skip to main content

Title: Cherenkov Telescope Array sensitivity to the putative millisecond pulsar population responsible for the Galactic Centre excess
ABSTRACT The leading explanation of the Fermi Galactic Centre γ-ray excess is the extended emission from an unresolved population of millisecond pulsars (MSPs) in the Galactic bulge. Such a population would, along with the prompt γ-rays, also inject large quantities of electrons/positrons (e±) into the interstellar medium. These e± could potentially inverse-Compton (IC) scatter ambient photons into γ-rays that fall within the sensitivity range of the upcoming Cherenkov Telescope Array (CTA). In this article, we examine the detection potential of CTA to this signature by making a realistic estimation of the systematic uncertainties on the Galactic diffuse emission model at TeV-scale γ-ray energies. We forecast that, in the event that e± injection spectra are harder than E−2, CTA has the potential to robustly discover the IC signature of a putative Galactic bulge MSP population sufficient to explain the Galactic Centre excess for e± injection efficiencies in the range of ≈2.9–74.1 per cent, or higher, depending on the level of mismodelling of the Galactic diffuse emission components. On the other hand, for spectra softer than E−2.5, a reliable CTA detection would require an unphysically large e± injection efficiency of ${\gtrsim} 158{{\ \rm per\ cent}}$. However, even this pessimistic conclusion may be avoided in more » the plausible event that MSP observational and/or modelling uncertainties can be reduced. We further find that, in the event that an IC signal were detected, CTA can successfully discriminate between an MSP and a dark matter origin for the radiating e±. « less
Authors:
; ; ; ; ;
Award ID(s):
1914409 1908960
Publication Date:
NSF-PAR ID:
10279272
Journal Name:
Monthly Notices of the Royal Astronomical Society
Volume:
506
Issue:
2
Page Range or eLocation-ID:
1741 to 1760
ISSN:
0035-8711
Sponsoring Org:
National Science Foundation
More Like this
  1. ABSTRACT Intermediate-mass black holes (IMBHs, $10^{3\!-\!6} \, {\rm M_\odot }$), are typically found at the centre of dwarf galaxies and might be wandering, thus far undetected, in the Milky Way (MW). We use model spectra for advection-dominated accretion flows to compute the typical fluxes, in a range of frequencies spanning from radio to X-rays, emitted by a putative population of $10^5 \, {\rm M_\odot }$ IMBHs wandering in five realistic volume-weighted MW environments. We predict that $\sim 27{{\ \rm per\ cent}}$ of the wandering IMBHs can be detected in the X-ray with Chandra, $\sim 37{{\ \rm per\ cent}}$ in the near-infrared with the Roman Space Telescope, $\sim 49{{\ \rm per\ cent}}$ in the sub-mm with CMB-S4, and $\sim 57{{\ \rm per\ cent}}$ in the radio with ngVLA. We find that the brightest fluxes are emitted by IMBHs passing through molecular clouds or cold neutral medium, where they are always detectable. We propose criteria to facilitate the selection of candidates in multiwavelength surveys. Specifically, we compute the X-ray to optical ratio (αox) and the optical to sub-mm ratio, as a function of the accretion rate of the IMBH. We show that at low rates the sub-mm emission of IMBHs is significantlymore »higher than the optical, UV, and X-ray emission. Finally, we place upper limits on the number N• of these objects in the MW: N• < 2000 and N• < 100, based on our detectability expectations and current lack of detections in molecular clouds and cold neutral medium, respectively. These predictions will guide future searches of IMBHs in the MW, which will be instrumental to understanding their demographics and evolution.« less
  2. Abstract The joint detection of gravitational waves and the gamma-ray counterpart of a binary neutron star merger event, GW170817, unambiguously validates the connection between short gamma-ray bursts and compact binary object (CBO) mergers. We focus on a special scenario where short gamma-ray bursts produced by CBO mergers are embedded in disks of active galactic nuclei (AGNs), and we investigate the γ -ray emission produced in the internal dissipation region via synchrotron, synchrotron self-Compton, and external inverse Compton (EIC) processes. In this scenario, isotropic thermal photons from the AGN disks contribute to the EIC component. We show that a low-density cavity can be formed in the migration traps, leading to the embedded mergers producing successful GRB jets. We find that the EIC component would dominate the GeV emission for typical CBO mergers with an isotropic-equivalent luminosity of L j ,iso = 10 48.5 erg s −1 that are located close to the central supermassive black hole. Considering a long-lasting jet of duration T dur ∼ 10 2 –10 3 s, we find that the future Cherenkov Telescope Array (CTA) will be able to detect its 25–100 GeV emission out to a redshift z = 1.0. In the optimistic case, it ismore »possible to detect the on-axis extended emission simultaneously with GWs within one decade using MAGIC, H.E.S.S., VERITAS, CTA, and LHAASO-WCDA. Early diagnosis of prompt emissions with Fermi-GBM and HAWC can provide valuable directional information for the follow-up observations.« less
