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In the recent paper of Hooper & Goodenough (1010.2752) it was reported that gamma-ray emission from the Galactic Center region contains an excess compared to the contributions from the large-scale diffuse emission and known point sources. This excess was argued to be consistent with a signal from annihilation of Dark Matter with a power law density profile. We reanalyze the Fermi data and find instead that it is consistent with the "standard model" of diffuse emission and of known point sources. The main reason for the discrepancy with the interpretation of 1010.2752 is different (as compared to the previous works) spectrum of the point source at the Galactic Center assumed in 1010.2752. We discuss possible reasons for such an interpretation. In the recent paper of Hooper and Goodenough (2010) [10] it was reported that γ -ray emission from the Galactic Center region contains an excess compared to the contributions from the large-scale diffuse emission and known point sources. This excess was argued to be consistent with a signal from annihilation of Dark Matter with a power law density profile. We reanalyze the Fermi data and find instead that it is consistent with the “standard model” of diffuse emission and of known point sources. The main reason for the discrepancy with the interpretation of Hooper and Goodenough (2010) [10] is different (as compared to the previous works) spectrum of the point source at the Galactic Center assumed by Hooper and Goodenough (2010) [10] . We discuss possible reasons for such an interpretation. In the recent paper of Hooper & Goodenough (1010.2752) it was reported that gamma-ray emission from the Galactic Center region contains an excess compared to the contributions from the large-scale diffuse emission and known point sources. This excess was argued to be consistent with a signal from annihilation of Dark Matter with a power law density profile. We reanalyze the Fermi data and find instead that it is consistent with the 'standard model' of diffuse emission and of known point sources. The main reason for the discrepancy with the interpretation of 1010.2752 is different (as compared to the previous works) spectrum of the point source at the Galactic Center assumed in 1010.2752. We discuss possible reasons for such an interpretation.

The detection of stochastic background of gravitational waves (GWs), produced by cosmological phase transitions (PTs), is of fundamental importance because allows to probe the physics related to PT energy scales. Motivated by the decisive role of non-zero quark chemical potential towards understanding physics in the core of neutron stars, quark stars and heavy-ion collisions, in this paper we qualitatively explore the stochastic background of GW spectrum generated by a cosmological source such as high-density QCD first order PT during the early Universe. Specifically, we calculate the frequency peak $f_{peak}$ redshifted at today time and the fractional energy density $\Omega_{gw}h^2$ in light of equation-of-state improved by the finite quark (baryon) chemical potential (we consider an effective three flavor chiral quarks model of QCD). Our calculations reveal a striking increase in $f_{peak}$ and $\Omega_{gw}h^2$ due to the quark chemical potential, which means to improve the chances of detection, in possible future observations (in particular SKA/PTA experiments), of the stochastic background of GWs from QCD first order PT. Even if the improvements could be weak, by updating the sensitivity of relevant detectors in the future, we can still remain hopeful. Concerning the phenomenological contribution of QCD equation-of-state, and in particular the possibility to detect a stochastic GW signal, we further show that the role of the quark chemical potential is model-dependent. This feature allows to discriminate among possible QCD effective models depending on their capability to shed light on the dynamic of QCD-PT through future observations of primordial GWs. In this perspective, the results are indeed encouraging to employ the GWs to study the QCD PT in high density strong interaction matter., Comment: 12 pages, 4 figures, to appear in Phys. Lett. B

Flat spectrum radio quasar (FSRQ) is the most luminous blazar at the GeV energies. In this paper, we probe the photon-axion-like particle (ALP) oscillation effect on the latest very-high-energy (VHE) $\gamma$-ray observations of the FSRQ 4C+21.35 (PKS 1222+216). The $\gamma$-ray spectra are measured by the collaborations Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC), Very Energetic Radiation Imaging Telescope Array System (VERITAS), and Fermi Large Area Telescope (Fermi-LAT), which cover two activity VHE flares of 4C+21.35 in 2010 and 2014. We show the spectral energy distributions (SEDs) of these two phases under the null and ALP hypotheses, and set the combined limit on the ALP parameter space. The 95% $\rm C.L.$ combined limit set by the FSRQ 4C+21.35 observations measured by MAGIC, VERITAS, and Fermi-LAT in the $m_a-g_{a\gamma}$ plane is roughly at the photon-ALP coupling $g_{a\gamma} \gtrsim 8\times 10^{-12} \rm \, GeV^{-1}$ for the ALP mass $[\,2\times 10^{-10}\, {\rm eV} \lesssim m_a \lesssim 2\times 10^{-8}\, \rm eV\,]$. Compared with the constraint of NGC 1275 set by Fermi-LAT, no stringent limit result is derived with the photon-ALP coupling $g_{a\gamma}$ from the FSRQ 4C+21.35, while this result could slightly broaden the ALP mass $m_a$ limit at the low-mass region., Comment: 9 pages, 5 figures. Published in PLB

