[Paper Review] Dark matter subhalos as Fermi gamma-ray sources and first candidates in the 1FGL catalog

H.-S. Zechlin, M. V. Fernandes+2 more|arXiv (Cornell University)|Oct 31, 2011

Astrophysics and Cosmic Phenomena2 references3 citations

TL;DR

This paper proposes that dark matter subhalos in the Milky Way could emit detectable gamma rays via WIMP self-annihilation, with one candidate—1FGL J0030.7+0724—showing characteristics consistent with a dark matter subhalo. Using Fermi-LAT 1FGL data and Swift XRT follow-up, the source exhibits non-variable flux and moderate spatial extent, though it also aligns with a high-energy-peaked blazar, leaving the origin ambiguous.

ABSTRACT

Predicted by hierarchical structure formation, Milky Way-type galaxies are anticipated to host numerous dark matter subhalos with masses between 10^{10} and a cut-off of 10^{-6} solar masses, or even less. In self-annihilating dark matter scenarios, these objects could be visible in the gamma-ray band as faint and non-variable sources without astrophysical counterpart. In accordance with realistic subhalo models and current observational constraints on self-annihilating dark matter, we predict that about one massive Galactic subhalo may have already been detected in the 11-months catalog of Fermi-LAT data (1FGL). Selection cuts applied to the 1FGL reveal twelve possible candidates, and in-depth studies of the most promising object, 1FGL J0030.7+0724, are presented. In a dedicated X-ray follow-up observation with the Swift XRT, seven point-like X-ray sources have been discovered. Within the positional uncertainty derived from the 24-months Fermi-LAT data, we consider the unidentified radio source NVSS J003119+072456, coinciding with one of the discovered Swift sources, as the most promising counterpart candidate for 1FGL J0030.7+0724. The broad-band spectral energy distribution is consistent with a high-energy-peaked blazar. However, flux and extent of the Fermi source may also be compatible with a dark matter subhalo. A discrimination between the two scenarios requires further multi-wavelength observations. Strategies for identifying gamma-ray sources associated with self-annihilating DM subhalos are discussed.

Motivation & Objective

• To assess the detectability of dark matter subhalos via gamma-ray emission from WIMP self-annihilation in the Fermi-LAT 11-month survey.
• To identify and characterize potential dark matter subhalo candidates in the 1FGL catalog using astrophysical and statistical criteria.
• To test the hypothesis that 1FGL J0030.7+0724 could be a dark matter subhalo by comparing its gamma-ray, X-ray, and radio properties with known astrophysical sources.
• To evaluate the viability of a dark matter origin versus a blazar origin for the most promising candidate through multi-wavelength analysis.

Proposed method

• Modeling subhalo gamma-ray emission using the Navarro-Frenk-White (NFW) density profile and the self-annihilation rate formula involving the thermally-averaged cross section ⟨σv⟩_eff and WIMP mass m_χ.
• Applying selection cuts to the 1FGL catalog to identify 12 candidate sources, focusing on non-variable, faint, and spatially extended gamma-ray sources without known counterparts.
• Conducting an unbinned Kolmogorov-Smirnov test to assess flux constancy over time, supporting the non-variable nature of 1FGL J0030.7+0724.
• Using a likelihood-ratio test to compare point-source and extended-source hypotheses, yielding a best-fit angular extent of θ_s = 0.14° with 95% upper limit ≤ 0.72°.
• Performing Swift-XRT follow-up observations to detect X-ray counterparts, resulting in the discovery of seven new X-ray sources in the field.
• Comparing the broad-band spectral energy distribution (SED) of the radio, X-ray, and gamma-ray data with models of high-energy-peaked blazars (HBL) and dark matter subhalos.

Experimental results

Research questions

• RQ1 Can self-annihilating dark matter subhalos in the Milky Way produce detectable gamma-ray signals in the Fermi-LAT 1FGL catalog?
• RQ2 What are the key observational signatures that distinguish a dark matter subhalo from a conventional astrophysical source like a blazar?
• RQ3 Is the candidate source 1FGL J0030.7+0724 consistent with a dark matter subhalo origin, given its flux, extent, and multi-wavelength properties?
• RQ4 To what extent can variability and astrometric precision help discriminate between a dark matter subhalo and a blazar for 1FGL J0030.7+0724?
• RQ5 How do the X-ray and radio counterparts of 1FGL J0030.7+0724 constrain the physical origin of the gamma-ray emission?

Key findings

• Twelve potential dark matter subhalo candidates were identified in the 1FGL catalog, with 1FGL J0030.7+0724 being the most promising due to its non-variable flux and moderate spatial extent.
• The gamma-ray source 1FGL J0030.7+0724 shows no significant variability, with a Kolmogorov-Smirnov test confirming a steady flux at a 50% probability.
• The angular extent of the source is θ_s = 0.14°^{+0.20}_{-0.12}°, consistent with an extended source but not significantly different from a point source, with a 95% confidence upper limit of 0.72°.
• Swift-XRT observations discovered seven new X-ray sources, including SWIFT J003119.8+072454, which spatially coincides with the radio source NVSS J003119+072456 and the optical counterpart SDSS J003119.71+072453.5.
• The broad-band SED of the radio, X-ray, and gamma-ray data is consistent with a high-energy-peaked blazar at redshift z = 0.39 ± 0.03, but also compatible with a dark matter subhalo of mass 10^6–10^8 M_⊙ at a distance of 1.7–2.4 kpc.
• For a 150 GeV WIMP annihilating to τ⁺τ⁻, a boost factor of 3–13 is required to explain the flux, depending on subhalo concentration, suggesting that sub-substructures or cuspier profiles could reduce the required boost.