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A. Bamba

Publications and source records attributed to A. Bamba.

At least 55 records · Page 3Linked to original sources
arXiv2015

Search for new supernova remnant shells in the Galactic plane with H.E.S.S.

Amongst the population of TeV gamma-ray sources detected with the High Energy Stereoscopic System (H.E.S.S.) in the Galactic plane, clearly identified supernova remnant (SNR) shells constitute a small but precious source class. TeV-selected SNRs are prime candidates for sources of efficient cosmic-ray acceleration. In this work, we present new SNR candidates that have been identified in the entire H.E.S.S. phase I data set of the Galactic plane recorded over the past ten years. Identification with a known SNR shell candidate was successful for one new source, HESS J1534-571. In other cases, TeV-only shell candidates are challenging to firmly identify as SNRs due to their lack of detected non-thermal emission in lower energy bands. We will discuss how these objects may present an important link between young and evolved SNRs, since their shell emission may be dominated by hadronic processes.

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arXiv2015

Simulating Cherenkov Telescope Array observation of RX J1713.7-3946

We perform simulations of Cherenkov Telescope Array (CTA) observations of a young supernova remnant RX J1713.7-3946. This target is not only one of the brightest sources ever discovered in very high-energy gamma rays but also well observed in other wavebands. In X-rays, the emission is dominated by synchrotron radiation, which links directly to the existence of high-energy electrons. Radio observations of CO and HI gas have revealed a highly inhomogeneous medium surrounding the SNR, such as clumpy molecular clouds. Therefore gamma rays from hadronic interactions are naturally expected. However, the spectrum in GeV energy range measured by Fermi/LAT indicates more typical of leptonic emission from accelerated electrons. Despite lots of multi-wavelength information, the competing interpretations have led to much uncertainty in the quest of unraveling the true origin of the gamma-ray emission from RX~J1713.7--3946. CTA will achieve highest performance ever in sensitivity, angular resolution, and energy resolution. We estimate CTA capability to examine the emission mechanisms of the gamma rays through simulated spatial distribution, spectra, and their time variation.

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arXiv2014

ASTRO-H White Paper - Older Supernova Remnants and Pulsar Wind Nebulae

Most supernova remnants (SNRs) are old, in the sense that their structure has been profoundly modified by their interaction with the surrounding interstellar medium (ISM). Old SNRs are very heterogenous in terms of their appearance, reflecting differences in their evolutionary state, the environments in which SNe explode and in the explosion products. Some old SNRs are seen primarily as a result of a strong shock wave interacting with the ISM. Others, the so-called mixed-morphology SNRs, show central concentrations of emission, which may still show evidence of emission from the ejecta. Yet others, the pulsar wind nebulae (PWNe), are seen primarily as a result of emission powered by a pulsar; these SNRs often lack the detectable thermal emission from the primary shock. The underlying goal in all studies of old SNRs is to understand these differences, in terms of the SNe that created them, the nature of the ISM into which they are expanding, and the fundamental physical processes that govern their evolution. Here we identify three areas of study where ASTRO-H can make important contributions. These are constraining abundances and physical processes in mature limb-brightened SNRs, understanding the puzzling nature of mixed-morphology SNRs, and exploring the nature of PWNe. The Soft X-ray Spectrometer (SXS) on-board ASTRO-H will, as a result of its high spectral resolution, be the primary tool for addressing problems associated with old SNRs, supported by hard X-ray observations with the Hard X-ray Imager (HXI) to obtain broad band X-ray coverage.

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arXiv2014

ASTRO-H White Paper - Young Supernova Remnants

Thanks to the unprecedented spectral resolution and sensitivity of the Soft X-ray Spectrometer (SXS) to soft thermal X-ray emission, ASTRO-H will open a new discovery window for understanding young, ejecta-dominated, supernova remnants (SNRs). In particular we study how ASTRO-H observations will address, comprehensively, three key topics in SNR research: (1) using abundance measurements to unveil SNR progenitors, (2) using spatial and velocity distribution of the ejecta to understand supernova explosion mechanisms, (3) revealing the link between the thermal plasma state of SNRs and the efficiency of their particle acceleration.