We consider crossed electric and a magnetic fields $\left(\vec{B}=B\,\hat{z},~\vec{E}=E\,\hat{y}\right)$, with $E/B, Comment: 8 pages, 6 figures. The paper has been accepted for publication in Physics Letters B

The polytropic hydrodynamic vortex describes an effective $(2+1)$-dimensional acoustic spacetime with an inner reflecting boundary at $r=r_{\text{c}}$. This physical system, like the spinning Kerr black hole, possesses an ergoregion of radius $r_{\text{e}}$ and an inner non-pointlike curvature singularity of radius $r_{\text{s}}$. Interestingly, the fundamental ratio $r_{\text{e}}/r_{\text{s}}$ which characterizes the effective geometry is determined solely by the dimensionless polytropic index $N_{\text{p}}$ of the circulating fluid. It has recently been proved that, in the $N_{\text{p}}=0$ case, the effective acoustic spacetime is characterized by an {\it infinite} countable set of reflecting surface radii, $\{r_{\text{c}}(N_{\text{p}};n)\}^{n=\infty}_{n=1}$, that can support static (marginally-stable) sound modes. In the present paper we use {\it analytical} techniques in order to explore the physical properties of the polytropic hydrodynamic vortex in the $N_{\text{p}}>0$ regime. In particular, we prove that in this physical regime, the effective acoustic spacetime is characterized by a {\it finite} discrete set of reflecting surface radii, $\{r_{\text{c}}(N_{\text{p}},m;n)\}^{n=N_{\text{max}}}_{n=1}$, that can support the marginally-stable static sound modes (here $m$ is the azimuthal harmonic index of the acoustic perturbation field). Interestingly, it is proved analytically that the dimensionless outermost supporting radius $r^{\text{max}}_{\text{c}}/r_{\text{e}}$, which marks the onset of superradiant instabilities in the polytropic hydrodynamic vortex, increases monotonically with increasing values of the integer harmonic index $m$ and decreasing values of the dimensionless polytropic index $N_{\text{p}}$., 13 pages

Non-standard neutrino interactions (NSI) affect both their propagation through matter and their detection, with bounds on NSI parameters coming from various astrophysical and terrestrial neutrino experiments. In this paper, we show that NSI can be probed in future direct dark matter detection experiments through both elastic neutrino-electron scattering and coherent neutrino-nucleus scattering, and that these channels provide complementary probes of NSI. We show NSI can increase the event rate due to solar neutrinos, with a sharp increase for lower nuclear recoil energy thresholds that are within reach for upcoming detectors. We also identify an interference range of NSI parameters for which the rate is reduced by approximately 40\%. Finally, we show that the "dark side" solution for the solar neutrino mixing angle may be discovered at forthcoming direct detection experiments., 12 pages, 5 figures

Some quantum-gravity theories suggest that the absorbing horizon of a classical black hole should be replaced by a reflective surface which is located a microscopic distance above the would-be classical horizon. Instead of an absorbing black hole, the resulting horizonless spacetime describes a reflective exotic compact object. Motivated by this intriguing prediction, in the present paper we explore the physical properties of exotic compact objects which are linearly coupled to stationary bound-state massive scalar field configurations. In particular, solving the Klein-Gordon wave equation for a stationary scalar field of proper mass $\mu$ and spheroidal harmonic indices $(l,m)$ in the background of a rapidly-rotating exotic compact object of mass $M$ and angular momentum $J=Ma$, we derive a compact analytical formula for the {\it discrete} radii $\{r_{\text{c}}(\mu,l,m,M,a;n)\}$ of the exotic compact objects which can support the stationary bound-state massive scalar field configurations. We confirm our analytical results by direct numerical computations., Comment: 11 pages. arXiv admin note: text overlap with arXiv:1704.05856

IceCube observed high-energy neutrino flux in the energy region from TeV to PeV. The decay of a massive long-lived particle in the early universe can be the origin of the IceCube neutrino events, which we call an "early decay scenario." In this paper, we construct a particle physics model that contains such a massive long-lived particle based on the Peccei-Quinn model. We calculate the present neutrino flux, taking account of realistic initial energy distributions of particles produced by the decay of the massive long-lived particle. We show that the early decay scenario naturally fits into the Peccei-Quinn model, and that the neutrino flux observed by IceCube can be explained in such a framework. We also see that, based on that model, a consistent cosmological history that explains the abundance of the massive long-lived particle is realized., Comment: 20 pages, 2 figures; v2: references added; v3: version accepted for publication in PLB

