HESS J0632 Research Articles

HESS J1809-193: Gamma-Ray Emission by Cosmic Rays from a Past Explosion

The very-high-energy γ-ray source HESS J1809-193 has been detected by the LHAASO and HAWC observatory beyond 100 TeV energy. It is an interesting candidate for exploring the underlying mechanisms of γ-ray production due to the presence of supernova remnants, pulsars, and molecular clouds close to it. We have considered the injection of the energetic cosmic rays from a past explosion, whose reminiscent may be SNR G011.0-00.0, which is located within the extended γ-ray source HESS J1809-193. We explain the multiwavelength data from the region of HESS J1809-193 with synchrotron, inverse Compton, and bremsstrahlung emission of cosmic-ray electrons and secondary γ-ray production in interactions of cosmic-ray protons with the cold protons in the local molecular clouds within a time-dependent framework including the diffusion loss of cosmic rays. The observational data have been modeled with the secondary photons produced by the time-evolved cosmic-ray spectrum assuming the age of the explosion is 4500 yr.

Hybrid stars and the stiffness of the nuclear equation of state in light of the HESS J1731-347 remnant

The recent observation of the extremely compact neutron star in the HESS J1731-347 remnant has challenged our understanding concerning the nature of dense nuclear matter. In particular, the low radius of the aforementioned compact object favors soft nuclear equations of state. However, the neutron skin thickness of 208Pb extracted from the long-awaited PREX-II experiment favors stiff equations of state which may be associated with larger radii for low mass stellar configurations. In this contribution we present our recent work on the possible reconciliation of the HESS J1731-347 observation in the framework of hybrid stars, under the assumption of a stiff low-density phase which may be favored by the PREX-II results. In addition, we examine the compatibility of the resulting hybrid models with recent constraints based on the observation of PSR J0030+0451, PSR J0952-0607 and GW190814.

Quark matter and quark star in color-flavor-locked phase

In this work, we investigate the thermodynamical properties of strange quark matter (SQM) and color-flavor-locked (CFL) quark matter under strong magnetic fields by using a quasiparticle model. We calculate the energy density and the corresponding anisotropic pressure of both SQM and CFL quark matter. Our results indicate that CFL quark matter exhibits greater stability than the SQM, and the pressure of CFL quark matter increases with the energy gap constant <inline-formula><tex-math id="Z-20240223200049">\begin{document}$\varDelta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200049.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200049.png"/></alternatives></inline-formula> increasing. We also observe that the oscillation effects coming from the lowest Landau level can be reduced by increasing the energy gap constant <inline-formula><tex-math id="Z-20240223200123">\begin{document}$ \varDelta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200123.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200123.png"/></alternatives></inline-formula>, which cannot be observed in SQM under a similar strong magnetic field. The equivalent quark mass for u, d, and s quark and the chemical potential for each flavor of quarks decrease with the energy gap constant <inline-formula><tex-math id="Z-20240223200141">\begin{document}$ \varDelta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200141.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200141.png"/></alternatives></inline-formula> increasing, which matches the conclusion that CFL quark matter is more stable than SQM. From the calculations of the magnetars with SQM and CFL quark matter, we find that the maximum mass of magnetars increases with the energy gap constant <inline-formula><tex-math id="Z-20240223200204">\begin{document}$\varDelta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200204.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200204.png"/></alternatives></inline-formula> increasing for both the longitudinal and the transverse orientation distribution of magnetic field. Additionally, the tidal deformability of the magnetars increases with the <inline-formula><tex-math id="Z-20240223200231">\begin{document}$\varDelta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200231.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200231.png"/></alternatives></inline-formula> increasing. On the other hand, the central baryon density of the maximum mass of the magnetars decreases with the <inline-formula><tex-math id="Z-20240223200245">\begin{document}$\varDelta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200245.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231649_Z-20240223200245.png"/></alternatives></inline-formula> increasing. The results also indicate that the mass-radius lines of the CFL quark star can also satisfy the new estimates of the mass-radius region from PSR J0740 + 6620, PSR J0030 + 0451, and HESS J1731-347.