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arXiv2014

Suzaku Observation of HESS J1507-622

HESS J1507-622 is one of the bright unidentified TeV objects. HESS J1507-622 is unique, since the location of the object is off the Galactic disk. We observed the HESS J1507-622 region with the Suzaku XIS, and found no obvious counterpart although there is no severe interstellar extinction. However, there are two interesting X-ray objects; SRC1 is a bright extended source, and SRC2 is a faint diffuse object. If either of them is a counterpart, the flux ratio between TeV and X-ray is large, and HESS J1507-622 is a real dark particle accelerator.

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arXiv2014

Extreme Blazars Studied with Fermi-LAT and Suzaku: 1ES 0347-121 and Blazar Candidate HESS J1943+213

We report on our study of high-energy properties of two peculiar TeV emitters: the "extreme blazar" 1ES 0347-121 and the "extreme blazar candidate" HESS J1943+213 located near the Galactic Plane. Both objects are characterized by quiescent synchrotron emission with flat spectra extending up to the hard X-ray range, and both were reported to be missing GeV counterparts in the Fermi-LAT 2-year Source Catalog. We analyze a 4.5 year accumulation of the Fermi-LAT data, resulting in the detection of 1ES 0347-121 in the GeV band, as well as in improved upper limits for HESS J1943+213. We also present the analysis results of newly acquired Suzaku data for HESS J1943+213. The X-ray spectrum is well represented by a single power law extending up to 25 keV with photon index 2.00+/-0.02 and a moderate absorption in excess of the Galactic value, in agreement with previous X-ray observations. No short-term X-ray variability was found over the 80 ks duration of the Suzaku exposure. Under the blazar hypothesis, we modeled the spectral energy distributions of 1ES 0347-121 and HESS J1943+213, and derived constraints on the intergalactic magnetic field strength and source energetics. We conclude that although the classification of HESS J1943+213 has not yet been determined, the blazar hypothesis remains the most plausible option, since in particular the broad-band spectra of the two analyzed sources along with the source model parameters closely resemble each other, and the newly available WISE and UKIDSS data for HESS J1943+213 are consistent with the presence of an elliptical host at the distance of approximately ~600 Mpc.

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arXiv2013

The Cherenkov Telescope Array Large Size Telescope

The two arrays of the Very High Energy gamma-ray observatory Cherenkov Telescope Array (CTA) will include four Large Size Telescopes (LSTs) each with a 23 m diameter dish and 28 m focal distance. These telescopes will enable CTA to achieve a low-energy threshold of 20 GeV, which is critical for important studies in astrophysics, astroparticle physics and cosmology. This work presents the key specifications and performance of the current LST design in the light of the CTA scientific objectives.

astro-ph.IM↗
arXiv2013

Development of the Photomultiplier-Tube Readout System for the CTA Large Size Telescope

We have developed a prototype of the photomultiplier tube (PMT) readout system for the Cherenkov Telescope Array (CTA) Large Size Telescope (LST). Two thousand PMTs along with their readout systems are arranged on the focal plane of each telescope, with one readout system per 7-PMT cluster. The Cherenkov light pulses generated by the air showers are detected by the PMTs and amplified in a compact, low noise and wide dynamic range gain block. The output of this block is then digitized at a sampling rate of the order of GHz using the Domino Ring Sampler DRS4, an analog memory ASIC developed at Paul Scherrer Institute. The sampler has 1,024 capacitors per channel and four channels are cascaded for increased depth. After a trigger is generated in the system, the charges stored in the capacitors are digitized by an external slow sampling ADC and then transmitted via Gigabit Ethernet. An onboard FPGA controls the DRS4, trigger threshold, and Ethernet transfer. In addition, the control and monitoring of the Cockcroft-Walton circuit that provides high voltage for the 7-PMT cluster are performed by the same FPGA. A prototype named Dragon has been developed that has successfully sampled PMT signals at a rate of 2 GHz, and generated single photoelectron spectra.

