arXiv subjects

Masaki Mori

Publications and source records attributed to Masaki Mori.

At least 37 records · Page 2Linked to original sources

arXiv2014

Fermi LAT detection of two high Galactic latitude gamma-ray sources, Fermi J1049.7+0435 and J1103.2+1145

The Second Fermi LAT source catalog (Nolan et al. 2012) includes as many as 1,873 sources, but initial attempts to identify counterparts at other wavelengths resulted in 575 sources remaining unidentified. The 2FGL catalog is based on the first 24 months of LAT observation since its launch in 2008, but the LAT has now accumulated more than 5 years of high-energy gamma-ray data almost flawlessly, presenting the possibility of finding new sources which were too faint to be detected in the first two years of data or showed flaring activity after the catalog was created. In this paper we report on two new gamma-ray sources serendipitously discovered in the constellation Leo and discuss possible counterparts based on radio observations.

astro-ph.HE↗

arXiv2013

Time series analysis of gamma-ray blazars and implications for the central black-hole mass

Radiation from the blazar class of of active galactic nuclei (AGN) exhibits fast time variability which is usually ascribed to instabilities in the emission region near the central supermassive black hole. The variability time scale is generally faster in higher energy region, and data recently provided by the {\it Fermi} Gamma-ray Space Telescope in the GeV energy band enable a detailed study of the temporal behavior of AGN. Due to its wide field-of-view in the scanning mode, most sky regions are observed for several hours per day and daily light curves of many AGN have been accumulated for more than 4 years. In this paper we investigate the time variability of 15 well-detected AGNs by studying the normalized power spectrum density (NPSD) of their light curves in the GeV energy band. One source, 3C 454.3, shows a specific time scale of $6.8\times10^5$ s, and this value suggests, assuming the internal shock model, a mass for the central black hole of $(10^8$--$10^{10})M_\odot$ which is consistent with other estimates. It also indicates the typical time interval of ejected blobs is $(7$--$70)$ times the light crossing time of the Schwarzschild radius.

astro-ph.HE↗

arXiv2013

Lower Bounds on Magnetic Fields in Intergalactic Voids from Long-Term GeV-TeV Light Curves of the Blazar Mrk 421

Lower bounds are derived on the amplitude B of intergalactic magnetic fields (IGMFs) in the region between Galaxy and the blazar Mrk 421, from constraints on the delayed GeV pair-echo flux that are emitted by secondary e^-e^+ produced in γγinteractions between primary TeV gamma-rays and the cosmic infrared background. The distribution of galaxies mapped by the Sloan Digital Sky Survey shows that this region is dominated by a large intergalactic void. We utilize data from long-term, simultaneous GeV-TeV observations by the Fermi Large Area Telescope and the ARGO-YBJ experiment extending over 850 days. For an assumed value of B, we evaluate the daily GeV pair-echo flux expected from the TeV data, select the dates where this exceeds the Fermi 2-σsensitivity, compute the probability that this flux is excluded by the Fermi data for each date, and then combine the probabilities using the inverse normal method. Consequently, we exclude B < 10^{-20.5} G for a field coherence length of 1 kpc at 4- σlevel, as long as plasma instabilities are unimportant for cooling of the pair beam. This is much more significant than the 2-σbounds we obtained previously from observations of Mrk 501, by virtue of more extensive data from the ARGO-YBJ, as well as improved statistical analysis. Compared with most other studies of IGMF bounds, the evidence we present here for a non-zero IGMF is more robust as it does not rely on unproven assumptions on the primary TeV emission during unobserved periods.

astro-ph.CO↗

arXiv2013

Fermi-LAT study of two gamma-ray binaries, HESS J0632+057 and AGL J2241+4454

GeV gamma-ray emission from two gamma-ray binary candidates, HESS J0632+057 and AGL J2241+4454, which were recently reported by H.E.S.S. and AGILE, respectively, have been searched for using the Fermi-LAT archival dataset. Spatial and temporal distribution of gamma-ray events are studied, but there was no evidence for GeV gamma-ray signal from either sources.

astro-ph.HE↗

arXiv2012

On representation categories of wreath products in non-integral rank

For an arbitrary commutative ring k and t in k, we construct a 2-functor S_t which sends a tensor category to a new tensor category. By applying it to the representation category of a bialgebra we obtain a family of categories which interpolates the representation categories of the wreath products of the bialgebra. This generalizes the construction of Deligne's category Rep(S_t,k) for representation categories of symmetric groups.

math.RT↗

arXiv2011

GeV Gamma-ray Emission from the Binary PSR B1259-63/SS2883 During the 2010 Periastron Passage

PSR B1259-63/SS2883 is a binary system which consists of a 48-ms radio pulsar and a massive star in a highly eccentric orbit with a period of about 3.4 years. Non-pulsed and non-thermal emissions from this binary have been reported in the radio, X-ray and TeV gamma-ray energy ranges. Light curves in the radio and X-ray bands showed characteristic double-peaked features which can be attibuted to the interactions of the pulsar wind and the Be disk during the crossings by the pulsar. The TeV light curves around periastron differ between 2004 and 2007 observations, and the feature is not conclusive. We report a detection of GeV gamma-ray emission around the periastron passage in December 2010 with Fermi-LAT.

