Gamma-ray Sources Research Articles

Ultra-high energy inverse Compton emission from Galactic electron accelerators

It is generally held that >100 TeV emission from astrophysical objects unambiguously demonstrates the presence of PeV protons or nuclei, due to the unavoidable Klein–Nishina suppression of inverse Compton emission from electrons. However, in the presence of inverse Compton dominated cooling, hard high-energy electron spectra are possible. We show that the environmental requirements for such spectra can naturally be met in spiral arms, and in particular in regions of enhanced star formation activity, the natural locations for the most promising electron accelerators: powerful young pulsars. Leptonic scenarios are applied to gamma-ray sources recently detected by the High-Altitude Water Cherenkov Observatory (HAWC) and the Large High Altitude Air Shower Observatory (LHAASO). We show that these sources can indeed be explained by inverse Compton emission.

Low-energy Electron-track Imaging for a Liquid Argon Time-projection-chamber Telescope Concept Using Probabilistic Deep Learning

The GammaTPC is an MeV-scale single-phase liquid argon time-projection-chamber gamma-ray telescope concept with a novel dual-scale pixel-based charge-readout system. It promises to enable a significant improvement in sensitivity to MeV-scale gamma rays over previous telescopes. The novel pixel-based charge readout allows for imaging of the tracks of electrons scattered by Compton interactions of incident gamma rays. The two primary contributors to the accuracy of a Compton telescope in reconstructing an incident gamma-ray’s original direction are its energy and position resolution. In this work, we focus on using deep learning to optimize the reconstruction of the initial position and direction of electrons scattered in Compton interactions, including using probabilistic models to estimate predictive uncertainty. We show that the deep-learning models are able to predict locations of Compton scatters of MeV-scale gamma rays from simulated 500 μm pixel-based data to better than 1 mm rms error and are sensitive to the initial direction of the scattered electron. We compare and contrast different deep-learning uncertainty estimation algorithms for reconstruction applications. Additionally, we show that event-by-event estimates of the uncertainty of the locations of the Compton scatters can be used to select those events that were reconstructed most accurately, leading to improvement in locating the origin of gamma-ray sources on the sky.

Prospective study on observations of gamma-ray emission from active galactic nuclei using the HADAR experiment

The High Altitude Detection of Astronomical Radiation (HADAR) experiment is a refracting terrestrial telescope array based on the atmospheric Cherenkov imaging technique. It is a hybrid array consisting of four water-lens telescopes and a surrounding scintillation detector array for observing Cherenkov light induced by 10 GeV–10 TeV cosmic rays and gamma rays in the atmosphere. The water-lens telescope mainly consists of a hemispherical lens with a diameter of 5 m acting as a Cherenkov light collector, a cylindrical metal tank with a 4 m radius and 7 m height, and an imaging system at the bottom of the tank. The sky region covered by HADAR is much larger than the current generation of Imaging Atmospheric Cherenkov Telescopes, and even the CTA. The field-of-view (FOV) of HADAR can reach up to 60 degrees. The HADAR experiment possesses the advantages of a large field-of-view and low energy threshold, so it can continuously scan wide portions of the sky and easily observe extragalactic gamma-ray sources. The majority of the extragalactic gamma-ray sources detected at very high energy (VHE) energies are active galactic nuclei (AGNs). In this study, we present the potential of using the HADAR experiment for detecting AGN. Based on the AGN catalog sources of the Fermi Large Area Telescope (Fermi-LAT), the observed energy is extrapolated to the VHE range. The VHE gamma rays propagating over cosmological distances can interact with the low-energy of the extragalactic background light (EBL) and produce electron-positron pairs. Therefore, we consider the absorption effects of different EBL models when calculating the expected gamma ray spectra of the AGN sample. We select the sample with redshift measurements and locations inside the FOV of HADAR from 4LAC catalog. In total, there are 375 BL Lacertae objects (BL Lacs) and 289 flat-spectrum radio quasars (FSRQs) satisfying the selection conditions. The integral gamma ray spectra are derived and compared with the sensitivity curve of HADAR, the number of sources with fluxes above the sensitivity of HADAR is counted. Further, we calculate the statistical significance of HADAR for AGN source observation based on the equi-zenith angle sky scanning analysis method. The simulation results reveal that a total of 31 sources of Fermi-LAT AGN can be detected by HADAR with a significance greater than five standard deviations over a one-year survey period, most of which are BL Lacs.