astro-ph.IM↗
arXiv2013

Proper Motions of H-alpha filaments in the Supernova Remnant RCW 86

We present a proper motion study of the eastern shock-region of the supernova remnant RCW 86 (MSH 14-63, G315.4-2.3), based on optical observations carried out with VLT/FORS2 in 2007 and 2010. For both the northeastern and southeastern regions, we measure an average proper motion of H-alpha filaments of 0.10 +/- 0.02 arcsec/yr, corresponding to 1200 +/- 200 km/s at 2.5kpc. There is substantial variation in the derived proper motions, indicating shock velocities ranging from just below 700 km/s to above 2200 km/s. The optical proper motion is lower than the previously measured X-ray proper motion of northeastern region. The new measurements are consistent with the previously measured proton temperature of 2.3 +/- 0.3 keV, assuming no cosmic-ray acceleration. However, within the uncertainties, moderately efficient (< 27 per cent) shock acceleration is still possible. The combination of optical proper motion and proton temperature rule out the possibility that RCW 86 has a distance less than 1.5kpc. The similarity of the proper motions in the northeast and southeast is peculiar, given the different densities and X-ray emission properties of the regions. The northeastern region has lower densities and the X-ray emission is synchrotron dominated, suggesting that the shock velocities should be higher than in the southeastern, thermal X-ray dominated, region. A possible solution is that the H-alpha emitting filaments are biased toward denser regions, with lower shock velocities. Alternatively, in the northeast the shock velocity may have decreased rapidly during the past 200yr, and the X-ray synchrotron emission is an afterglow from a period when the shock velocity was higher.

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arXiv2013

The Hot and Energetic Universe: The astrophysics of supernova remnants and the interstellar medium

The study of both supernova remnants and the hot and cold phases of the interstellar medium are essential for understanding the final stages of stellar evolution and their feedback on the evolution of galaxies through injection of energy and heavy elements. These studies are also crucial for understanding the physics of supernovae, their cosmological implication, and the origin of galactic cosmic rays. The unique capabilities of Athena+ will allow us to explore a new parameter space. Spatially-resolved high-resolution spectroscopy using Athena+ X-IFU of young remnants will allow to characterize individual parcels of ejected material in the line of sight in terms of kinematics, ionization and composition, providing access to the three dimensional geometry of the explosion. Athena+ will also allow studying shock physics and particle acceleration in supernova remnants, as well as their interaction with their environment. Athena+ X-IFU will also characterize the ionization mechanisms competing in forming the complex structures of the hot interstellar medium, likely to keep the echo of past star formation activity, both in our Galaxy and nearby ones. For the first time the dust and gas of the densest cold medium, like in the Galactic Centre environment, will be studied. Athena+ X-IFU will observe, along with the Mg K and Si K edges, which are the main tracers of the silicates content of the ISM, the Fe K edge with unprecedented sensitivity and energy-resolution. This will allow us to study for the first time the nature of Fe-bearing dust in such regions.