astro-ph.HE↗

arXiv2011

Lower Bounds on Intergalactic Magnetic Fields from Simultaneously Observed GeV-TeV Light Curves of the Blazar Mrk 501

We derive lower bounds on intergalactic magnetic fields (IGMFs) from upper limits on the pair echo emission from the blazar Mrk 501, that is, delayed GeV emission from secondary $e^{-}e^{+}$ pairs produced via interactions of primary TeV gamma rays with the cosmic infrared background. Utilizing only simultaneous GeV-TeV light curves observed by VERITAS, MAGIC and {\it Fermi}-LAT during a multiwavelength campaign in 2009 that included a TeV flare, bounds are deduced on the IGMF strength of $B \gtrsim 10^{-20} {\rm G}$ at 90% confidence level for a field coherence length of 1 kpc. Since our analysis is based firmly on the observational data alone and nearly free of assumptions concerning the primary TeV flux in unobserved periods or spectral bands, our evaluation of the pair echo flux is conservative and the evidence for a non-zero IGMF is more robust compared to previous studies.

astro-ph.CO↗

arXiv2010

Prospects for Very High Energy Blazar Survey by the Next Generation Cherenkov Telescopes

The prospects for future blazar surveys by next-generation very high energy (VHE) gamma-ray telescopes, such as Advanced Gamma-ray Imaging System (AGIS) and Cherenkov Telescope Array (CTA), are investigated using the latest model of blazar luminosity function and its evolution which is in good agreement with the flux and redshift distribution of observed blazars as well as the extragalactic gamma-ray background. We extend and improve the template of spectral energy distributions (SEDs) based on the blazar SED sequence paradigm, to make it reliable also in the VHE bands (above 100 GeV) by comparing with the existing VHE blazar data. Assuming the planned CTA sensitivities, a blind survey using a total survey time of ~100 hrs could detect ~3 VHE blazars, with larger expected numbers for wider/shallower surveys. We also discuss following-up of Fermi blazars. Detectability of VHE blazars in the plane of Fermi flux and redshift is presented, which would be useful for future survey planning. Prospects and strategies are discussed to constrain the extragalactic background light (EBL) by using the absorption feature of brightest blazar spectra, as well as cut-offs in the redshift distribution. We will be able to get useful constraints on EBL by VHE blazars at different redshifts ranging 0.3-1 TeV corresponding to z=0.10-0.36.

astro-ph.HE↗

arXiv2009

Search for GeV gamma-ray emission from clusters of galaxies studied by TeV telescopes

A cluster of galaxies is a huge system bounded by gravitation, and cosmic rays are thought to be confined in the system, thus it should contain much non-thermal components. Many theories predict significant gamma-ray emission that could be detectable by state-of-the-art gamma-ray telescopes. Some clusters have already been studied by using Fermi gamma-ray space telescope in the GeV band and Cherenkov telescopes in the TeV band, but most clusters are not studied in both energy bands. Here I present results on GeV gamma-ray emission from clusters of galaxies which have been given upper limits by Cherenkov telescopes using Fermi archival data.

astro-ph.HE↗

arXiv2009

Nuclear enhancement factor in calculation of Galactic diffuse gamma-rays: A new estimate with DPMJET-3

A new calculation of nuclear enhancement factor, used in estimation of Galactic diffuse gamma-ray flux from proton-proton interaction in order to take account of heavy nuclei included in cosmic-rays and interstellar matter, is presented by use of a Monte Carlo simulator, DPMJET-3. A new value of 1.8-2.0 in the energy range of 6-1000 GeV/nucleon, slightly increasing with kinetic energy of projectile cosmic rays, is about 20% larger than previous estimates.

astro-ph.HE↗

arXiv2008

Recent Topics on Very High Energy Gamma-ray Astronomy

With the advent of imaging atmospheric Cherenkov telescopes in late 1980's, ground-based observation of TeV gamma-rays came into reality after struggling trials by pioneers for twenty years, and the number of gamma-ray sources detected at TeV energies has increased to be over seventy now. In this review, recent findings from ground-based very-high-energy gamma-ray observations are summarized (as of 2008 March), and up-to-date problems in this research field are presented.

astro-ph↗

arXiv2004

Gamma-ray Spectra due to Cosmic-ray Interactions with Dense Gas Clouds

Gamma-ray spectra from cosmic-ray proton and electron interactions with dense gas clouds have been calculated using a Monte Carlo event simulation code, GEANT4. Such clouds are postulated as a possible form of baryonic dark matter in the Universe. The simulation fully tracks the cascade and transport processes that are important in a dense medium, and the resulting gamma-ray spectra are computed as a function of cloud column-density. These calculations are used for predicting the Galactic diffuse gamma-ray spectrum that may be contributed by baryonic dark matter; the results are compared with data from the EGRET instrument, and used to constrain the fraction of Galactic dark matter that may be in the form of dense gas clouds. In agreement with previous authors, we find useful constraints on the fraction of Galactic dark matter that may be in the form of low column-density clouds ($Σ\la 10 g cm^{-2}$). However, this fraction rises steeply in the region $Σ\sim 10^2 g cm^{-2}$, and for $Σ\ga 200 g cm^{-2}$ we find that baryonic dark matter models are virtually unconstrained by the existing gamma-ray data.

