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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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Features in cosmic-ray lepton data unveil the properties of nearby cosmic accelerators

[Ottavio Fornieri, Daniele Gaggero, Dario Grasso]

Written by Ottavio Fornieri.

Summary of the paper with the same title recently submitted to the journal.

arXiv: 1907.03696

The origin and transport properties of leptonic cosmic rays (CRs) have intrigued scientists for decades. Their propagation in the Galaxy is characterized not only by diffusion, but also by strong energy losses. Given the E^2 scaling of this loss rate, the effective horizon associated to CR leptons progressively shrinks as energy increases, hence the stochastic nature of the sources is expected to emerge. On the other hand, CR protons and nuclei can originate from a much larger galactic volume and give fundamental information on the propagation details. The experimental effort in measuring CR spectra (and in particular their spectral features) has lately reached an unprecedented precision, offering valuable clues on the source ages/positions as well as on the details of the transport.

The standard propagation model that considers positrons as of secondary origin exclusively is in contrast with well-established experimental observations (PAMELA and AMS Collaborations): The data clearly show a positron excess above ~ 40 GeV, and an even larger excess in the electron flux. In this context, the main challenge is to remove this tension within a unified picture that includes all the available observables.

To this aim, we first perform a multi-channel fit of the available data on CR protons and nuclei, by solving the transport equation with the DRAGON numerical code, to set the relevant propagation parameters: we find a set of values compatible with single-observable dedicated analyses.

With this transport setup at hand, we invoke the pulsar explanation to reproduce the positron flux, propagating particles coming from several sources (with a burst-like or constant-luminosity injection, and broken power law energy spectrum), possibly injected by one of the two closest observed pulsars, Geminga and Monogem. We find that each of these possibilities is able to fit the data, not allowing to pinpoint the dominating antimatter source.

Nevertheless, the key point to keep in mind is that pulsars are pair e+/e- factories. Therefore, since positrons are ~O(10) less than electrons, other additional source(s) of e- only are needed. Hence, we consider nearby observed SNRs, fitting their contribution to the e+ + e- spectrum measured by the H.E.S.S. Collaboration up to ~ 20 TeV: we find that, under certain assumptions, those known sources are not able to reproduce the spectral feature at ~ 1 TeV. In order to get a good fit of that part of the spectrum as well, we invoke the contribution from an old SNR t_age ~ 1e5 yr that, having reached its final radiative phase, is no longer visible with a multi-wavelength analysis. The properties of this hidden sources are characterized by our fitting procedure, and possible future strategies for its identification are discussed.

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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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Properties of subhalos in the interacting dark matter scenario

[Moliné, Ángeles; Schewtschenko, Jascha A.; Sánchez-Conde, Miguel A.; Aguirre-Santaella, Alejandra; Cora, Sofía A.; Abadi, Mario]

Written by A. Moliné

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

arXiv: 1907.12531

The current standard model of cosmology is based on a cosmological constant to explain the late-time accelerated expansion of the Universe and a cold dark matter (CDM) component to account for the required additional gravitational attraction to form and support the galaxies and larger structures we observe today. In this framework, the structure of the Universe is formed via a hierarchical, bottom-up scenario with small primordial density perturbations growing to the point where they collapse into the filaments, walls and eventually dark matter (DM) halos that form the underlying large-scale-structure filamentary web of the Universe. The galaxies are embedded in these massive, extended DM halos teeming with self-bound substructure. Any viable cosmological model has to successfully predict both the abundance and internal properties of these structures and their substructures, and match the observational data on a wide range of scales.

One natural deviation from the collisionless CDM in the standard model is the assumption of the existence of interactions between DM and the standard model (SM) particles we know about, in particular, photons or neutrinos. This does not only affect the formation of DM structures on small scales, but also provides an explanation for the exact relic abundance of DM found in the Universe today. Such possible interacting dark matter (IDM) model would imply a suppression of small-scale structures due to a large collisional damping effect, even though the weakly interacting massive particle (WIMP) can still be the DM candidate. Because of this, IDM models can help alleviate alleged tensions between standard CDM predictions and observations at small mass scales.

Using a high-resolution cosmological N-body simulation specifically run within this alternative model, we investigate the properties of DM halo substructure or subhalos formed in. We also run its CDM counterpart, which allowed us to compare subhalo properties in both cosmologies. We show that, in the lower mass range covered by our simulation runs, both subhalo concentrations and abundances as a function of the distance to the host halo center and subhalo mass (or, alternatively maximum circular velocity) are systematically lower in IDM compared to the CDM scenario. Yet, as in CDM, we find that median IDM subhalo concentration values increase towards the innermost regions of their hosts for same mass subhalos. Also similarly to CDM, we find IDM subhalos to be more concentrated than field halos of the same mass.

