HESS J0632 Research Articles

Constraining neutron star properties and dark matter admixture with the NITR-I equation of state: Insights from observations and universal relations

A recent observational study has set a constraint on the maximum mass of neutron stars (NSs), specifically focusing on PSR J0952-0607 and the compact star remnant HESS J1731-347, particularly within the low-mass regime. In our recent study, Ref. 1 , we developed an energy density functional named NITR, which successfully produced the mass limit of the aforementioned pulsar but did not fully meet other observational constraints, such as those from NICER+XMM and GW170817. In this study, we introduce a new EDF named “NITR-I”, which not only reproduces the mass limit of PSR J0952-0607 but also aligns its canonical radius with NICER+XMM data, and its canonical dimensionless tidal deformability is consistent with the GW170817 event, thereby demonstrating the robustness of our model. The low-mass constraint associated with HESS J1731-347 suggests various possible compositions for the NS. The NITR-I model alone does not satisfy the HESS J1731-347 constraint; thus, we explore the possibility of incorporating dark matter (DM) inside the NS to meet this constraint. This approach proves successful when a specific value of Fermi momentum is considered. We also examine the impact of DM with varying Fermi momentum on different NS properties, such as tidal deformability and nonradial f-mode oscillation, using various relativistic mean-field (RMF) models. For the NITR-I EOS, the f-mode frequency is about 2.15[Formula: see text]kHz at 1.4 [Formula: see text] when [Formula: see text][Formula: see text]GeV, and it slightly increases to around 2.32[Formula: see text]kHz with [Formula: see text][Formula: see text]GeV. This increase in frequency due to DM suggests a possible reduction in tidal deformability, indicating that neutron stars with higher DM content are less susceptible to deformation by tidal forces which could be detectable in gravitational wave signals from neutron star mergers. Finally, we explore various universal relations (URs) for DM-admixed NSs, such as the relation between compactness and tidal deformability, the f-mode frequency and tidal deformability, and estimate the canonical values corresponding to both compactness and f-mode frequency using the GW170817 data.

Observation of the Galactic Center PeVatron beyond 100 TeV with HAWC

We report an observation of ultrahigh-energy (UHE) gamma rays from the Galactic center (GC) region, using 7 yr of data collected by the High-Altitude Water Cherenkov (HAWC) Observatory. The HAWC data are best described as a point-like source (HAWC J1746-2856) with a power-law spectrum ( dN/dE=ϕE/26TeVγ ), where γ = −2.88 ± 0.15stat − 0.1sys and ϕ = 1.5 × 10−15 (TeV cm2 s)−1 ±0.3stat−0.13sys+0.08sys extending from 6 to 114 TeV. We find no evidence of a spectral cutoff up to 100 TeV using HAWC data. Two known point-like gamma-ray sources are spatially coincident with the HAWC gamma-ray excess: Sgr A* (HESS J1745-290) and the Arc (HESS J1746-285). We subtract the known flux contribution of these point sources from the measured flux of HAWC J1746-2856 to exclude their contamination and show that the excess observed by HAWC remains significant (>5σ), with the spectrum extending to >100 TeV. Our result supports that these detected UHE gamma rays can originate via hadronic interaction of PeV cosmic-ray protons with the dense ambient gas and confirms the presence of a proton PeVatron at the GC.

Color-flavor locked strange stars admixed with mirror dark matter and the observations of compact stars

We investigate the structure and the tidal deformability of the color-flavor locked strange stars admixed with mirror dark matter. Assuming the stars in the GW170817 event have a mirror-dark-matter core or a mirror-dark-matter halo, the observations of the central compact object within the supernova remnant HESS J1731-347 and the compact objects in the GW190814 and GW170817 events could be explained simultaneously with a pairing gap much smaller than 200 MeV. In contrast, a pairing gap larger than about 200 MeV must be employed without the consideration of a mirror-dark-matter core (halo). More importantly, we find that for the case of the quartic coefficient a 4 < 0.589, if the mass fraction of the mirror dark matter (fD ) of the compact stars in GW170817 is in a certain range (eg., 22.8% < fD < 77.2% for a 4 = 0.55), the minimum allowed value of the pairing gap could be less than 46.5 MeV (i.e., one half of the value of the strange quark mass which is taken as 93 MeV in this paper), which leads to the result that all astrophysical observations mentioned above could be satisfied without violating the conformal bound or the recently proposed positive trace anomally bound.