MeV Gamma-Ray Source Contribution to the Inner Galactic Diffuse Emission

The origin of the inner Galactic emission, measured by COMPTEL with a flux of ∼10−2 MeV cm−2 s−1 sr−1 in the 1–30 MeV energy range from the inner Galactic region, has remained unsettled since its discovery. In this paper, we elaborate on a model of individual MeV gamma-ray sources unresolved by COMPTEL. This is conducted for sources crossmatched between the Swift-BAT and Fermi-LAT catalogs by interpolating the energy spectra in the hard X-ray and GeV gamma-ray ranges, as well as unmatched sources between the two catalogs. We find that the source contribution to the COMPTEL emission would be at least ∼20%. Combined with the Galactic diffuse emission, which is not well constrained, the COMPTEL emission can be roughly reproduced in some cases.

Comparison of Characteristics of Gamma-Ray Imager Based on Coded Aperture by Varying the Thickness of the BGO Scintillator

Background: The conventional cerium-doped Gd2Al2Ga3O12 (GAGG(Ce)) scintillator-based gamma-ray imager has a bulky detector, which can lead to incorrect positioning of the gammaray source if the shielding against background radiation is not appropriately designed. In addition, portability is important in complex environments such as inside nuclear power plants, yet existing gamma-ray imager based on a tungsten mask tends to be weighty and therefore difficult to handle. Motivated by the need to develop a system that is not sensitive to background radiation and is portable, we changed the material of the scintillator and the coded aperture.Materials and Methods: The existing GAGG(Ce) was replaced with Bi4Ge3O12 (BGO), a scintillator with high gamma-ray detection efficiency but low energy resolution, and replaced the tungsten (W) used in the existing coded aperture with lead (Pb). Each BGO scintillator is pixelated with 144 elements (12 × 12), and each pixel has an area of 4 mm × 4 mm and the scintillator thickness ranges from 5 to 20 mm (5, 10, and 20 mm). A coded aperture consisting of Pb with a thickness of 20 mm was applied to the BGO scintillators of all thicknesses.Results and Discussion: Spectroscopic characterization, imaging performance, and image quality evaluation revealed the 10 mm-thick BGO scintillators enabled the portable gamma-ray imager to deliver optimal performance. Although its performance is slightly inferior to that of existing GAGG(Ce)-based gamma-ray imager, the results confirmed that the manufacturing cost and the system’s overall weight can be reduced.Conclusion: Despite the spectral characteristics, imaging system performance, and image quality is slightly lower than that of GAGG(Ce), the results show that BGO scintillators are preferable for gamma-ray imaging systems in terms of cost and ease of deployment, and the proposed design is well worth applying to systems intended for use in areas that do not require high precision.

Neutron and Gamma-Ray Detection System Coupled to a Multirotor for Screening of Shipping Container Cargo.

In order to detect special nuclear materials and other radioactive materials in Security and Defense scenarios, normally, a combination of neutron and gamma-ray detection systems is used. In particular, to avoid illicit traffic of special nuclear materials and radioactive sources/materials, radiation portal monitors are placed at seaports to inspect shipping-container cargo. Despite their large volume (high efficiency), these detection systems are expensive, and therefore only a fraction of these containers are inspected. In this work, a novel mobile radiation detection system is presented, based on an EJ-200 plastic scintillator for the detection of gamma rays and beta particles, and a neutron detector EJ-426HD plastic scintillator (with 6Li) embedded in a compact and modular moderator. The use of silicon photomultipliers in both detectors presented advantages such as lightweight, compactness, and low power consumption. The developed detection system was integrated in a highly maneuverable multirotor. Monte Carlo simulations were validated by laboratory measurements and field tests were performed using real gamma-ray and neutron sources. The detection and localization within one meter was achieved using a maximum likelihood estimation algorithm for 137Cs sources (4 MBq), as well as the detection of 241Am-beryllium (1.45 GBq) source placed inside the shipping container.

