arXiv subjects

E. Kasai

Publications and source records attributed to E. Kasai.

52 records · Page 3Linked to original sources

arXiv2021

TeV emission of Galactic plane sources with HAWC and H.E.S.S

The High Altitude Water Cherenkov (HAWC) observatory and the High Energy Stereoscopic System (H.E.S.S.) are two leading instruments in the ground-based very-high-energy gamma-ray domain. HAWC employs the water Cherenkov detection (WCD) technique, while H.E.S.S. is an array of Imaging Atmospheric Cherenkov Telescopes (IACTs). The two facilities therefore differ in multiple aspects, including their observation strategy, the size of their field of view and their angular resolution, leading to different analysis approaches. Until now, it has been unclear if the results of observations by both types of instruments are consistent: several of the recently discovered HAWC sources have been followed up by IACTs, resulting in a confirmed detection only in a minority of cases. With this paper, we go further and try to resolve the tensions between previous results by performing a new analysis of the H.E.S.S. Galactic plane survey data, applying an analysis technique comparable between H.E.S.S. and HAWC. Events above 1 TeV are selected for both datasets, the point spread function of H.E.S.S. is broadened to approach that of HAWC, and a similar background estimation method is used. This is the first detailed comparison of the Galactic plane observed by both instruments. H.E.S.S. can confirm the gamma-ray emission of four HAWC sources among seven previously undetected by IACTs, while the three others have measured fluxes below the sensitivity of the H.E.S.S. dataset. Remaining differences in the overall gamma-ray flux can be explained by the systematic uncertainties. Therefore, we confirm a consistent view of the gamma-ray sky between WCD and IACT techniques.

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arXiv2021

Southern African Large Telescope Spectroscopy of BL Lacs for the CTA project

In the last two decades, very-high-energy gamma-ray astronomy has reached maturity: over 200 sources have been detected, both Galactic and extragalactic, by ground-based experiments. At present, Active Galactic Nuclei (AGN) make up about 40% of the more than 200 sources detected at very high energies with ground-based telescopes, the majority of which are blazars, i.e. their jets are closely aligned with the line of sight to Earth and three quarters of which are classified as high-frequency peaked BL Lac objects. One challenge to studies of the cosmological evolution of BL Lacs is the difficulty of obtaining redshifts from their nearly featureless, continuum- dominated spectra. It is expected that a significant fraction of the AGN to be detected with the future Cherenkov Telescope Array (CTA) observatory will have no spectroscopic redshifts, compromising the reliability of BL Lac population studies, particularly of their cosmic evolution. We started an effort in 2019 to measure the redshifts of a large fraction of the AGN that are likely to be detected with CTA, using the Southern African Large Telescope (SALT). In this contribution, we present two results from an on-going SALT program focused on the determination of BL Lac object redshifts that will be relevant for the CTA observatory.

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arXiv2021

LMC N132D: A mature supernova remnant with a power-law gamma-ray spectrum extending beyond 8 TeV

We analyzed 252 hours of High Energy Stereoscopic System (H.E.S.S.) observations towards the supernova remnant (SNR) LMC N132D that were accumulated between December 2004 and March 2016 during a deep survey of the Large Magellanic Cloud, adding 104 hours of observations to the previously published data set to ensure a > 5 sigma detection. To broaden the gamma-ray spectral coverage required for modeling the spectral energy distribution, an analysis of Fermi-LAT Pass 8 data was also included. We unambiguously detect N132D at very high energies (VHE) with a significance of 5.7 sigma. We report the results of a detailed analysis of its spectrum and localization based on the extended H.E.S.S. data set. The joint analysis of the extended H.E.S.S and Fermi-LAT data results in a spectral energy distribution in the energy range from 1.7 GeV to 14.8 TeV, which suggests a high luminosity of N132D at GeV and TeV energies. We set a lower limit on a gamma-ray cutoff energy of 8 TeV with a confidence level of 95%. The new gamma-ray spectrum as well as multiwavelength observations of N132D when compared to physical models suggests a hadronic origin of the VHE gamma-ray emission. SNR N132D is a VHE gamma-ray source that shows a spectrum extending to the VHE domain without a spectral cutoff at a few TeV, unlike the younger oxygen-rich SNR Cassiopeia A. The gamma-ray properties of N132D may be affected by an interaction with a nearby molecular cloud that partially lies inside the 95% confidence region of the source position. [Abridged]