HESS J1745−290 spectrum explained by a transition in the diffusion regime of PeV cosmic rays in the Sgr A* accretion flow

HESS J1745−290 spectrum explained by a transition in the diffusion regime of PeV cosmic rays in the Sgr A* accretion flow

Modeling compact objects with quark matter and dark energy: A comparative study of the radial oscillation modes of HESS J1731-347 and PSR J0740+6620

We model the massive pulsar J0740+6620 and the light HESS J1731-347 compact object assuming (i) quark matter and (ii) dark energy stars. Within Einstein’s General Relativity, and assuming objects made of isotropic matter, we adopt analytic equations-of-state for both matter contents. Although the inner composition of the objects is very different in each case, both equations-of-state imply qualitatively very similar mass-to-radius relationships. We compute the spectra of radial equations for all four cases and the corresponding wave functions as well as the large frequency separations. A comparison between the different matter contents is made as well.

TeV Analysis of a Source-rich Region with the HAWC Observatory: Is HESS J1809-193 a Potential Hadronic PeVatron?

HESS J1809-193 is an unidentified TeV source, first detected by the High Energy Stereoscopic System (H.E.S.S.) collaboration. The emission originates in a source-rich region that includes several supernova remnants (SNRs) and pulsars including SNR G11.1+0.1, SNR G11.0-0.0, and the young radio pulsar PSR J1809-1917. Originally classified as a pulsar wind nebula candidate, recent studies show the peak of the TeV region overlapping with a system of molecular clouds. This resulted in the revision of the original leptonic scenario to look for alternate hadronic scenarios. Marked as a potential PeVatron candidate, this region has been studied extensively by H.E.S.S. due to its emission extending up to several tens of TeV. In this work, we use 2398 days of data from the High Altitude Water Cherenkov (HAWC) observatory to carry out a systematic source search of the HESS J1809-193 region. We were able to resolve emission detected as an extended component (modelled as a symmetric Gaussian with a 1σ radius of 0.°21) with no clear cutoff at high energies and emitting photons up to 210 TeV. We model the multiwavelength observations for the region around HESS J1809-193 using a time-dependent leptonic model and a lepto-hadronic model. Our model indicates that both scenarios could explain the observed data within the region of HESS J1809-193.

Could a slow stable hybrid star explain the central compact object in HESS J1731-347?

Could a slow stable hybrid star explain the central compact object in HESS J1731-347?

CFL quark stars as a candidate for the HESS J1731-347 object with a trace anomaly and GW190814 bound implementation

CFL quark stars as a candidate for the HESS J1731-347 object with a trace anomaly and GW190814 bound implementation

Extended gamma-ray emission from particle escape in pulsar wind nebulae. Application to HESS J1809-193 and HESS J1825-137

There is growing evidence from gamma-ray observations at high and very high energies that particle escape is a key aspect shaping the morphological properties of pulsar wind nebulae (PWNe) at various evolutionary stages. We aim to provide a simple model for the gamma-ray emission from these objects including the transport of particles across the different components of the system. We applied it to sources HESS J1809$-$193 and HESS J1825$-$137. We developed a multi-zone framework applicable to dynamically young PWNe, taking into account the diffusive escape of relativistic electron-positron pairs out of the nebula into the parent supernova remnant (SNR) and their confinement downstream of the magnetic barrier of the forward shock until an eventual release into the surrounding interstellar medium (ISM). For a wide range of turbulence properties in the nebula, the GeV-TeV inverse-Compton radiation from pairs that escaped into the remnant can be a significant if not dominant contribution to the emission from the system. It may dominate the pion-decay radiation from cosmic rays accelerated at the forward shock and advected downstream of it. In the TeV-PeV range, the contribution from particles escaped into the ISM can exceed by far that of the SNR+PWN components. Applied to HESS J1809$-$193 and HESS J1825$-$137, we found that spatially extended GeV-TeV emission components can be accounted for mostly from particles escaped into the ISM, while morphologically more compact components above $50-100$ are ascribed to the PWNe. In these two cases, the model suggests high turbulence in the nebula and a forward shock accelerating cosmic rays up to $ at most. The model provides the temporal and spectral properties of the flux of particles originally energized by the pulsar wind and ultimately released in the ISM. It can be used to constrain the transport of particles in the vicinity of pulsar-PWN-SNR systems from broadband gamma-ray observations, or in studies of the contribution of pulsar-related systems to the local positron flux.

Reconciling constraints from the supernova remnant HESS J1731-347 with the parity doublet model

Reconciling constraints from the supernova remnant HESS J1731-347 with the parity doublet model

Is the Compact Object Associated with HESS J1731-347 a Strange Quark Star? A Possible Astrophysical Scenario for Its Formation

The analysis of the central compact object within the supernova (SN) remnant HESS J1731-347 suggests that it has a small radius and, even more interestingly, a mass of the order or smaller than 1 M ⊙. This raises the question of which astrophysical process could lead to such a small mass, since the analysis of various types of SN explosions indicate that is it not possible to produce a neutron star with a mass smaller than about 1.17 M ⊙. Here we show that masses of the order or smaller than 1 M ⊙ can be obtained in the case of strange quark stars and that it is possible to build a coherent model explaining not only the mass and the radius of that object, but also its slow cooling suggested in various analyses. We also show that an astrophysical path exists, which leads to the formation of such an object, and we discuss the role played in that scenario by strangelets assumed to constitute the dark matter.