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arXiv2012

Prospect of Studying Hard X- and Gamma-Rays from Type Ia Supernovae

We perform multi-dimensional, time-dependent radiation transfer simulations for hard X-ray and gamma-ray emissions, following radioactive decays of 56Ni and 56Co, for two-dimensional delayed detonation models of Type Ia supernovae (SNe Ia). The synthetic spectra and light curves are compared with the sensitivities of current and future observatories for an exposure time of 10^6 seconds. The non-detection of the gamma-ray signal from SN 2011fe at 6.4 Mpc by SPI on board INTEGRAL places an upper limit for the mass of 56Ni of \lesssim 1.0 Msun, independently from observations in any other wavelengths. Signals from the newly formed radioactive species have not been convincingly measured yet from any SN Ia, but the future X-ray and gamma-ray missions are expected to deepen the observable horizon to provide the high energy emission data for a significant SN Ia sample. We predict that the hard X-ray detectors on board NuStar (launched in 2012) or ASTRO-H (scheduled for launch in 2014) will reach to SNe Ia at \sim15 Mpc, i.e., one SN every few years. Furthermore, according to the present results, the soft gamma-ray detector on board ASTRO-H will be able to detect the 158 keV line emission up to \sim25 Mpc, i.e., a few SNe Ia per year. Proposed next generation gamma-ray missions, e.g., GRIPS, could reach to SNe Ia at \sim20 - 35 Mpc by MeV observations. Those would provide new diagnostics and strong constraints on explosion models, detecting rather directly the main energy source of supernova light.

astro-ph.HE↗
arXiv2012

Gamma-ray signatures of cosmic ray acceleration, propagation, and confinement in the era of CTA

Galactic cosmic rays are commonly believed to be accelerated at supernova remnants via diffusive shock acceleration. Despite the popularity of this idea, a conclusive proof for its validity is still missing. Gamma-ray astronomy provides us with a powerful tool to tackle this problem, because gamma rays are produced during cosmic ray interactions with the ambient gas. The detection of gamma rays from several supernova remnants is encouraging, but still does not constitute a proof of the scenario, the main problem being the difficulty in disentangling the hadronic and leptonic contributions to the emission. Once released by their sources, cosmic rays diffuse in the interstellar medium, and finally escape from the Galaxy. The diffuse gamma-ray emission from the Galactic disk, as well as the gamma-ray emission detected from a few galaxies is largely due to the interactions of cosmic rays in the interstellar medium. On much larger scales, cosmic rays are also expected to permeate the intracluster medium, since they can be confined and accumulated within clusters of galaxies for cosmological times. Thus, the detection of gamma rays from clusters of galaxies, or even upper limits on their emission, will allow us to constrain the cosmic ray output of the sources they contain, such as normal galaxies, AGNs, and cosmological shocks. In this paper, we describe the impact that the Cherenkov Telescope Array, a future ground-based facility for very-high energy gamma-ray astronomy, is expected to have in this field of research.

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arXiv2012

Constraints on cosmic-ray efficiency in the supernova remnant RCW 86 using multi-wavelength observations

Several young supernova remnants (SNRs) have recently been detected in the high-energy and very-high-energy gamma-ray domains. As exemplified by RX J1713.7-3946, the nature of this emission has been hotly debated, and direct evidence for the efficient acceleration of cosmic-ray protons at the SNR shocks still remains elusive. We analyzed more than 40 months of data acquired by the Large Area Telescope (LAT) on-board the Fermi Gamma-Ray Space Telescope in the HE domain, and gathered all of the relevant multi-wavelength (from radio to VHE gamma-rays) information about the broadband nonthermal emission from RCW 86. For this purpose, we re-analyzed the archival X-ray data from the ASCA/Gas Imaging Spectrometer (GIS), the XMM-Newton/EPIC-MOS, and the RXTE/Proportional Counter Array (PCA). Beyond the expected Galactic diffuse background, no significant gamma-ray emission in the direction of RCW 86 is detected in any of the 0.1-1, 1-10 and 10-100 GeV Fermi-LAT maps. In the hadronic scenario, the derived HE upper limits together with the HESS measurements in the VHE domain can only be accommodated by a spectral index Gamma <= 1.8, i.e. a value in-between the standard (test-particle) index and the asymptotic limit of theoretical particle spectra in the case of strongly modified shocks. The interpretation of the gamma-ray emission by inverse Compton scattering of high energy electrons reproduces the multi-wavelength data using a reasonable value for the average magnetic field of 15-25 muG. For these two scenarios, we assessed the level of acceleration efficiency. We discuss these results in the light of existing estimates of the magnetic field strength, the effective density and the acceleration efficiency in RCW 86.