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arXiv2021

Search for dark matter annihilation signals from unidentified Fermi-LAT objects with H.E.S.S

Cosmological $N$-body simulations show that Milky Way-sized galaxies harbor a population of unmerged dark matter subhalos. These subhalos could shine in gamma-rays and be eventually detected in gamma-ray surveys as unidentified sources. We performed a thorough selection among unidentified Fermi-LAT Objects (UFOs) to identify them as possible TeV-scale dark matter subhalo candidates. We search for very-high-energy (E $\gtrsim$ 100 GeV) gamma-ray emissions using H.E.S.S. observations towards four selected UFOs. Since no significant very-high-energy gamma-ray emission is detected in any dataset of the four observed UFOs nor in the combined UFO dataset, strong constraints are derived on the product of the velocity-weighted annihilation cross section $\langle σv \rangle$ by the $J$-factor for the dark matter models. The 95% C.L. observed upper limits derived from combined H.E.S.S. observations reach $\langle σv \rangle J$ values of 3.7$\times$10$^{-5}$ and 8.1$\times$10$^{-6}$ GeV$^2$cm$^{-2}$s$^{-1}$ in the $W^+W^-$ and $τ^+τ^-$ channels, respectively, for a 1 TeV dark matter mass. Focusing on thermal WIMPs, the H.E.S.S. constraints restrict the $J$-factors to lie in the range 6.1$\times$10$^{19}$ - 2.0$\times$10$^{21}$ GeV$^2$cm$^{-5}$, and the masses to lie between 0.2 and 6 TeV in the $W^+W^-$ channel. For the $τ^+τ^-$ channel, the $J$-factors lie in the range 7.0$\times$10$^{19}$ - 7.1$\times$10$^{20}$ GeV$^2$cm$^{-5}$ and the masses lie between 0.2 and 0.5 TeV. Assuming model-dependent predictions from cosmological N-body simulations on the $J$-factor distribution for Milky Way-sized galaxies, the dark matter models with masses greater than 0.3 TeV for the UFO emissions can be ruled out at high confidence level.

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arXiv2021

Evidence of 100 TeV $γ$-ray emission from HESS J1702-420: A new PeVatron candidate

The identification of PeVatrons, hadronic particle accelerators reaching the knee of the cosmic ray spectrum (few $10^{15}$ eV), is crucial to understand the origin of cosmic rays in the Galaxy. We provide an update on the unidentified source HESS J1702-420, a promising PeVatron candidate. We present new observations of HESS J1702-420 made with the High Energy Stereoscopic System (H.E.S.S.), and processed using improved analysis techniques. The analysis configuration was optimized to enhance the collection area at the highest energies. We applied a three-dimensional (3D) likelihood analysis to model the source region and adjust non thermal radiative spectral models to the $γ$-ray data. We also analyzed archival data from the Fermi Large Area Telescope (LAT) to constrain the source spectrum at $γ$-ray energies >10 GeV. We report the detection of a new source component called HESS J1702-420A, that was separated from the bulk of TeV emission at a $5.4σ$ confidence level. The power law $γ$-ray spectrum of HESS J1702-420A extends with an index of $Γ=1.53\pm0.19_\text{stat}\pm0.20_\text{sys}$ and without curvature up to the energy band 64-113 TeV, in which it was detected by H.E.S.S. at a $4.0σ$ confidence level. This brings evidence for the source emission up to $100\,\text{TeV}$, which makes HESS J1702-420A a compelling candidate site for the presence of extremely high energy cosmic rays. Remarkably, in a hadronic scenario, the cut-off energy of the proton distribution powering HESS J1702-420A is found to be higher than 0.5 PeV at a 95% confidence level. HESS J1702-420A becomes therefore one of the most solid PeVatron candidates detected so far in H.E.S.S. data, altough a leptonic origin of its emission could not be ruled out either.