Self-calibration of LHAASO-KM2A electromagnetic particle detectors using single particles within extensive air showers

In the Large High Altitude Air Shower Observatory (LHAASO), the square kilometer array, with 5249 electromagnetic particle detectors (EDs) and 1188 muon detectors, is deployed to explore the gamma-ray sources above 30 TeV with unprecedented sensitivity and to measure primary cosmic rays in the energy range from 10 TeV to 100 PeV. The energetic particles produced by extensive air showers can serve as a continuously available source for calibration of the numerous EDs over a large area. In this study, the detector untriggered probability is first proposed to estimate the particle density at different distances from the shower core and distinguish the characteristic single-particle signal detected by each ED. This method uses science data directly, and does not require prior knowledge of the cosmic-ray elemental composition or hadronic interaction model. Experimental results show that this self-calibration can be used to determine the number of particles detected by each ED with an accuracy better than 2% within a time scale of hours, which is adequate to meet the physics requirements of the LHAASO experiment. With this high efficiency and accuracy, this calibration also provides an ideal method to monitor the detector performance throughout an expected lifetime of >10 years.9 MoreReceived 15 April 2022Accepted 21 November 2022DOI:https://doi.org/10.1103/PhysRevD.106.122004© 2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCosmic rays & astroparticlesTechniquesAstrophysical & cosmological simulationsCosmic ray & astroparticle detectorsMonte Carlo methodsProbability theoryScintillatorsAccelerators & BeamsGravitation, Cosmology & AstrophysicsParticles & Fields

Ultra-relativistic electron beams deflection by quasi-mosaic crystals

This paper provides an explanation of the key effects behind the deflection of ultra-relativistic electron beams by means of oriented ‘quasi-mosaic’ Bent Crystals (qmBC). It is demonstrated that accounting for specific geometry of the qmBC and its orientation with respect to a collimated electron beam, its size and emittance is essential for an accurate quantitative description of experimental results on the beam deflection by such crystals. In an exemplary case study, a detailed analysis of the recent experiment at the SLAC facility is presented. The methodology developed has enabled to understand the peculiarities in the measured distributions of the deflected electrons. Also, this achievement constitutes an important progress in the efforts toward the practical realization of novel gamma-ray crystal-based light sources and puts new challenges for the theory and experiment in this research area.

A gamma ray spectrometer with Compton suppression on the HL-2A tokamak.

A new broad-energy, high-resolution gamma ray spectrometer (GRS) with Compton suppression function has been developed recently in the HL-2A tokamak to obtain the gamma ray information in the energy range of 0.1-10MeV. This is the first time to develop an anti-Compton GRS for a magnetic confinement fusion device. The anticoincidence detector consists of a large-volume high purity germanium (HPGe) crystal (Φ63 × 63mm2) as the primary detector and eight trapezoidal bismuth germinate (BGO) scintillators (trapezoid crystal with 30mm thickness) as the secondary detector. The anti-coincidence data processing is implemented by a digital-based data acquisition system with fast digitization and software signal processing technology. Using radioisotope gamma ray sources and Monte Carlo N-Particle code, the energy and efficiency of the spectrometer have been calibrated and quantitatively tested. The Compton continuum suppression factor reaches 4.2, and the energy resolution (Full Width at Half Maximum) of the 1.332MeV full energy peak for 60Co is 2.1keV. Measurements of gamma ray spectra with Compton suppression using the spectrometer have been successfully performed during HL-2A discharges with different conditions. The performance of the spectrometer and the first experimental results are presented in this paper.

A possible explanation of the TeV emission from the pulsar wind nebula HESS J1825-137

ABSTRACT Pulsar wind nebula (PWN) HESS J1825-137 is one of the largest and brightest gamma-ray sources in Milky Way. It was found that the morphology of the nebula strongly depends on energy and the photon spectrum varies obviously with distance from the pulsar in the >0.1 TeV energy band. In the frame of a spatially-dependent particle transport model, we investigate the TeV gamma-ray observed features of HESS J1825-137 by using the Markov chain Monte Carlo method. Our results are as follows: (i) the observed features can be reasonably explained by the competition between the transport (advection and diffusion) and energy cooling losses (adiabatic, synchrotron, and inverse Compton) of the high energy electrons, and that a parameter ξ ≡ τp/τC is introduced to describe this competition, which is the function of electron energy and position within the nebula, where τp and τC are total transport and energy cooling timescales, respectively; (ii) for the high energy electrons (Ee > 0.1 TeV) within the nebula, the particle diffusion dominates over its advection, and the radiative cooling mostly dominates over adiabatic cooling; and (iii) the change of the radial extent with photon energy Eγ can be expressed as $R\propto E^{-\beta }_\gamma$, where the index β is a function of Eγ and the Klein−Nishina effect has a very important role for the variation of β with Eγ. From the modelling results, the initial rotation period, braking index, and age of the pulsar are predicted to be 12.16 ms, 1.79, and 44 kyr, respectively.