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arXiv2011

Diffraction-limited Subaru imaging of M82: sharp mid-infrared view of the starburst core

We present new imaging at 12.81 and 11.7 microns of the central ~40"x30" (~0.7x0.5 kpc) of the starburst galaxy M82. The observations were carried out with the COMICS mid-infrared (mid-IR) imager on the 8.2m Subaru telescope, and are diffraction-limited at an angular resolution of <0".4. The images show extensive diffuse structures, including a 7"-long linear chimney-like feature and another resembling the edges of a ruptured bubble. This is the clearest view to date of the base of the kpc-scale dusty wind known in this galaxy. These structures do not extrapolate to a single central point, implying multiple ejection sites for the dust. In general, the distribution of dust probed in the mid-IR anticorrelates with the locations of massive star clusters that appear in the near-infrared. The 10-21 micron mid-IR emission, spatially-integrated over the field of view, may be represented by hot dust with temperature of ~160 K. Most discrete sources are found to have extended morphologies. Several radio HII regions are identified for the first time in the mid-IR. The only potential radio supernova remnant to have a mid-IR counterpart is a source which has previously also been suggested to be a weak active galactic nucleus. This source has an X-ray counterpart in Chandra data which appears prominently above 3 keV and is best described as a hot (~2.6 keV) absorbed thermal plasma with a 6.7 keV Fe K emission line, in addition to a weaker and cooler thermal component. The mid-IR detection is consistent with the presence of strong [NeII]12.81um line emission. The broad-band source properties are complex, but the X-ray spectra do not support the active galactic nucleus hypothesis. We discuss possible interpretations regarding the nature of this source.

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arXiv2010

X-ray observation of AM Herculis in a very low state with Suzaku

The X-ray observation of AM Her in a very low state was performed with {\it Suzaku} in October 2008. One flare event with a time scale of $\sim$ 3700 sec was detected at the X-ray luminosity of $6.0 \times 10^{29} {\rm ~erg ~sec}^{-1}$ in the 0.5 -- 10 keV band assuming at a distance of 91 pc. The X-ray spectrum is represented by a thermal plasma emission model with a temperature of $8.67_{-1.14}^{+1.31}$ keV. During the quiescence out of the flare interval, {\it Suzaku} also detected significant X-rays at a luminosity of $1.7 \times 10^{29} {\rm ~erg ~sec}^{-1}$ in the 0.5 -- 10 keV band, showing a clear spin modulation at a period of 0.1289273(2) days at BJD 2454771.581. The X-ray spectra in the quiescence were represented by a MEKAL + Power Law (PL) model or a single CEMEKL model, which are also supported by phase-resolved analyses. A correlation between the temperature and the volume emission measure was found together with historical X-ray measurements of AM Her in various states. In order to account for a possible non-thermal emission from AM Her, particle acceleration mechanisms in the AM Her system are also discussed, including a new proposal of a shock acceleration process on the top of the accretion column.

astro-ph.HE↗
arXiv2010

Searching for Diffuse Nonthermal X-Rays from the Superbubbles N11 and N51D in the Large Magellanic Cloud

We report on observations of the superbubbles (SBs) N11 and N51D in the Large Magellanic Cloud (LMC) with Suzaku and XMM-Newton. The interior of both SBs exhibits diffuse X-ray emission, which is well represented by thin thermal plasma models with a temperature of 0.2-0.3keV. The presence of nonthermal emission, claimed in previous works, is much less evident in our careful investigation. The 3-sigma upper limits of 2-10keV flux are 3.6*10^{-14}ergs/cm^2/s and 4.7*10^{-14}ergs/cm^2/s for N11 and N51D, respectively. The previous claims of the detection of nonthermal emission are probably due to the inaccurate estimation of the non X-ray background. We conclude that no credible nonthermal emission has been detected from the SBs in the LMC, with the exception of 30 Dor C.

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