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arXiv2021

Search for dark matter annihilation in the dwarf irregular galaxy WLM with H.E.S.S

We search for an indirect signal of dark matter through very high-energy gamma rays from the Wolf-Lundmark-Melotte (WLM) dwarf irregular galaxy. The pair annihilation of dark matter particles would produce Standard Model particles in the final state such as gamma rays, which might be detected by ground-based Cherenkov telescopes. Dwarf irregular galaxies represent promising targets as they are dark matter dominated objects with well measured kinematics and small uncertainties on their dark matter distribution profiles. In 2018, the H.E.S.S. five-telescope array observed the dwarf irregular galaxy WLM for 18 hours. We present the first analysis based on data obtained from an imaging atmospheric Cherenkov telescope for this subclass of dwarf galaxy. As we do not observe any significant excess in the direction of WLM, we interpret the result in terms of constraints on the velocity-weighted cross section for dark matter pair annihilation as a function of the dark matter particle mass for various continuum channels as well as the prompt gamma-gamma emission. For the $τ^+τ^-$ channel the limits reach a $\langle σv \rangle$ value of about $4\times 10^{-22}$ cm3s-1 for a dark matter particle mass of 1 TeV. For the prompt gamma-gamma channel, the upper limit reaches a $\langle σv \rangle$ value of about $5 \times10^{-24}$ cm3s-1 for a mass of 370 GeV. These limits represent an improvement of up to a factor 200 with respect to previous results for the dwarf irregular galaxies for TeV dark matter search.

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arXiv2021

Optical spectroscopy of Blazars for the Cherenkov Telescope Array

Context. Blazars are the most numerous class of High Energy (HE; E about 50 MeV - few 100 GeV) and Very High Energy (VHE; E about 100 GeV - 10 TeV) gamma-ray emitters. As of today, a measured spectroscopic redshift is available for only about 50% of gamma-ray BL Lacs, mainly due to the difficulty of measuring reliable redshifts from their nearly featureless, continuum-dominated optical spectra. The knowledge of the redshift is fundamental for understanding the emission from blazars, for population studies and also for indirect studies of the extragalactic background light and searches for Lorentz invariance violation and axion-like particles using blazars. Aims. This paper is the first of a series of papers which aim to measure the redshift of a sample of blazars likely to be detected with the upcoming Cherenkov Telescope Array (CTA), a ground based gamma-ray observatory. Methods. Monte Carlo simulations were performed to select those hard spectrum gamma-ray blazars detected with the Fermi-LAT telescope still lacking redshift measurements but likely to be detected by CTA in 30 hours of observing time or less. Optical observing campaigns involving deep imaging and spectroscopic observations were organised to efficiently constrain their redshifts. We performed deep medium to high resolution spectroscopy of nineteen blazar optical counterparts using the ESI spectrograph at Keck, the RSS spectrograph at the SALT telescope, and the EFOSC2 spectrograph at the ESO NTT. We searched systematically for spectral features and, when possible, we estimated the contribution of the host galaxy to the total flux. Results. We measured eleven firm spectroscopic redshifts with values ranging from 0.1116 to 0.482. one tentative redshift, three redshift lower limits including one at z > 0.449 and another at z > 0.868. There were four objects found to have featureless spectra.

