark subhalos

Written by Javier Coronado-Blázquez.

Summary of the paper with the same title submitted to PDU.

arXiv: 2101.10003

In this work, we study the potential of the Cherenkov Telescope Array (CTA) for the detection of Galactic dark matter (DM) subhalos, focusing on low-mass subhalos – not massive enough to retain any baryonic content – therefore lacking any multiwavelength counterpart. As in previous papers, devoted to the Fermi-LAT and HAWC instruments, If the DM is made of weakly interacting massive particles (WIMPs), these dark subhalos may appear in the gamma-ray sky as unidentified sources. We perform a detailed characterization of CTA instrumental response to dark subhalos, using the ctools analysis software, simulating CTA observations under different array configurations and pointing strategies.

We distinguish three different observational modes: i) a key science project, the extragalactic survey (codename EGAL). This will observe a fourth of sky at high-latitudes with uniform exposure, providing unprecedented coverage at very high energies; ii) a proposed deep-field campaign (DEEP), which would point at a blank spot of the sky aiming to serendipitously find new sources, such as dark subhalos, due to the extreme sensitivity; and iii) a chance of finding a dark subhalo in the field of view of any of CTA’s science operations through accumulated exposure the years, so-called overall exposure (EXPO).

To be able to compute the latter strategy, one has to estimate the sky coverage in, e.g., 10 years of operation, as well as the median exposure time. We did so by extrapolating the MAGIC telescope operations, which share location with the CTA-North. In this way, we get a realistic estimation of the accumulated observations, which turn out to be a factor 2 more area and a factor 10 more time than the EGAL survey. This, together with information on the subhalo population as inferred from N-body cosmological simulations, allows us to predict the CTA detectability of dark subhalos, i.e., the expected number of subhalos in each of the considered observational scenarios.

In the absence of detection, for each observation strategy we set competitive limits to the annihilation cross section as a function of the DM particle mass, that are between one and two orders of magnitude away from the thermal cross section, for the bb and ττ annihilation channels. This way, CTA will offer the most constraining limits from subhalo searches in the intermediate range between 1−3 TeV, complementing previous results with Fermi-LAT and HAWC at lower and higher energies, respectively, as well as an independent probe of DM.

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SKA-Phase 1 sensitivity to synchrotron radio emission from multi-TeV Dark Matter candidates

Written by V. Gammaldi and M. Méndez-Isla.

Summary of the paper with the same title published in PDU.

arXiv: 1905.11154

Dark matter constitutes a fundamental piece within the paradigm of modern Cosmology, comprising ~25% of the energy density of the universe. Despite numerous evidence of the existence of dark matter, its nature remains elusive. Based on the study of thermal relics in the Early Universe, one possibility would be conceiving dark matter as particles. Indeed, the energy density of dark matter today could be explained in terms of Weakly Interactive Massive Particles that were coupled with the primordial plasma. 

Considering dark matter annihilating in galactic halos, it would be reasonable to expect signatures in the sky that may be observed through different detectors. This fact would allow us to constrain the dark matter parameter space comparing theoretical predictions with diverse observational data. In fact, there exists the possibility of dark matter annihilating into Standard Model particles that subsequently would decay or hadronise into cosmic rays. In this scenario, dark matter not only would constitute an exotic source of cosmic rays but also of photons in a large range of frequencies. Such photons are the result of the interaction of the abovementioned cosmic rays with the interstellar medium. Indeed, one possibility could be dark matter annihilating into electrons/positrons whose interaction with galactic magnetic fields would produce synchrotron signals, in general, at radio frequencies. In this sense, high-sensitivity radio telescopes, such as the Square Kilometre Array, could be crucial to put tight constraints on both the dark matter mass M and its thermally averaged cross section.

With the purpose of constraining radio signals from TeV dark matter candidates with SKA, we compute the expected flux density for different annihilation channels in the Draco dwarf spheroidal galaxy and we compare it with the SKA sensitivity. Varying the dark matter mass M and thermally averaged cross section, we set sensitivity constraints, as shown in the Figure below. In such a Figure, the region above the orange and blue curves show the dark matter parameter space detectable by the SKA. Furthermore, the intersection between the orange curve and the dashed black line representing a cross section of 3e26 cm^3 / s shows that the maximum observable mass for thermal relics would lie around 10 TeV for dark matter annihilating into W+W- and b quarks.