Gamma Ray Source Localization for Time Projection Chamber Telescopes Using Convolutional Neural Networks

Diverse phenomena such as positron annihilation in the Milky Way, merging binary neutron stars, and dark matter can be better understood by studying their gamma ray emission. Despite their importance, MeV gamma rays have been poorly explored at sensitivities that would allow for deeper insight into the nature of the gamma emitting objects. In response, a liquid argon time projection chamber (TPC) gamma ray instrument concept called GammaTPC has been proposed and promises exploration of the entire sky with a large field of view, large effective area, and high polarization sensitivity. Optimizing the pointing capability of this instrument is crucial and can be accomplished by leveraging convolutional neural networks to reconstruct electron recoil paths from Compton scattering events within the detector. In this investigation, we develop a machine learning model architecture to accommodate a large data set of high fidelity simulated electron tracks and reconstruct paths. We create two model architectures: one to predict the electron recoil track origin and one for the initial scattering direction. We find that these models predict the true origin and direction with extremely high accuracy, thereby optimizing the observatory’s estimates of the sky location of gamma ray sources.

Inducing aluminum oxide growth at room temperature and atmospheric pressure through low dose gamma-ray irradiation

The corrosion resistance of the aluminum is well-known and it is attributed to passivation, a process that occurs when the metal surface reacts to the environment to form a spontaneously thin oxide film. Basically, the natural aluminum oxide film is composed of two layers, an internal layer composed entirely by Al2O3, and a second one made by Al(OH)3. In normal conditions of temperature and pressure, the Al(OH)3 film takes a long time to grow, something around several weeks or months. However, in the present work, through the use of low doses gamma-ray irradiation in an environment composed by air and water vapor, it was possible, in a few days, to increase the layer of Al(OH)3 on a sample of pure aluminum. Using a Cs-137 disk as a gamma-ray source, three aluminum samples were irradiated during 14, 8 and 5 days, receiving absorbed doses of 2.85 mGy, 1.49 mGy and 0.94 mGy, respectively. Comparing the Raman spectrum of the two irradiated samples with another non-irradiated sample, it was possible to verify that the sample irradiated during 14 days had the greatest values of peak intensity and peak area, indicating a higher concentration of aluminum hydroxide when compared to other samples. Moreover, the polarization curves of samples were obtained to analyze the nature of the film formed on the surface. From the results, it was possible to observe that the gamma-ray irradiation led to an improvement in passive film quality (higher resistance), once the irradiation resulted in a lower passivation current density (ipass). To confirm the consistence of previous analysis, more two samples were examined through X-ray photoelectron spectroscopy (XPS), where a sample irradiated during five days showed an increase in the percentage of aluminum hydroxide (related to XPS O 1s peak) in comparison to a non-irradiated sample. Then, it can be pointed that the irradiation, in a low dose, induced the growth of the aluminum hydroxide layer of passive film and improved the film's resistance in a few days. Moreover, the results show the utility of applying low doses of ionizing irradiation to change metal oxide surfaces.

A Comprehensive Review on Amplification of Laser Pulses via Stimulated Raman Scattering and Stimulated Brillouin Scattering in Plasmas

The demand for high-intensity lasers has grown ever since the invention of lasers in 1960, owing to their applications in the fields of inertial confinement fusion, plasma-based relativistic particle accelerators, complex X-ray and gamma-ray sources, and laboratory astrophysics. To create such high-intensity lasers, free-running lasers were either Q-switched or mode-locked to increase the peak power to the gigawatt range. Later, chirped pulse amplification was developed, allowing the generation of peak power up to 1012 W. However, the next generation of high-intensity lasers might not be able to be driven by the solid-state technology alone as they are already operating close to their damage thresholds. In this scenario, concepts of amplification based on plasmas has the potential to revolutionize the laser industry, as plasma is already a broken-down medium, and hence does not pose any problems related to the damage thresholds. On the other hand, there are many other aspects that need to be addressed before developing technologies based on plasma-based amplification, and they are being investigated via theoretical and numerical methods and supported by several experiments. In this report, we review the prospects of employing plasma as the medium of amplification by utilising stimulated scattering techniques, such as the stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) techniques, to modulate high-power laser pulses, which would possibly be the key to the next generation of high-power lasers. The 1980s saw the commencement of research in this field, and possibilities of obtaining high peak powers were verified theoretically with the help of numerical calculations and simulations. The extent of amplification by these stimulated scattering schemes are limited by a number of instabilities such as forward Raman scattering (FRS), filamentation, etc., and here, magnetised plasma played an important role in counteracting these parasitic effects. The current research combines all these factors to experimentally realise a large-scale plasma-based amplifier, which can impact the high-energy laser industry in the near future.