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arXiv2020

An extreme particle accelerator in the Galactic plane: HESS J1826$-$130

The unidentified very-high-energy (VHE; E $>$ 0.1 TeV) $γ$-ray source, HESS J1826$-$130, was discovered with the High Energy Stereoscopic System (HESS) in the Galactic plane. The analysis of 215 h of HESS data has revealed a steady $γ$-ray flux from HESS J1826$-$130, which appears extended with a half-width of 0.21$^{\circ}$ $\pm$ 0.02$^{\circ}_{\text{stat}}$ $\pm$ 0.05$^{\circ}_{\text{sys}}$. The source spectrum is best fit with either a power-law function with a spectral index $Γ$ = 1.78 $\pm$ 0.10$_{\text{stat}}$ $\pm$ 0.20$_{\text{sys}}$ and an exponential cut-off at 15.2$^{+5.5}_{-3.2}$ TeV, or a broken power-law with $Γ_{1}$ = 1.96 $\pm$ 0.06$_{\text{stat}}$ $\pm$ 0.20$_{\text{sys}}$, $Γ_{2}$ = 3.59 $\pm$ 0.69$_{\text{stat}}$ $\pm$ 0.20$_{\text{sys}}$ for energies below and above $E_{\rm{br}}$ = 11.2 $\pm$ 2.7 TeV, respectively. The VHE flux from HESS J1826$-$130 is contaminated by the extended emission of the bright, nearby pulsar wind nebula (PWN), HESS J1825$-$137, particularly at the low end of the energy spectrum. Leptonic scenarios for the origin of HESS J1826$-$130 VHE emission related to PSR J1826$-$1256 are confronted by our spectral and morphological analysis. In a hadronic framework, taking into account the properties of dense gas regions surrounding HESS J1826$-$130, the source spectrum would imply an astrophysical object capable of accelerating the parent particle population up to $\gtrsim$200 TeV. Our results are also discussed in a multiwavelength context, accounting for both the presence of nearby supernova remnants (SNRs), molecular clouds, and counterparts detected in radio, X-rays, and TeV energies.

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arXiv2019

First Cosmology Results Using Type Ia Supernovae From the Dark Energy Survey: Photometric Pipeline and Light Curve Data Release

We present griz light curves of 251 Type Ia Supernovae (SNe Ia) from the first 3 years of the Dark Energy Survey Supernova Program's (DES-SN) spectroscopically classified sample. The photometric pipeline described in this paper produces the calibrated fluxes and associated uncertainties used in the cosmological parameter analysis (Brout et al. 2018-SYS, DES Collaboration et al. 2018) by employing a scene modeling approach that simultaneously forward models a variable transient flux and temporally constant host galaxy. We inject artificial point sources onto DECam images to test the accuracy of our photometric method. Upon comparison of input and measured artificial supernova fluxes, we find flux biases peak at 3 mmag. We require corrections to our photometric uncertainties as a function of host galaxy surface brightness at the transient location, similar to that seen by the DES Difference Imaging Pipeline used to discover transients. The public release of the light curves can be found at https://des.ncsa.illinois.edu/releases/sn.

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arXiv2019

First Cosmology Results Using Type Ia Supernovae From the Dark Energy Survey: Analysis, Systematic Uncertainties, and Validation

We present the analysis underpinning the measurement of cosmological parameters from 207 spectroscopically classified type Ia supernovae (SNe Ia) from the first three years of the Dark Energy Survey Supernova Program (DES-SN), spanning a redshift range of 0.017<$z$<0.849. We combine the DES-SN sample with an external sample of 122 low-redshift ($z$<0.1) SNe Ia, resulting in a "DES-SN3YR" sample of 329 SNe Ia. Our cosmological analyses are blinded: after combining our DES-SN3YR distances with constraints from the Cosmic Microwave Background (CMB; Planck Collaboration 2016), our uncertainties in the measurement of the dark energy equation-of-state parameter, $w$, are .042 (stat) and .059 (stat+syst) at 68% confidence. We provide a detailed systematic uncertainty budget, which has nearly equal contributions from photometric calibration, astrophysical bias corrections, and instrumental bias corrections. We also include several new sources of systematic uncertainty. While our sample is <1/3 the size of the Pantheon sample, our constraints on $w$ are only larger by 1.4$\times$, showing the impact of the DES SN Ia light curve quality. We find that the traditional stretch and color standardization parameters of the DES SNe Ia are in agreement with earlier SN Ia samples such as Pan-STARRS1 and the Supernova Legacy Survey. However, we find smaller intrinsic scatter about the Hubble diagram (0.077 mag). Interestingly, we find no evidence for a Hubble residual step ( 0.007 $\pm$ 0.018 mag) as a function of host galaxy mass for the DES subset, in 2.4$σ$ tension with previous measurements. We also present novel validation methods of our sample using simulated SNe Ia inserted in DECam images and using large catalog-level simulations to test for biases in our analysis pipelines.