Quark models and radial oscillations: decoding the HESS J1731-347 compact object’s equation of state

We investigate the peculiar nature of strange stars through an analysis of different quark models, i.e. vBag model and CFL model equation of states at different parameter sets, and focus on understanding the equation of state governing the intriguing central compact object (CCO) within the supernova remnant HESS J1731-347, with a mass and radius of M=0.77-0.17+0.20M⊙\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M = 0.77^{+0.20}_{-0.17} M_{\\odot }$$\\end{document} and R=10.4-0.78+0.86\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$R = 10.4^{+0.86}_{-0.78}$$\\end{document} km, respectively. Additionally, we compare the radial oscillations of two models to determine the frequency of the HESS J1731-347 compact object at its maximum mass. The frequencies of radial oscillations are computed for each of the four EoSs considered. In total, the 10 lowest radial frequencies for each of those EoSs have been computed. By delving into these aspects, we aim at deepening our understanding of strange stars and their connection to the observed HESS J1731-347 mass-radius relationship.

What Is the Nature of the HESS J1731-347 Compact Object?

Once further confirmed in future analyses, the radius and mass measurement of HESS J1731-347 with and will be among the lightest and smallest compact objects ever detected. This raises many questions about its nature and opens up the window for different theories to explain such a measurement. In this article, we use the information from Doroshenko et al. on the mass, radius, and surface temperature together with the multimessenger observations of neutron stars to investigate the possibility that HESS J1731-347 is one of the lightest observed neutron star, a strange quark star, a hybrid star with an early deconfinement phase transition, or a dark matter–admixed neutron star. The nucleonic and quark matter are modeled within realistic equation of states (EOSs) with a self-consistent calculation of the pairing gaps in quark matter. By performing the joint analysis of the thermal evolution and mass–radius constraint, we find evidence that within a 1σ confidence level, HESS J1731-347 is consistent with the neutron star scenario with the soft EOS as well as with a strange and hybrid star with the early deconfinement phase transition with a strong quark pairing and neutron star admixed with dark matter.

Expansion and ongoing cosmic ray acceleration in HESS J1731−347

Diffusive shock acceleration in supernova remnants (SNRs) is considered one of the prime mechanisms of galactic cosmic ray (GCR) acceleration. It is still unclear, however, whether SNRs can contribute to the GCR spectrum up to the “knee” (1 PeV) band as acceleration to such energies requires an efficient magnetic field amplification process around the shocks. The presence of such a process is challenging to test observationally. Here, we report on the detection of fast variability in the X-ray synchrotron emission from the forward shock in the SNR HESS J1731−347, which implies the presence of a strong (∼0.2 mG) field exceeding background values, and thus of effective field amplification. We also report a direct measurement of the high forward shock expansion velocity of 4000–5500 km s−1, confirming that the SNR is expanding in a tenuous wind bubble blown by the SNR progenitor, is significantly younger (2.4–9 kyr) than previously assumed by some authors, and only recently started interacting with the dense material outside of the bubble. We finally conclude that there is strong evidence for ongoing hadronic GCR acceleration in this SNR.

Relativistic mean-field model for the ultracompact low-mass neutron star HESS J1731-347

Relativistic mean-field model for the ultracompact low-mass neutron star HESS J1731-347

VERITAS highlights of observations and results

Located in southern Arizona, VERITAS is amongst the most sensitive detectors for astrophysical very high energy (VHE; E>100 GeV) gamma rays and has been operational since April 2007. We highlight some recent results from VERITAS observations. These include the long-term observations of the gamma-ray binaries HESS J0632+057 and LS I +61° 303, the observations of the Galactic Center region, and of the supernova remnant Cas A. We discuss the results from a decade of multi-wavelength observations of the blazar 1ES 1215+303, the EHT 2017 campaign on the M87 galaxy, the discovery of 3C 264 in VHE, and the observation of three flaring quasars. Brief highlights of the indirect dark matter searches and targets-of-opportunity (ToO) observations are also discussed. The ToO observations allow for rapid follow-up of multi-messenger alerts and astrophysical transients.