astro-ph↗

arXiv2003

Microwave anisotropies from the Galactic halo

Models in which a large fraction of the Galactic dark matter takes the form of cold gas clouds imply that there is thermal microwave emission from the Galactic dark halo. Such models can therefore be directly constrained by data on the microwave sky, and in particular the very sensitive observations of microwave anisotropies which are now being made. To this end we have computed the anisotropy power-spectrum expected for a Galactic dark halo made of cold, dense gas clouds, including the effects of clustering with a CDM-like mass spectrum of mini-halo substructure. The power-spectrum displays two peaks: one, at l~50, is the Poisson noise for the mini-halos, and the second, much larger and at much higher l, is the Poisson noise of the individual clouds. Because it appears on small (milli-arcsecond) angular scales, where the instrumental sensitivity is inevitably very poor, the latter signal is not directly detectable. By contrast, clusters of cold gas clouds may contribute significantly to the observed anisotropies if their emission has a grey-body spectrum. In this case the peak fluctuation, at l~50, amounts to 4/|sin b| micro-K in the Rayleigh-Jeans limit, and is the dominant Galactic foreground between 40 and 80 GHz. It will be possible to constrain this foreground component using low-latitude data from the MAP satellite, providing that its spectrum conforms to a grey-body. If the spectrum is ``dusty'' there will be relatively little power at frequencies below the thermal peak, and in this case the predicted anisotropies are shown to be negligible.

astro-ph↗

arXiv2003

Dense gas clouds and the Unidentified EGRET sources

Cold, dense gas clouds have been proposed as a major component of the Galactic dark matter; such clouds can be revealed by the gamma-ray emission which arises from cosmic-rays interacting with the gas. If this dark matter component is clustered then highly luminous GeV sources result, preferentially at low to mid Galactic latitudes where they lie within the cosmic-ray disk. The predicted emission for such clusters is steady, continuum emission with peak power emerging at several hundred MeV. These sources would not have obvious counterparts at other wavelengths and are thus of interest in connection with the UnIDentified (UID) EGRET sources. Here we present a Monte Carlo simulation of the gamma-ray source population due to cold gas clouds, assuming a Cold-Dark-Matter-like mass spectrum for the clustering. We find that ~280 EGRET sources are predicted by this model, with a median Galactic latitude of 12 degrees for the population, and a median angular size of 2.2 degrees. The latitude and size distributions are consistent with the UID EGRET source data, but the source counts are clearly overpredicted. On the basis of these results we propose that clusters of cold gas clouds comprise the majority population of the observed UID EGRET sources. Our interpretation implies that there should be microwave counterparts to most of the UID sources, and will thus be strongly constrained by data from the present generation of microwave anisotropy experiments.

astro-ph↗

arXiv2001

GeV-TeV Gamma-ray Astronomy

Recent results of GeV and TeV observations of gamma-rays from the Universe are briefly reviewed. Topics include observational technique, diffuse gamma-rays, pulsars, unidentified sources, plerions, supernova remnants and AGNs.

astro-ph↗

arXiv2001

Cangaroo-II and Cangaroo-III

Preliminary results from CANGAROO-II, a 10 m imaging Cherenkov telescope, in Woomera, South Australia are presented. They include the confirmation of detections of TeV gamma-ray sources we have reported using a 3.8 m telescope, CANGAROO-I. Also the status of the construction of an array of four 10 m telescopes, called CANGAROO-III, is reported. The first telescope of the array was upgraded from the 7 m telescope and the second one is being constructed in this year. The full array will be operating in 2004.

astro-ph↗

arXiv2000

Status of the CANGAROO-III Project

The CANGAROO-III project, a system of four 10 m telecopes dedicated for gamma-ray astrophysics, has started in 1999 and is expected to complete in 2004 in Woomera, South Australia. We report the construction work during the first year, which includes the completion of the first 10 m telescope built as a 7 m telescope in 1999, and the work in progess to increase the performance by constructing three more similar telescopes based on the experience gained during the construction of the first telescope.

astro-ph↗

arXiv1999

The Cangaroo-III Project

The CANGAROO-III project,which consists of an array of four 10 m imaging Cherenkov telescopes,has just started being constructed in Woomera, South Australia,in a collaboration between Australia and Japan. The first stereoscopic observation of celestial high-energy gamma-rays in the 100 GeV region with two telescopes will start in 2002,and the four telescope array will be completed in 2004. The concept of the project and the expected performance are discussed.

astro-ph↗