Our work has a direct application on studies aimed at the indirect detection of DM where subhalos are expected to boost the DM signal of their host halos significantly. From our results, we conclude that the role of halo substructure in DM searches will be less important in interacting scenarios than in CDM, but is nevertheless far from being negligible.

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Looking for ultralight dark matter near supermassive black holes

[Bar, Nitsan; Blum, Kfir; Lacroix, Thomas; Panci, Paolo]

Written by T. Lacroix

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

arXiv: 1905.11745

Ultralight dark matter (ULDM) has gained interest in the past few years both as an alternative to thermally produced dark matter (DM) candidates, and as a promising way to alleviate possible small-scale tensions within the cold DM paradigm, for masses between 1e-22 and 1e-21 eV. Although this particular mass range is now in tension with cosmological constraints, more general ULDM candidates still comprise the lightest possible DM candidates and as such warrant dedicated studies.

Now, measurements of the dynamical environment of supermassive black holes (SMBHs) have been steadily accumulating, becoming both abundant and precise. In this paper, we use such measurements to look for ULDM, which is predicted to form dense cores (“solitons”) in the center of galactic halos. More specifically, we search for the gravitational imprint of an ULDM soliton on stellar orbits near the supermassive black hole Sgr A* at the center of the Milky Way, and by combining stellar velocity measurements with Event Horizon Telescope imaging of M87*. Finding no positive evidence, we set limits on the soliton mass for different values of the ULDM particle mass m. The constraints we derive exclude the solitons predicted by a naive extrapolation of the soliton-halo relation, found in DM-only numerical simulations, for 2e-20 eV <~ m <~ 8e-19 eV (from Sgr A*) and m <~ 4e-22 eV (from M87*). However, we present theoretical arguments suggesting that an extrapolation of the soliton-halo relation may not be adequate: in some regions of the parameter space, the dynamical effects of the SMBH could cause this extrapolation to over-predict the soliton mass by orders of magnitude. This prevents constraints on solitons from being turned into exclusion of ranges of ULDM mass m. Dedicated numerical simulations accounting for these effects are therefore in order.

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X-ray and gamma-ray limits on the primordial black hole abundance from Hawking radiation

Written by J. Coronado-Blazquez

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

arXiv: 1906.10113

In the last few years, mainly due to LIGO’s detection of gravitational waves, an old idea has become quite popular once more: that the dark matter (DM) is not made of a new particle, but of black holes of primordial origin (PBHs), this is, produced during the inflationary phase of the Universe, shortly after the Big Bang. The range of masses of these objects are as low as 10^15g (much lighter than the Halley comet) up to thousands of times the Sun mass.

Stringent constraints have been put to f, quotient between the cosmological density of the PBHs versus the required DM one. If f=1, it means that all of the DM is in form of PBHs, while f=0.1 means only a 10% can be explained with them. Most of the ‘mass windows’ are now closed by different probes, but there is a very interesting one, ranging from 1e16 to 1e19g. No experiment has been able to constraint this mass window since the femtolensing limits placed in the region were found to be wrongly computed and no constraining at all.

A very popular and stringent constraint is the one achieved by the Hawking radiation: due to spontaneous particle creation near the event horizon, one member of the particle-antiparticle pair may fall into the PBH, leaving the other one without a companion to annihilate with. Therefore, the PBH seems to ‘emit’ particles, with approximately a blackbody spectrum at a given temperature inversely proportional to its mass. For a solar-mass PBH, this emission is completely negligible, but for very light PBHs, it can produce radiation as extreme as X- or gamma-rays.

This also makes the PBH to ‘evaporate’ in a given time. If this time is smaller than the age of the Universe (ca. 13.6 Gyr), they cannot be the DM, as this is expected to be stable, at least up to today. For this reason, PBHs below ~10^15g should have been evaporated now and cannot be the DM. But, if we lower the energy a bit, therefore increasing the mass, maybe they are stable emitters which can in fact be detected. As the emission we see will be the sum of all of these PBHs at cosmological scales, we should see an isotropic background.

This was done for the gamma-ray isotropic background, therefore for PBHs in the verge of the evaporation limits, but we extend these constraints to X-rays, being able to reach larger masses. Not only that, we realized that most of the isotropic emission is expected to be due to unresolved Active Galactic Nuclei (AGNs), such as BL-Lacertae or Quasars. Taking into account some models for the extrapolation of the luminosity function of these objects over cosmological scales and times, the space for the PBHs is greatly reduced, being able to constrain PBH DM up to large masses.

Finally, we perform some prospects for next-generation experiments such as E-Astrogam. As a future experiment will have a better sensitivity, it will resolve sources with a lower flux. The photons corresponding to these sources will no longer contribute to the isotropic background, lowering its intensity, and being easier to detect a PBH above it. We foresee two scenarios, a 90% and 99% background reduction, which again improve the constraints both in mass range and f.

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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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