Hybrid Star Models in the Light of New Multimessenger Data

Recent astrophysical mass inferences of compact stars HESS J1731-347 and PSR J0952-0607, with extremely small and large masses respectively, as well as the measurement of the neutron skin of Ca in the CREX experiment challenge and constrain the models of dense matter. We examine the concept of hybrid stars—objects containing quark cores surrounded by nucleonic envelopes—as models that account for these new data along with other inferences. We employ a family of 81 nucleonic equations of state (EOSs) with variable skewness and slope of symmetry energy at saturation density and a constant speed-of-sound EOS for quark matter. For each nucleonic EOS, a family of hybrid EOSs is generated by varying the transition density, the energy jump, and the speed of sound. These models are tested against the data from GW170817 and J1731-347, which favor low-density soft EOS and J0592-0607 and J0740+6620, which require high-density stiff EOS. The addition of J0592-0607's mass measurement to the constraints has no significant impact on the parameter space of the admissible EOS, but allows us to explore the potential effect of pulsars more massive than J0740+6620, if such exists. We then examine the occurrence of twin configurations and quantify the ranges of masses and radii that they can possess. It is shown that including J1731-347 data favors EOSs that predict low-mass twins with M ≲ 1.3 M ⊙ that can be realized if the deconfinement transition density is low. If combined with large speed of sound in quark matter such models allow for maximum masses of hybrid stars in 2.0–2.6 M ⊙.

Two-flavor Color Superconducting Quark Stars May Not Exist

Large uncertainties in the determinations of the equation of state of dense stellar matter allow for the intriguing possibility that the bulk quark matter in beta equilibrium might be the true ground state of the matter at zero pressure. Also, quarks will form Cooper pairs very readily since the dominant interaction between quarks is attractive in some channels. As a result, quark matter will generically exhibit color superconductivity, with the favored pairing pattern at intermediately high densities being two-flavor pairing. In the light of several possible candidates for such self-bound quark stars, including the very low-mass central compact object in supernova remnant HESS J1731-347 reported recently, we carry out a one-field theoretic model, the Nambu–Jona–Lasinio model, to investigate the stability of the beta-stable two-flavor color superconducting (2SC) phase of quark matter, but find no physically allowed parameter space for the existence of 2SC quark stars.

Revision of the GeV γ-Ray Emission in the Region of HESS J1813-178 with Fermi-LAT

HESS J1813-178 is one of the brightest and most compact TeV γ-ray sources, and whether its γ-ray emission is associated with supernova remnant (SNR), pulsar wind nebula (PWN), or young stellar cluster (YSC) is still under debate. By analyzing the GeV γ-ray data in the field of HESS J1813-178 using 14 yr of PASS 8 data recorded by the Fermi Large Area Telescope (Fermi-LAT), we report the discovery of three different sources with different spectra in this region. The hard source with a PL spectral index of 2.11 ± 0.08 has a small size extension, which is spatially and spectrally coincident with the TeV γ-ray emission from HESS J1813-178. CO observations display the dense molecular clouds surrounding HESS J1813-178 in the velocity range of 45–60 km s−1. The possible origins of the γ-ray emission from HESS J1813-178 are discussed, including SNR G12.820.02, the PWN driven by the energetic X-ray pulsar PSR J1813-1749 and YSC Cl 1813-178. However, none of them can be ruled out clearly. Note that the maximum energy of protons in the hadronic model should exceed a few hundred TeV, which makes HESS J1813-178 a promising PeVatron. A detailed LHAASO data analysis about the morphology and spectrum would be helpful to investigate the origin of the γ-ray emission in this region and test its PeVatron nature.

2FHL J1745.1–3035: A Newly Discovered, Powerful Pulsar Wind Nebula Candidate

We present a multi-epoch, multi-observatory X-ray analysis for 2FHL J1745.1–3035, a newly discovered very high-energy Galactic source detected by the Fermi Large Area Telescope (LAT) located in close proximity to the Galactic Center (l = 358.°5319; b = −0.°7760). The source shows a very hard γ-ray photon index above 50 GeV, Γ γ = 1.2 ± 0.4, and is found to be a TeV emitter by the Fermi–LAT. We conduct a joint XMM-Newton, Chandra, and NuSTAR observing campaign, combining archival XMM-Newton observations, to study the X-ray spectral properties of 2FHL J1745.1–3035 over a time span of over 20 yr. The joint X-ray spectrum is best fitted as a broken-power-law model with break energy E b ∼ 7 keV: the source is very hard at energies below 10 keV, with Γ1 ∼ 0.6, and significantly softer in the higher energy range measured by NuSTAR with Γ2 ∼ 1.9. We also perform a spatially resolved X-ray analysis with Chandra, finding evidence for marginal extension (up to an angular size r ∼ 5″), a result that supports a compact pulsar wind nebula scenario. Based on the X-ray and γ-ray properties, 2FHL J1745.1–3035 is a powerful pulsar wind nebula candidate. Given its nature as an extreme TeV emitter, further supported by the detection of a coincident TeV extended source HESS J1745-303, 2FHL J1745.1–3035 is an ideal candidate for a follow up with the upcoming Cherenkov Telescope Array.