Properties of color-flavor locked matter in a quasiparticle model

We investigate the thermodynamical properties of color-flavor locked (CFL) quark matter at zero temperature, finite temperature, and strong magnetic field by using quasiparticle model. We find that considering CFL quark phase can significantly change the equation of state (EOS) as well as the properties of quark matter in quark stars (QSs) at finite temperature or under magnetic field within quasiparticle model. In particular, our results have shown that we can provide the large QSs within CFL quark phase from quasiparticle model by satisfying both the upper limit of Lambda _{1.4}<800 for the low-spin priors of 1.4 solar mass pulsars from GW170817 and the new estimates of the mass-radius region from PSR J0740 + 6620, PSR J0030 + 0451, HESS J1731-347, and 4U 1702-429, which cannot be obtained by considering the QSs with SQM within quasiparticle model.

Multi-wavelength Study of HESS J1303-631 with 14 yr of Fermi-LAT Data

HESS J1303-631 is an extended TeV pulsar wind nebula powered by the pulsar PSR J1301-6305 detected with the High Energy Stereoscopic System. We present an analysis of the GeV γ-ray region of HESS J1303-631 with about 14 yr of Fermi Large Area Telescope data. The GeV γ-ray emission, coincident with the very-high-energy source, has a photon index of 1.69 ± 0.09 in 10–500 GeV band, and the GeV morphology has an extension to the same direction as indicated in the TeV band. Moreover, the observed multi-wavelength spectral energy distribution of the nebula is studied with a one-zone time-dependent leptonic model, in which the electrons/positrons injected into the nebula are assumed to have a broken power-law spectrum. The result indicates that the multi-wavelength non-thermal emission can be well reproduced via synchrotron radiation and inverse Compton scattering of the particles.

Color-flavor locked quark stars in light of the compact object in the HESS J1731-347 and the GW190814 event

Color-flavor locked quark stars in light of the compact object in the HESS J1731-347 and the GW190814 event

Probing the impact of WIMP dark matter on universal relations, GW170817 posterior, and radial oscillations

ABSTRACT In this study, we investigate the impact of weakly interacting massive particles (WIMPs) dark matter (DM) on C–Λ universal relations, GW170817 posterior, and radial oscillations of neutron stars (NSs) by considering the interactions of uniformly trapped neutralinos as a DM candidate with the hadronic matter through the exchange of the Higgs boson within the framework of the Next-to-Minimal Supersymmetric Standard Model (NMSSM). The hadronic equation of state (EOS) is modelled using the relativistic mean-field (RMF) formalism with IOPB-I, G3, and quark–meson coupling (QMC)-RMF series parameter sets. The presence of DM softens the EOS at both the background and the perturbation levels that implies a small shift to the left in the posterior accompanied by a much larger jump in the left of the mass–radius curves with increasing DM mass. It is observed that EOSs with DM also satisfy the C–Λ universality relations among themselves but get slightly shifted to the right in comparison to that without considering DM. Additionally, we find that the inclusion of DM allows the mass–radius (M–R) curves to remain consistent with observational constraints for HESS J1731−347, indicating the possibility of classifying it as a dark matter-admixed neutron star (DMANS). Moreover, we explore the impact of DM on the radial oscillations of pulsating stars and investigate the stability of NSs. The results demonstrate a positive correlation between the mass of DM and the frequencies of radial oscillation modes.

Anisotropic strange stars in the spotlight: unveiling constraints through observational data

ABSTRACT Motivated by the recent suggestions that very massive pulsar (PSR J0952−0607) and very light compact object (HESS J1731−347) exist, in this article, we revisit the possibility of such objects being strange stars instead of the standard hadronic neutron stars. We study the possible presence of local anisotropy and how it affects the macroscopic properties of strange stars and compare our results with the recent constraints presented in the literature. We found that the presence of anisotropy increases the maximum mass, the radius of the canonical star, and its tidal deformability for positive values of λBL and the opposite for negative values. We also show that although we cannot rule out the possibility of very compact objects being standard hadronic neutron stars, strange stars easily fulfill most of the observational constraints.