  3. Context. The Shapley Supercluster (⟨ z ⟩≈0.048) contains several tens of gravitationally bound clusters and groups, making it an ideal subject for radio studies of cluster mergers. Aims. We used new high sensitivity radio observations to investigate the less energetic events of mass assembly in the Shapley Supercluster from supercluster down to galactic scales. Methods. We created total intensity images of the full region between A3558 and A3562, from ∼230 to ∼1650 MHz, using ASKAP, MeerKAT and the GMRT, with sensitivities ranging from ∼6 to ∼100 μJy beam −1 . We performed a detailed morphological and spectral study of the extended emission features, complemented with ESO-VST optical imaging and X-ray data from XMM-Newton . Results. We report the first GHz frequency detection of extremely low brightness intercluster diffuse emission on a ∼1 Mpc scale connecting a cluster and a group, namely: A3562 and the group SC 1329–313. It is morphologically similar to the X-ray emission in the region. We also found (1) a radio tail generated by ram pressure stripping in the galaxy SOS 61086 in SC 1329–313; (2) a head-tail radio galaxy, whose tail is broken and culminates in a misaligned bar; (3) ultrasteep diffuse emission at the centremore »of A3558. Finally (4), we confirm the ultra-steep spectrum nature of the radio halo in A3562. Conclusions. Our study strongly supports the scenario of a flyby of SC 1329–313 north of A3562 into the supercluster core. This event perturbed the centre of A3562, leaving traces of this interaction in the form of turbulence between A3562 and SC 1329–313, at the origin of the radio bridge and eventually affecting the evolution of individual supercluster galaxies by triggering ram pressure stripping. Our work shows that minor mergers can be spectacular and have the potential to generate diffuse radio emission that carries important information on the formation of large-scale structures in the Universe.« less
  4. ABSTRACT Shocks in γ-ray emitting classical novae are expected to produce bright thermal and non-thermal X-rays. We test this prediction with simultaneous NuSTAR and Fermi/LAT observations of nova V906 Car, which exhibited the brightest GeV γ-ray emission to date. The nova is detected in hard X-rays while it is still γ-ray bright, but contrary to simple theoretical expectations, the detected 3.5–78 keV emission of V906 Car is much weaker than the simultaneously observed >100 MeV emission. No non-thermal X-ray emission is detected, and our deep limits imply that the γ-rays are likely hadronic. After correcting for substantial absorption (NH ≈ 2 × 1023 cm−2), the thermal X-ray luminosity (from a 9 keV optically thin plasma) is just ∼2 per cent of the γ-ray luminosity. We consider possible explanations for the low thermal X-ray luminosity, including the X-rays being suppressed by corrugated, radiative shock fronts or the X-rays from the γ-ray producing shock are hidden behind an even larger absorbing column (NH > 1025 cm−2). Adding XMM–Newton and Swift/XRT observations to our analysis, we find that the evolution of the intrinsic X-ray absorption requires the nova shell to be expelled 24 d after the outburst onset. The X-ray spectra show that the ejecta are enhanced in nitrogen and oxygen, and the nova occurredmore »on the surface of a CO-type white dwarf. We see no indication of a distinct supersoft phase in the X-ray light curve, which, after considering the absorption effects, may point to a low mass of the white dwarf hosting the nova.« less
  5. Extragalactic background light (EBL) plays an important role in cosmology since it traces the history of galaxy formation and evolution. Such diffuse radiation from near-UV to far-infrared wavelengths can interact with γ -rays from distant sources such as active galactic nuclei (AGNs), and is responsible for the high-energy absorption observed in their spectra. However, probing the EBL from γ -ray spectra of AGNs is not trivial due to internal processes that can mimic its effect. Such processes are usually taken into account in terms of curvature of the intrinsic spectrum. Hence, an improper choice of parametrization for the latter can seriously affect EBL reconstruction. In this paper, we propose a statistical approach that avoids a priori assumptions on the intrinsic spectral curvature and that, for each source, selects the best-fit model on a solid statistical basis. By combining the Fermi -LAT observations of 490 blazars, we determine the γ -ray-inferred level of EBL for various state-of-the-art EBL models. We discuss the EBL level obtained from the spectra of both BL Lacs and flat spectrum radio quasars (FSRQ) in order to investigate the impact of internal absorption in different classes of objects. We further scrutinize constraints on the EBL evolution frommore »γ -ray observations by reconstructing the EBL level in four redshift ranges, up to z  ∼ 2.5. The approach implemented in this paper, carefully addressing the question of the modeling of the intrinsic emission at the source, can serve as a solid stepping stone for studies of hundreds of high-quality spectra acquired by next-generation γ -ray instruments.« less