How do the dynamics of the Milky Way–Large Magellanic Cloud system affect gamma-ray constraints on particle dark matter?

ABSTRACT Previous studies on astrophysical dark matter (DM) constraints have all assumed that the Milky Way’s (MW) DM halo can be modelled in isolation. However, recent work suggests that the MW’s largest dwarf satellite, the Large Magellanic Cloud (LMC), has a mass of 10–20${{\ \rm per\ cent}}$ that of the MW and is currently merging with our Galaxy. As a result, the DM haloes of the MW and LMC are expected to be strongly deformed. We here address and quantify the impact of the dynamical response caused by the passage of the LMC through the MW on the prospects for indirect DM searches. Utilizing a set of state-of-the-art numerical simulations of the evolution of the MW–LMC system, we derive the DM distribution in both galaxies at the present time based on the basis function expansion formalism. Consequently, we build $\mathcal {J}$-factor all-sky maps of the MW–LMC system to study the impact of the LMC passage on gamma-ray indirect searches for thermally produced DM annihilating in the outer MW halo as well as within the LMC halo stand-alone. We conduct a detailed analysis of 12 yr of the Fermi Large Area Telescope data that incorporates various large-scale gamma-ray emission components and we quantify the systematic uncertainty associated with the imperfect knowledge of the astrophysical gamma-ray sources. We find that the dynamical response caused by the LMC passage can alter the constraints on the velocity-averaged annihilation cross-section for weak-scale particle DM at a level comparable to the existing observational uncertainty of the MW halo’s density profile and total mass.

A New Sample of Gamma-Ray Emitting Jetted Active Galactic Nuclei

We considered the fourth catalog of gamma-ray point sources produced by the Fermi Large Area Telescope (LAT) and selected only jetted active galactic nuclei (AGN) or sources with no specific classification, but with a low-frequency counterpart. Our final list is composed of 2980 gamma-ray point sources. We then searched for optical spectra in all the available literature and publicly available databases, to measure redshifts and to confirm or change the original LAT classification. Our final list of gamma-ray emitting jetted AGN is composed of BL Lac Objects (40%), flat-spectrum radio quasars (23%), misaligned AGN (2.8%), narrow-line Seyfert 1, Seyfert, and low-ionization nuclear emission-line region galaxies (1.9%). We also found a significant number of objects changing from one type to another, and vice versa (changing-look AGN, 1.1%). About 30% of gamma-ray sources still have an ambiguous classification or lack one altogether.

Evidence for neutrino emission from the nearby active galaxy NGC 1068.

A supermassive black hole, obscured by cosmic dust, powers the nearby active galaxy NGC 1068. Neutrinos, which rarely interact with matter, could provide information on the galaxy's active core. We searched for neutrino emission from astrophysical objects using data recorded with the IceCube neutrino detector between 2011 and 2020. The positions of 110 known gamma-ray sources were individually searched for neutrino detections above atmospheric and cosmic backgrounds. We found that NGC 1068 has an excess of [Formula: see text] neutrinos at tera-electron volt energies, with a global significance of 4.2σ, which we interpret as associated with the active galaxy. The flux of high-energy neutrinos that we measured from NGC 1068 is more than an order of magnitude higher than the upper limit on emissions of tera-electron volt gamma rays from this source.

Low intensity gamma-ray monitor for in-situ solids fraction measurements in liquids

A novel detection system has been designed to measure the settling of solids in tailing ponds in order to facilitate the reuse of water in oil sands extraction processing, tailings excavation, transportation and management. The system is based on a weak gamma ray source and an inexpensive scintillator-based detector. The system measures gamma ray photons which are transmitted through the material of interest which are detected by a combined scintillator and Multi Pixel Photon Counter (MPPC) detector. This non-destructive measurement allows the determination of the solids contents profile versus depth in a tailing pond. The attenuation of gamma radiation depends on the density of the solids within the fluid tailings and thus varies with the weight fraction of solids content of the tailings. Modelling of the system was carried out by Geant4 simulations. The system was deployed using a weak 133Ba gamma-ray source together with a simple microprocessor controller readout circuit to analyze the pulse height response of a Cerium doped Lutetium Yttrium Orthosilicate (LYSO (Ce)) scintillator crystal. The detection system is calibrated with the known samples and can measure solids content with a relative precision of within ∼2%.