Nuclear and hybrid equations of state in light of the low-mass compact star in HESS J1731-347

Nuclear and hybrid equations of state in light of the low-mass compact star in HESS J1731-347

The Nature of γ-Ray Emission from HESS J1912+101

Since the discovery of HESS J1912+101 at teraelectronvolt energies, its nature has been extensively studied. Due to the absence of X-ray and radio counterparts, whether its γ-ray emission is produced by relativistic electrons or ions is still a matter of debate. We reanalyze its megaelectronvolt to gigaelectronvolt γ-ray emission using 14 yr of Pass 8 data of the Fermi-LAT, and find that the gigaelectronvolt γ-ray emission is more extended than the teraelectronvolt shell detected by H. E. S. S. and flux above 10 GeV from the northern half is much higher than that from the southern half, where there is evident interaction between shocks and molecular clouds. As a consequence, the gigaelectronvolt spectrum of the northern half (with an index of 2.19 ± 0.12) is much harder than that in the south (with an index of 2.72 ± 0.08), and the overall gigaelectronvolt spectrum shows a concave shape, which is distinct from most γ-ray supernova remnants (SNRs). In combination with the teraelectronvolt spectrum, the overall γ-ray spectrum can be fitted with a broken power-law model for trapped ions and a low energy component due to escaping ions. The diffusion coefficient for escaping ions however needs to be proportional to the energy, implying that the low energy component may also be attributed to ions accelerated via recent shock–cloud interactions. A hadronic origin for the γ-ray emission is therefore favored and the overall emission properties are consistent with ion acceleration by SNR shocks. On the other hand, it is still undeniable that stellar cluster or PWN may have some contribution in some parts of this extended source.

Baryonic models of ultra-low-mass compact stars for the central compact object in HESS J1731-347

The recent attempt on mass and radius inference of the central compact object within the supernova remnant HESS J1731-347 suggests for this object an unusually low mass of M=0.77−0.17+0.20M⊙ and a small radius of R=10.4−0.78+0.86km. We explore the ways such a result can be accommodated within models of dense matter with heavy baryonic degrees of freedom which are constrained by the multi-messenger observations. We find that to do so using only purely nucleonic models, one needs to assume a rather small value of the slope of symmetry energy Lsym. Once heavy baryons are included higher values of the slope Lsym become acceptable at the cost of a slightly reduced maximum mass of static configuration. These two scenarios are distinguished by the particle composition and will undergo different cooling scenarios. In addition, we show that the universalities of the I-Love-Q relations for static configurations can be extended to very low masses without loss in their accuracy.

Nonstrange quark stars within resummed QCD

The recently developed resummation technique known as {\it renormalization group optimized perturbation theory} (RGOPT) is employed in the evaluation of the EoS describing non-strange cold quark matter at NLO. Inspired by recent investigations, which suggest that stable quark matter can be made only of up and down quarks, the mass-radius relation for two flavor pure quark stars is evaluated and compared with the predictions from perturbative QCD (pQCD) at NNLO. This comparison explicitly shows that by being imbued with renormalization group properties, and a variational optimization procedure, the method allows for an efficient resummation of the perturbative series. Remarkably, when the renormalization scale is chosen so as to reproduce maximum mass stars with M=$2-2.3M_\odot$, one obtains a mass-radius curve compatible with the masses and radii of the pulsars PSR J0740+6620, PSR J0030+0451, and the compact object HESS J1731-347. Moreover, the scale dependence of the EoS (and mass-radius relation) obtained with the RGOPT is greatly improved when compared to that of pQCD. This seminal application to the description of quark stars shows that the RGOPT represents a robust alternative to pQCD when describing compressed quark matter.