A similar analysis is performed for extra-dimensional Brane-world DM candidates, dubbed branons, i.e., new degrees of freedom appearing in flexible Brane-world models. This particular case is analysed for usual astrophysical scenarios as well as alternative ones in which the synchrotron signal would be enhanced by the presence of an intermediate-mass black hole. This latter possibility could be the key to observe dark matter masses beyond the 10 TeV detectable in conventional scenarios.

Finally, we compare the SKA facilities in dark matter searches with other detectors for different ranges of frequencies. Even though SKA is expected to be the most sensitivity telescope in radio frequencies, the most suitable frequency range to detect dark matter would be affected by the annihilation channel. In this regard, in our work we also analyse the role played by detectors such as GBT, VLA or LOFAR for TeV dark matter multi-wavelength searches.

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Constraints to dark matter annihilation from high-latitude HAWC unidentified sources

Written by Javier Coronado-Blázquez.

Summary of the paper with the same title published by Galaxies.

arXiv: 2001.02536

Unidentified gamma-ray sources (unIDs) have proven to be competitive targets for indirect DM detection, via the ΛCDM-predicted subhalos in our own Galaxy, not massive enough to retain baryons and become visible. These may shine in gamma rays provided that the dark matter (DM) is made of weakly interacting massive particles (WIMPs), that would self-annihilate and would appear as unIDs.

In previous works, we used Fermi-LAT due to the all-sky coverage, which maximizes the probability of finding a DM subhalo within the unIDs pool. One can play the same game for Very High Energy (VHE) observatories, such as the Imaging Atmospheric Cherenkov Telescopes (IACTs). Some of them are MAGIC, H.E.S.S., VERITAS or MILAGRO. The “problem” of these observatories is that, unlikely Fermi-LAT, are pointing telescopes, i.e., have a very narrow field of view, and therefore, the sky coverage is very small.

Another interesting observatory is HAWC, which works very similar to IACTs, but using water as medium for the Cherenkov radiation. HAWC is not a pointing telescope, and due to the rotation of the Earth it observes ~2/3 of the sky with uniform exposure. Combining all these observatories produces the TeVCat, a VHE source catalog. As we are interested in high-latitude sources, due to the Galactic astrophysical confusion at low latitudes.

In the TeVCat we find three interesting high-latitude (|b|≥10º) sources present in HAWC’s 2HWC catalog, with no associations at other wavelengths. Indeed, only one of these sources, 2HWC J1040+308, is found to be above the HAWC detection threshold when considering 760 days of data, a factor 1.5 more exposure time than in the original 2HWC catalog. Other instruments such as Fermi-LAT or VERITAS at lower energies do not detect this source.

Also, this unID is reported as spatially extended, which is supposed to be a “smoking gun” in a DM search context. While waiting for more data that may shed further light on the nature of this source, we can set competitive upper limits on the annihilation cross section by comparing this HAWC unID to expectations based on state-of-the-art N-body cosmological simulations of the Galactic subhalo population. We find these constraints to be particularly competitive for heavy WIMPs.

Although far from the thermal relic cross section value, the obtained limits are independent and nicely complementary to those from radically different DM analyses and targets, demonstrating again the high potential of this DM search approach.

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Spectral and spatial analysis of the dark matter subhalo candidates among Fermi Large Area Telescope unidentified sources

[Javier Coronado-Blázquez, Miguel A. Sánchez-Conde, Mattia Di Mauro, Alejandra Aguirre-Santaella, Ioana Ciucă, Alberto Domínguez, Daisuke Kawata, Néstor Mirabal]

Written by Javier Coronado-Blázquez.

Summary of the paper with the same title accepted by JCAP.

arXiv: 1910.14429

Fermi-LAT unidentified sources (unIDs) are very promising targets for indirect dark matter (DM) searches of subhalos, which would appear as this kind of objects in the gamma-ray sky. In a previous work, we found that among the 1235 unIDs in Fermi-LAT catalogs (3FGL, 2FHL and 3FHL) only 44 are DM subhalos candidates (see https://projects.ift.uam-csic.es/damasco/?p=61). Here, we perform a detailed spectral analysis to test whether these remaining sources are compatible or not with a DM origin. This analysis is executed using almost 10 years of Pass 8 Fermi-LAT data, which maximizes the response and sensitivity of the instrument.