It has recently been shown that the Hawking evaporation process of $(3+1)$-dimensional Schwarzschild black holes is characterized by the dimensionless ratio $\eta\equiv\tau_{\text{gap}}/\tau_{\text{emission}}\gg1$, where $\tau_{\text{gap}}$ is the characteristic time gap between the emissions of successive Hawking quanta and $\tau_{\text{emission}}$ is the characteristic timescale required for an individual Hawking quantum to be emitted from the Schwarzschild black hole. This strong inequality implies that the Hawking cascade of gravitons from a $(3+1)$-dimensional Schwarzschild black hole is extremely {\it sparse}. In the present paper we explore the semi-classical Hawking evaporation rates of {\it higher}-dimensional Schwarzschild black holes. We find that the dimensionless ratio $\eta(D)\equiv{{\tau_{\text{gap}}}/{\tau_{\text{emission}}}}$, which characterizes the Hawking emission of gravitons from the $(D+1)$-dimensional Schwarzschild black holes, is a {\it decreasing} function of the spacetime dimension. In particular, we show that higher-dimensional Schwarzschild black holes with $D\gtrsim 10$ are characterized by the relation $\eta(D), Comment: 6 pages in Physics Letters B 756, 133 (2016)

The well-known superradiant amplification mechanism allows a charged scalar field of proper mass $\mu$ and electric charge $q$ to extract the Coulomb energy of a charged Reissner-Nordstr\"om black hole. The rate of energy extraction can grow exponentially in time if the system is placed inside a reflecting cavity which prevents the charged scalar field from escaping to infinity. This composed black-hole-charged-scalar-field-mirror system is known as the {\it charged black-hole bomb}. Previous numerical studies of this composed physical system have shown that, in the linearized regime, the inequality $q/\mu>1$ provides a necessary condition for the development of the superradiant instability. In the present paper we use analytical techniques to study the instability properties of the charged black-hole bomb in the regime of linearized scalar fields. In particular, we prove that the lower bound ${{q}\over{\mu}}>\sqrt{{{r_{\text{m}}/r_--1}\over{r_{\text{m}}/r_+-1}}}$ provides a necessary condition for the development of the superradiant instability in this composed physical system (here $r_{\pm}$ are the horizon radii of the charged Reissner-Nordstr\"om black hole and $r_{\text{m}}$ is the radius of the confining mirror). This {\it analytically} derived lower bound on the superradiant instability regime of the composed black-hole-charged-scalar-field-mirror system is shown to agree with direct {\it numerical} computations of the instability spectrum., Comment: 9 pages

Rotating black holes can support quasi-stationary (unstable) bound-state resonances of massive scalar fields in their exterior regions. These spatially regular scalar configurations are characterized by instability timescales which are much longer than the timescale $M$ set by the geometric size (mass) of the central black hole. It is well-known that, in the small-mass limit $\alpha\equiv M\mu\ll1$ (here $\mu$ is the mass of the scalar field), these quasi-stationary scalar resonances are characterized by the familiar hydrogenic oscillation spectrum: $\omega_{\text{R}}/\mu=1-\alpha^2/2{\bar n}^2_0$, where the integer $\bar n_0(l,n;\alpha\to0)=l+n+1$ is the principal quantum number of the bound-state resonance (here the integers $l=1,2,3,...$ and $n=0,1,2,...$ are the spheroidal harmonic index and the resonance parameter of the field mode, respectively). As it depends only on the principal resonance parameter $\bar n_0$, this small-mass ($\alpha\ll1$) hydrogenic spectrum is obviously degenerate. In this paper we go beyond the small-mass approximation and analyze the quasi-stationary bound-state resonances of massive scalar fields in rapidly-spinning Kerr black-hole spacetimes in the regime $\alpha=O(1)$. In particular, we derive the non-hydrogenic (and, in general, non-degenerate) resonance oscillation spectrum ${{\omega_{\text{R}}}/{\mu}}=\sqrt{1-(\alpha/{\bar n})^2}$, where $\bar n(l,n;\alpha)=\sqrt{(l+1/2)^2-2m\alpha+2\alpha^2}+1/2+n$ is the generalized principal quantum number of the quasi-stationary resonances. This analytically derived formula for the characteristic oscillation frequencies of the composed black-hole-massive-scalar-field system is shown to agree with direct numerical computations of the quasi-stationary bound-state resonances., Comment: 7 pages