Search for the Galactic accelerators of cosmic rays up to the knee with the Pevatron test statistic

ABSTRACT The Pevatron test statistic (PTS) is applied to data from γ-ray observatories to test for the origin of cosmic rays (CRs) at energies around the knee of the CR spectrum. Several sources are analysed within hadronic emission models. Previously derived results for RX J1713.7−3946, Vela Jr, and HESS J1745−290 are confirmed to demonstrate the concept, reliability, and advantages of the PTS. It is excluded with a significance more than 5σ that the sources RX J1713.7−3946 and Vela Jr are Pevatrons, while strong indications exceeding 4σ are found for excluding HESS J1745−290 as a Pevatron. The importance to resolve source confusion with high angular resolution observations for Pevatrons searches is demonstrated using PTS for the region containing the SNR G106.3+2.7 and the Boomerang nebula. No statistically significant conclusion with respect to Pevatron associations could be drawn from this region, for the diffuse γ-ray emission around the Galactic Centre, and the unidentified γ-ray sources LHAASO J2108+5157, HESS J1702−420A, and MGRO J1908+06. Assuming the entire γ-ray emission from MGRO J1908+06 and the tail region of SNR G106.3+2.7 are hadronic, a statistical indication exceeding 3σ is found for the underlying proton spectrum to extend beyond 350–400 TeV as a power law. This result can indicate that these sources are proton and helium Pevatrons, in which the accelerated particles contribute to the knee of proton and helium spectra observed at Earth.

Revisiting the Chandra Observation on the Region of PSR J1J1809–193: Indication of the Existence of an X-Ray Halo and Implication for the Origin of HESS J1809–193

HESS J1809–193 is an extended TeV γ-ray source and the origin of its γ-ray emission remains ambiguous. The pulsar wind nebula (PWN) of PSR J1J1809–193 lying inside the extended γ-ray emission is a possible candidate. Powered by the central pulsar, ultrarelativistic electrons in the PWN can produce radio to X-ray emission through synchrotron and γ-ray emission by inverse Compton (IC) scattering. To check whether this PWN is the counterpart of HESS J1809–193, we analyzed the Chandra X-ray radial intensity profile and spectral index profile of this PWN. We then adopted a one-zone isotropic diffusion model to fit the keV and TeV data. We found diffuse nonthermal X-ray emission extending beyond the PWN, which is likely an X-ray halo radiated by escaping electron/positron pairs from the PWN. A relatively strong magnetic field of ∼20 μG is required to explain the spatial evolution of the X-ray spectrum (i.e., the significant softening of the spectrum with increasing distance from the pulsar), which, however, would suppress the IC radiation of pairs. Our result implies that a hadronic component may be needed to explain HESS J1809–193.

Expected polarization properties of nonmagnetized CCOs

Central compact objects (CCOs) are neutron stars found close to the center of some supernova remnants. A certain number of them are presumably covered by carbon envelopes. Their unpulsed thermal X-ray emission can originate either from the entire surface covered by a carbon atmosphere or alternatively from a nonuniformly emitting hydrogen atmosphere. However, the latter scenario appears unlikely given the available upper limits on the amplitude of pulsations. Here we explore a possibility to further discriminate between the two scenarios using X-ray polarimetric observations. We compute the polarization degree (PD) for nonmagnetized pure-carbon and pure-hydrogen atmospheres with effective temperatures of between 1 and 6 MK and find that it can reach up to 25% and 40% for hydrogen and carbon atmospheres, respectively, in the photon energy band 1–10 keV. However, given the available constraints on possible inhomogeneities of the temperature distribution deduced from models of the X-ray spectrum of the CCO in HESS J1731−347, the integrated PD appears to be very low for both carbon (&lt; 0.25%) and hydrogen (a few percent) compositions in the energy band of 2–8 keV covered by the recently launched Imaging X-ray Polarimetry Explorer. We therefore conclude that polarization from CCOs is not expected to be detectable by current facilities, but future detection would strongly support nonuniform hydrogen composition models.