New Identifications and Multiwavelength Properties of Extragalactic Fermi Gamma-Ray Sources in the SPT-SZ Survey Field

The fourth Fermi Large Area Telescope catalog (4FGL) contains 5064 γ-ray sources detected at high significance, but 26% of them still lack associations at other wavelengths. The SPT-SZ survey, conducted between 2008 and 2011 with the South Pole Telescope (SPT), covers 2500 deg2 of the southern sky in three millimeter-wavelength (mm) bands and was used to construct a catalog of nearly 5000 emissive sources. In this study, we introduce a new cross-matching scheme to search for multiwavelength counterparts of extragalactic γ-ray sources using a mm catalog. We apply a Poissonian probability to evaluate the rate of spurious false associations and compare the multiwavelength associations from the radio, mm, near-infrared, and X-ray with 4FGL γ-ray sources. In the SPT-SZ survey field, 85% of 4FGL sources are associated with mm counterparts. These mm sources include 94% of previously associated 4FGL sources and 56% of previously unassociated 4FGL sources. The latter group contains 40 4FGL sources for which SPT has provided the first identified counterparts. Nearly all of the SPT-associated 4FGL sources can be described as flat-spectrum radio quasars or blazars. We find that the mm band is the most efficient wavelength for detecting γ-ray blazars when considering both completeness and purity. We also demonstrate that the mm band correlates better to the γ-ray band than the radio or X-ray bands. With the next generation of CMB experiments, this technique can be extended to greater sensitivities and more sky area to further complete the identifications of the remaining unknown γ-ray blazars.

All-sky Medium Energy Gamma-ray Observatory eXplorer mission concept

The All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X) is designed to identify and characterize gamma rays from extreme explosions and accelerators. The main science themes include: supermassive black holes and their connections to neutrinos and cosmic rays; binary neutron star mergers and the relativistic jets they produce; cosmic ray particle acceleration sources including Galactic supernovae; and continuous monitoring of other astrophysical events and sources over the full sky in this important energy range. AMEGO-X will probe the medium energy gamma-ray band using a single instrument with sensitivity up to an order of magnitude greater than previous telescopes in the energy range 100 keV to 1 GeV that can be only realized in space. During its three-year baseline mission, AMEGO-X will observe nearly the entire sky every two orbits, building up a sensitive all-sky map of gamma-ray sources and emission. AMEGO-X was submitted in the recent 2021 NASA MIDEX Announcement of Opportunity.

Statistics for dark matter subhalo searches in gamma rays from a kinematically constrained population model: Fermi-LAT-like telescopes

Cold dark matter subhalos are expected to populate galaxies in numbers. If dark matter self-annihilates, these objects turn into prime targets for indirect searches, in particular with gamma-ray telescopes. Incidentally, the Fermi-LAT catalog already contains many unidentified sources that might be associated with subhalos. In this paper, we determine the probability for subhalos to be identified as gamma-ray pointlike sources from their predicted distribution properties. We use a semi-analytical model for the Galactic subhalo population, which, in contrast to cosmological simulations, can be made fully consistent with current kinematic constraints in the Milky Way and has no resolution limit. The model incorporates tidal stripping effects from a realistic distribution of baryons in the Milky Way. The same baryonic distribution contributes a diffuse gamma-ray foreground which adds up to that, often neglected in subhalo searches, generated by the smooth dark matter and the unresolved subhalos. This configuration implies a correlation between pointlike subhalo signals and diffuse background. Based on this semi-analytical modeling, we generate mock gamma-ray data assuming an idealized telescope resembling Fermi-LAT and perform a likelihood analysis to estimate the current and future sensitivity to subhalos in the relevant parameter space. We find a number of detectable subhalos of order ${\cal O}(<1)$ for optimistic model parameters and a WIMP mass of 100~GeV, maximized for a cored host halo. This barely provides support to the current interpretation of several Fermi unidentified sources as subhalos. We also find it more likely to detect the smooth Galactic halo itself before subhalos, should dark matter in the GeV-TeV mass range self-annihilate through $s$-wave processes.