Hybrid stars in light of the HESS J1731-347 remnant and the PREX-II experiment

Hybrid stars in light of the HESS J1731-347 remnant and the PREX-II experiment

Spherically symmetric anisotropic strange stars

In this work, we made an extensive study about the possible presence of anisotropies in strange stars. To accomplish this task, we use three different configurations for the strange matter: the unpaired matter, a two-flavor super-conducting (2SC) strange matter, and a fully three-flavor super-conducting strange matter (CFL). For each configuration, we calculate the relevant quantities for the strange stars, such as the mass-radius relation, the dimensionless tidal parameter, the moment of inertia, and the surface curvature for different degrees of anisotropies. Whenever possible, we compare our results with constraints found in the literature, especially focusing on the existence of very massive pulsars (PSR J0952-0607), as well as very light compact objects (HESS J1731-347).

Energy-dependent Analyses of the Gamma-Ray Emission from HESS J1857+026 with Fermi-LAT

We report the discovery of the energy-dependent morphology of the GeV gamma-ray emission from HESS J1857+026 with more than 13 yr of Fermi Large Area Telescope data. The GeV gamma-ray emission from this region is composed of two extended components. The hard component with an index of 1.74 ± 0.07 in the energy range of 0.5–500 GeV is spatially coincident with HESS J1857+026, and its 68% containment radius varies from ∼0.°44 below 40 GeV to ∼0.°30 above 140 GeV. The hard GeV gamma-ray spectrum and the energy-dependent morphology of HESS J1857+026 make it favor a pulsar wind nebula origin, which is associated with the energetic pulsar PSR J1856+0245. The soft component with an index of 2.70 ± 0.16 and another extended gamma-ray source detected in this region, 4FGL J1857.9+0313e, with an index of 2.55 ± 0.07, are spatially coincident with two molecular clumps in the northeast and southwest of HESS J1857+026, which favors the hadronic process, and the protons could be accelerated by the hypothetical supernova remnant associated with PSR J1856+0245.

A 3D diffusive and advective model of electron transport applied to the pulsar wind nebula HESS J1825 − 137

ABSTRACT HESS J1825 − 137 is one of the most powerful and luminous TeV gamma-ray pulsar wind nebulae, making it an excellent laboratory to study particle transportation around pulsars. We present a model of the (diffusive and advective) transport and radiative losses of electrons from the pulsar PSR J1826 − 1334 powering HESS J1825 − 137 using interstellar medium gas (ISM) data, soft photon fields, and a spatially varying magnetic field. We find that for the characteristic age of $21\, \mathrm{k}\mathrm{yr}$, PSR J1826 − 1334 is unable to meet the energy requirements to match the observed X-ray and gamma-ray emission. An older age of $40\, \mathrm{k}\mathrm{yr}$, together with an electron conversion efficiency of 0.14 and advective flow of v = 0.002c, can reproduce the observed multiwavelength emission towards HESS J1825 − 137. A turbulent ISM with magnetic field of $B=20 \,{\rm to}\, 60 \,\mathrm{\mu }{\rm G}$ to the north of HESS J1825 − 137 (as suggested by ISM observations) is required to prevent significant gamma-ray contamination towards the northern $\mathrm{T}\mathrm{e\mathrm{V}}$ source HESS J1826 − 130.

HESS J1731-347 and the existence of exotic matter: Kaon condensation in neutron stars

The recent observation of a compact star with the lowest observable mass of 0.77+0.20 –0.17 M⊙ within the supernova remnant HESS J1731-347 has redefined our understanding for the dense nuclear matter equation of state and enhances the existence of exotic matter. In the present work, we investigate the possible existence of kaon condensation in hadronic neutron stars through the framework of the Momentum-Dependent Interaction nuclear model describing the dense nuclear matter. We concentrate on the strangeness content of the proton and its implications on both the microscopic and macroscopic properties of neutron stars. The aforementioned quantity is of interest as it is also directly related to the location of the phase transition to kaons. The simultaneous description of the compact object within the HESS J1731-347 and the maximum observable neutron star mass imposes severe constraints on the strangeness content of the proton and the critical density for kaon condensation. The present study along with observations of neutron stars, may help to provide tighter constraints on the equation of state of dense nuclear matter and, even more, to shed light to the existence of an exotic core in neutron stars.