Following the interesting work of Ba\~nados, Silk, and West [Phys. Rev. Lett. {\bf 103}, 111102 (2009)], it is repeatedly stated in the physics literature that the center-of-mass energy, ${\cal E}_{\text{c.m}}$, of two colliding particles in a maximally rotating black-hole spacetime can grow unboundedly. For this extreme scenario to happen, the particles have to collide at the black-hole horizon. In this paper we show that Thorne's famous hoop conjecture precludes this extreme scenario from occurring in realistic black-hole spacetimes. In particular, it is shown that a new (and larger) horizon is formed {\it before} the infalling particles reach the horizon of the original black hole. As a consequence, the center-of-mass energy of the collisional Penrose process is {\it bounded} from above by the simple scaling relation ${\cal E}^{\text{max}}_{\text{c.m}}/2\mu\propto(M/\mu)^{1/4}$, where $M$ and $\mu$ are respectively the mass of the central black hole and the proper mass of the colliding particles., Comment: 5 pages

The elegant `no short hair' theorem states that, if a spherically-symmetric static black hole has hair, then this hair must extend beyond 3/2 the horizon radius. In the present paper we provide evidence for the failure of this theorem beyond the regime of spherically-symmetric static black holes. In particular, we show that rotating black holes can support extremely short-range stationary scalar configurations (linearized scalar `clouds') in their exterior regions. To that end, we solve analytically the Klein-Gordon-Kerr-Newman wave equation for a linearized massive scalar field in the regime of large scalar masses., Comment: 13 pages

Gamma-rays induced by annihilation or decay of dark matter can be its smoking gun signature. In particular, gamma-rays generated by internal bremsstrahlung of Majorana and real scalar dark matter is promising since it can be a leading emission of sharp gamma-rays. However in the case of Majorana dark matter, its cross section for internal bremsstrahlung cannot be large enough to be observed by future gamma-ray experiments if the observed relic density is assumed to be thermally produced. In this paper, we introduce some degenerate particles with Majorana dark matter, and show they lead enhancement of the cross section. As a result, increase of about one order of magnitude for the cross section is possible without conflict with the observed relic density, and it would be tested by the future gamma-ray experiments such as GAMMA-400 and Cherenkov Telescope Array (CTA). In addition, the constraints of perturbativity, positron observation by the AMS experiment and direct search for dark matter are discussed., 13 pages, 4 figures, version to appear in Physics Letters B

The recently observed excess in gamma-ray signal near the Galactic center suggests that dark matter particles may annihilate into charged fermions that produce gamma-ray to be observed. In this paper, we consider a leptonic dark matter, which annihilates into the standard model leptons, $\mu^+ \mu^-$ and $\tau^+ \tau^-$, by the interaction of the gauged lepton number ${\rm U(1)}_{L_\mu-L_\tau}$ and fits the observed excess. Interestingly, the necessary annihilation cross section for the observed gamma-ray flux provides a good fit to the value for the relic abundance of dark matter. We identify the preferred parameter space of the model after taking the existing experimental constraints from the precision measurements including the muon $(g-2)$, tau decay, neutrino trident production, dark matter direct detection, LHC, and LEP experiments., Comment: 18 pages, 8 figures, references added, matches published version

Kerrr in the title is not a typo. The third "r" stands for "regular", in the sense of pathology-free, rotating black hole. We exhibit a long search-for, exact, Kerr-like, solution of the Einstein equations with novel features: i) no curvature ring singularity; ii) no "anti-gravity" universe with causality violating timelike closed world-lines; iii) no "super-luminal" matter disk. The ring singularity is replaced by a classical, circular, rotating string with Planck tension representing the inner engine driving the rotation of all the surrounding matter. The resulting geometry is regular and smoothly interpolates among inner Minkowski space, borderline deSitter and outer Kerr universe. The key ingredient to cure all unphysical features of the ordinary Kerr black hole is the choice of a "noncommutative geometry inspired" matter source as the input for the Einstein equations, in analogy with spherically symmetric black holes described in earlier works., Comment: 16 pages, 2 figures. Paper accepted for publication in PLB.

The spheroidal harmonics $S_{lm}(\theta;c)$ have attracted the attention of both physicists and mathematicians over the years. These special functions play a central role in the mathematical description of diverse physical phenomena, including black-hole perturbation theory and wave scattering by nonspherical objects. The asymptotic eigenvalues $\{A_{lm}(c)\}$ of these functions have been determined by many authors. However, it should be emphasized that all previous asymptotic analyzes were restricted either to the regime $m\to\infty$ with a fixed value of $c$, or to the complementary regime $|c|\to\infty$ with a fixed value of $m$. A fuller understanding of the asymptotic behavior of the eigenvalue spectrum requires an analysis which is asymptotically uniform in both $m$ and $c$. In this paper we analyze the asymptotic eigenvalue spectrum of these important functions in the double limit $m\to\infty$ and $|c|\to\infty$ with a fixed $m/c$ ratio., Comment: 5 pages