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arXiv2019

First Cosmological Results using Type Ia Supernovae from the Dark Energy Survey: Measurement of the Hubble Constant

We present an improved measurement of the Hubble constant (H_0) using the 'inverse distance ladder' method, which adds the information from 207 Type Ia supernovae (SNe Ia) from the Dark Energy Survey (DES) at redshift 0.018 < z < 0.85 to existing distance measurements of 122 low redshift (z < 0.07) SNe Ia (Low-z) and measurements of Baryon Acoustic Oscillations (BAOs). Whereas traditional measurements of H_0 with SNe Ia use a distance ladder of parallax and Cepheid variable stars, the inverse distance ladder relies on absolute distance measurements from the BAOs to calibrate the intrinsic magnitude of the SNe Ia. We find H_0 = 67.8 +/- 1.3 km s-1 Mpc-1 (statistical and systematic uncertainties, 68% confidence). Our measurement makes minimal assumptions about the underlying cosmological model, and our analysis was blinded to reduce confirmation bias. We examine possible systematic uncertainties and all are below the statistical uncertainties. Our H_0 value is consistent with estimates derived from the Cosmic Microwave Background assuming a LCDM universe (Planck Collaboration et al. 2018).

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arXiv2019

First Cosmology Results using Type Ia Supernova from the Dark Energy Survey: Simulations to Correct Supernova Distance Biases

We describe catalog-level simulations of Type Ia supernova (SN~Ia) light curves in the Dark Energy Survey Supernova Program (DES-SN), and in low-redshift samples from the Center for Astrophysics (CfA) and the Carnegie Supernova Project (CSP). These simulations are used to model biases from selection effects and light curve analysis, and to determine bias corrections for SN~Ia distance moduli that are used to measure cosmological parameters. To generate realistic light curves, the simulation uses a detailed SN~Ia model, incorporates information from observations (PSF, sky noise, zero point), and uses summary information (e.g., detection efficiency vs. signal to noise ratio) based on 10,000 fake SN light curves whose fluxes were overlaid on images and processed with our analysis pipelines. The quality of the simulation is illustrated by predicting distributions observed in the data. Averaging within redshift bins, we find distance modulus biases up to 0.05~mag over the redshift ranges of the low-z and DES-SN samples. For individual events, particularly those with extreme red or blue color, distance biases can reach 0.4~mag. Therefore, accurately determining bias corrections is critical for precision measurements of cosmological parameters. Files used to make these corrections are available at https://des.ncsa.illinois.edu/releases/sn.

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arXiv2019

First Cosmology Results using Type Ia Supernovae from the Dark Energy Survey: Constraints on Cosmological Parameters

We present the first cosmological parameter constraints using measurements of type Ia supernovae (SNe Ia) from the Dark Energy Survey Supernova Program (DES-SN). The analysis uses a subsample of 207 spectroscopically confirmed SNe Ia from the first three years of DES-SN, combined with a low-redshift sample of 122 SNe from the literature. Our "DES-SN3YR" result from these 329 SNe Ia is based on a series of companion analyses and improvements covering SN Ia discovery, spectroscopic selection, photometry, calibration, distance bias corrections, and evaluation of systematic uncertainties. For a flat LCDM model we find a matter density Omega_m = 0.331 +_ 0.038. For a flat wCDM model, and combining our SN Ia constraints with those from the cosmic microwave background (CMB), we find a dark energy equation of state w = -0.978 +_ 0.059, and Omega_m = 0.321 +_ 0.018. For a flat w0waCDM model, and combining probes from SN Ia, CMB and baryon acoustic oscillations, we find w0 = -0.885 +_ 0.114 and wa = -0.387 +_ 0.430. These results are in agreement with a cosmological constant and with previous constraints using SNe Ia (Pantheon, JLA).