As a result, none of the unIDs are found to significantly prefer DM-induced emission compared to more conventional astrophysical sources. Previous constraints on the DM annihilation cross section are updated and improved with the new number of remaining DM subhalo candidates among unIDs, becoming closer to the famous LAT dSphs limits.

Additionally, in order to discriminate between pulsar and DM sources, which have a very similar spectrum, we developed a new method based on the source’s spectral curvature, peak energy, and its detection significance, which turns out to be specially useful for bright sources.

We also look for spatial extension, which may be a hint for a DM origin. In fact, according to our N-body simulation studies of the subhalo population, the brightest subhalos should appear as extended in the sky, maybe as large as 10º. Nevertheless, we did find no evidence of spatial extension for any of our best candidates.

Finally, we used Gaia DR2 data to search for a potential stellar counterpart to our best DM subhalo candidates, if one of them once hosted a dwarf galaxy. Due to tidal forces induced by gravitational interaction with the DM host halo, the subhalos can be torn into a stellar stream, a filament of stars orbiting the Milky Way halo. Finding evidence of an unID in one of these streams would reinforce the DM hypothesis. Although no firm associations could be found, one of our candidates coincides with the Sagittarius stellar stream.

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Unidentified gamma-ray sources as targets for indirect dark matter detection with the Fermi Large Area Telescope

Written by Javier Coronado-Blázquez.

Summary of the paper with the same title accepted by JCAP.

arXiv: 1906.11896

If the dark matter (DM) is made of WIMP particles, they should yield a gamma-ray annihilation flux. Provided the DM clumps, which are predicted by the bottom-up collapse scenario for structure formation within LCDM, are light enough, these are not expected to retain any baryons and, therefore to remain completely dark to standard telescopes.

Only in gamma rays we should detect this emission, with a highly curved and distinctive spectrum. Fermi-LAT, a space-based gamma-ray telescope, has roughly one third of detected sources of unknown nature, the so-called unidentified sources (unIDs). Are there subhalos hidden in the Fermi-LAT catalogs, just waiting to be properly classified? To answer this question we propose a ‘filtering’ of the unIDs, according to the expected characteristics of a DM subhalo. For example, a DM subhalo should be detected as a steady gamma-ray source, so if an unID is reported as variable, is removed from our pool of candidates. With these ‘filters’, we are able to reduce the DM candidates from 1235 to just 44 unIDs.

A correct computation of the expected annihilation fluxes (encoded in the so-called J-factor) requires a good understanding of the DM subhalo population in our Galaxy. To do so, we use a state-of-the-art N-body cosmological simulation, Via Lactea II (VL-II). Unfortunately, the resolution of this, and any, simulation is limited, meaning we are unable to resolve the smallest members of the subhalo population (which can be as light as the Earth). Nevertheless, some of these subhalos may be close enough to us and still be bright enough to be detected by Fermi-LAT. To take them into account, we ‘repopulate’ the parent simulation with subhalos, reaching 3 orders of magnitude better resolution in mass. As the Earth position is arbitrary, to ensure a proper statistical treatment of the J-factors, 1000 realizations are done.

Also, we must compute the sensitivity of the Fermi-LAT to this kind of objects. The reported instrumental sensitivity is for a power-law source, i.e., with a non-curved spectrum, while we deal with highly-curved spectra, which also vary with the annihilation channel and mass of the WIMP. Furthermore, taking into account the diffuse gamma-ray emission and the considered catalog, this sensitivity also varies with the sky position (e.g. a source close to the Galactic plane, where the diffuse emission is more intense, will be harder to detect than a high-latitude source) and observation parameters (such as the energy threshold, total exposure time, instrumental response functions, etc.). We compute a huge grid of configurations, changing all the above mentioned parameters and generating a sensitivity skymap for each annihilation channel, WIMP mass, and catalog.

Finally, we are able to set constraints in the cross section vs. WIMP mass parameter space. We do so comparing the predictions of our repopulated simulations with the actual gamma-ray candidates. The obtained constraints are competitive with other targets such as the CMB or the dSphs, but they can still be improved via new rejections of candidates.

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