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arXiv2019

Cosmological Constraints from Multiple Probes in the Dark Energy Survey

The combination of multiple observational probes has long been advocated as a powerful technique to constrain cosmological parameters, in particular dark energy. The Dark Energy Survey has measured 207 spectroscopically--confirmed Type Ia supernova lightcurves; the baryon acoustic oscillation feature; weak gravitational lensing; and galaxy clustering. Here we present combined results from these probes, deriving constraints on the equation of state, $w$, of dark energy and its energy density in the Universe. Independently of other experiments, such as those that measure the cosmic microwave background, the probes from this single photometric survey rule out a Universe with no dark energy, finding $w=-0.80^{+0.09}_{-0.11}$. The geometry is shown to be consistent with a spatially flat Universe, and we obtain a constraint on the baryon density of $Ω_b=0.069^{+0.009}_{-0.012}$ that is independent of early Universe measurements. These results demonstrate the potential power of large multi-probe photometric surveys and pave the way for order of magnitude advances in our constraints on properties of dark energy and cosmology over the next decade.

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arXiv2018

First Cosmology Results Using Type Ia Supernovae From the Dark Energy Survey: Survey Overview and Supernova Spectroscopy

We present spectroscopy from the first three seasons of the Dark Energy Survey Supernova Program (DES-SN). We describe the supernova spectroscopic program in full: strategy, observations, data reduction, and classification. We have spectroscopically confirmed 307 supernovae, including 251 type Ia supernovae (SNe Ia) over a redshift range of $0.017 < z < 0.85$. We determine the effective spectroscopic selection function for our sample, and use it to investigate the redshift-dependent bias on the distance moduli of SNe Ia we have classified. We also provide a full overview of the strategy, observations, and data products of DES-SN, which has discovered 12,015 likely supernovae during these first three seasons. The data presented here are used for the first cosmology analysis by DES-SN ('DES-SN3YR'), the results of which are given in DES Collaboration (2018a).

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arXiv2018

First Cosmology Results Using Type Ia Supernovae from the Dark Energy Survey: Effects of Chromatic Corrections to Supernova Photometry on Measurements of Cosmological Parameters

Calibration uncertainties have been the leading systematic uncertainty in recent analyses using type Ia Supernovae (SNe Ia) to measure cosmological parameters. To improve the calibration, we present the application of Spectral Energy Distribution (SED)-dependent "chromatic corrections" to the supernova light-curve photometry from the Dark Energy Survey (DES). These corrections depend on the combined atmospheric and instrumental transmission function for each exposure, and they affect photometry at the 0.01 mag (1%) level, comparable to systematic uncertainties in calibration and photometry. Fitting our combined DES and low-z SN Ia sample with Baryon Acoustic Oscillation (BAO) and Cosmic Microwave Background (CMB) priors for the cosmological parameters $Ω_{\rm m}$ (the fraction of the critical density of the universe comprised of matter) and w (the dark energy equation of state parameter), we compare those parameters before and after applying the corrections. We find the change in w and $Ω_{\rm m}$ due to not including chromatic corrections are -0.002 and 0.000, respectively, for the DES-SN3YR sample with BAO and CMB priors, consistent with a larger DES-SN3YR-like simulation, which has a w-change of 0.0005 with an uncertainty of 0.008 and an $Ω_{\rm m}$ change of 0.000 with an uncertainty of 0.002 . However, when considering samples on individual CCDs we find large redshift-dependent biases (approximately 0.02 in distance modulus) for supernova distances.

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