We have performed a systematic study of gamma-ray bursts (GRBs), which have various values in the peak energy of the F spectrum of prompt emission, Epeak, observed by the Swift/Burst Alert Telescope ...(BAT) and Fermi/Gamma-ray Burst Monitor, investigating their prompt and X-ray afterglow emissions. We cataloged long-lasting GRBs observed by Swift between 2004 December and 2014 February in three categories according to the classification by Sakamoto et al.: X-ray flashes (XRFs), X-ray-rich GRBs (XRRs), and classical GRBs (C-GRBs). We then derived , as well as if viable, of the Swift spectra of their prompt emission. We also analyzed their X-ray afterglows and found that GRB events with a lower , i.e., softer GRBs, are fainter in 0.3-10 keV X-ray luminosity and decay more slowly than harder GRBs. The intrinsic event rates of the XRFs, XRRs, and C-GRBs were calculated using the Swift/BAT trigger algorithm. Those of the XRRs and XRFs are larger than that of the C-GRBs. If we assume that the observational diversity of Epeak is explained using the off-axis model, these results yield a jet half-opening angle of Δθ ∼ 0 3, and a variance of the observing angles θobs 0 6. This implies that this tiny variance would be responsible for the Epeak diversity observed by Swift/BAT, which is unrealistic. Therefore, we conclude that the Epeak diversity is not explained with the off-axis model, but is likely to originate from some intrinsic properties of the jets.
Abstract
In order to investigate the far-infrared excess detected from the western hot spot of the radio galaxy Pictor A with the Herschel observatory, submillimeter photometry is performed with the ...Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter Array at Band 8 with the reference frequency of 405 GHz. A submillimeter source is discovered at the radio peak of the hot spot. Because the 405 GHz flux density of the source, 80.7 ± 3.1 mJy, agrees with the extrapolation of the synchrotron radio spectrum, the far-infrared excess is suggested to exhibit no major contribution at the ACA band. In contrast, by subtracting the power-law spectrum tightly constrained by the radio and ACA data, the significance of the excess in the Herschel band is well confirmed. No diffuse submillimeter emission is detected within the ACA field of view, and thus, the excess is ascribed to the western hot spot itself. In comparison to the previous estimate based on the Herschel data, the relative contribution of the far-infrared excess is reduced by a factor of ∼1.5. The spectrum of the excess below the far-infrared band is determined to be harder than that of the diffusive shock acceleration. This strengthens the previous interpretation that the excess originates via the magnetic turbulence in the substructures within the hot spot. The ACA data are utilized to evaluate the magnetic field strength of the excess and of diffuse radio structure associated with the hot spot.
ABSTRACT
The far-infrared counterpart of hotspot D, the terminal hotspot of the eastern jet hosted by the radio galaxy Cygnus A, is detected with Herschel Aperture photometry of the source performed ...in 5 photometric bands covering the wavelength range of 70–350 $\mu$m. After removing the contamination from another nearby hotspot, E, the far-infrared intensity of hotspot D is derived as 83 ± 13 and 269 ± 66 mJy at 160 and 350 $\mu$m, respectively. Since the far-infrared spectrum of the object smoothly connects to the radio one, the far-infrared emission is attributed to the synchrotron radiation from the radio-emitting electron population. The radio-to-near-infrared spectrum is confirmed to exhibit a far-infrared break feature at the frequency of $\nu _\mathrm{br}=2.0^{+1.2}_{-0.8} \times 10^{12}$ Hz. The change in energy index at the break (Δα = 0.5) is interpreted as the impact of radiative cooling on an electron distribution sustained by continuous injection from diffusive shock acceleration. By ascribing the derived break to this cooling break, the magnetic field, B, in the hotspot is determined as a function of its radius, R within a uniform one-zone model combined with the strong relativistic shock condition. An independent B–R constraint is obtained by assuming the X-ray spectrum is wholly due to synchrotron self-Compton emission. By combining these conditions, the two parameters are tightly determined as B = 120–150 μG and R = 1.3–1.6 kpc. A further investigation into the two conditions indicates the observed X-ray flux is highly dominated by the synchrotron self-Compton emission.
Abstract
Utilizing Chandra, XMM-Newton, and NuSTAR, a wide-band X-ray spectrum from 0.2 to 20 keV is reported from the western hotspot of Pictor A. In particular, the X-ray emission is significantly ...detected in the 3 to 20 keV band at 30σ by NuSTAR. This is the first detection of hard X-rays with energies above 10 keV from a jet termination hotspot of active galactic nuclei. The hard X-ray spectrum is well described with a power-law model with a photon index of Γ = 1.8 ± 0.2, and the flux is obtained to be (4.5 ± 0.4) × 10−13 erg s−1 cm−2 in the 3 to 20 keV band. The obtained spectrum is smoothly connected with those soft X-ray spectra observed by Chandra and XMM-Newton. The wide-band spectrum shows a single power-law spectrum with a photon index of Γ = 2.07 ± 0.03, excluding any cut-off/break features. Assuming the X-rays to be synchrotron radiation of the electrons, the energy index of the electrons is estimated as p = 2Γ − 1 = 3.14 ± 0.06 from the wide-band spectrum. Given that the X-ray synchrotron-emitting electrons quickly lose their initial energies via synchrotron radiation, the energy index of electrons at acceleration sites is estimated as pacc = p − 1 = 2.14 ± 0.06. This is consistent with the prediction of the diffusive shock acceleration. Since the spectrum has no cut-off feature up to 20 keV, the maximum electron energy is estimated to be no less than 40 TeV.
We present new measurements of the vertical density profile of the Earth's atmosphere at altitudes between 70 and 200 km, based on Earth occultations of the Crab Nebula observed with the X‐ray ...Imaging Spectrometer onboard Suzaku and the hard X‐ray Imager onboard Hitomi. X‐ray spectral variation due to the atmospheric absorption is used to derive tangential column densities of the absorbing species, that is, N and O including atoms and molecules, along the line of sight. The tangential column densities are then inverted to obtain the atmospheric number density. The data from 219 occultation scans at low latitudes in both hemispheres from September 15, 2005 to March 26, 2016 are analyzed to generate a single, highly averaged (in both space and time) vertical density profile. The density profile is in good agreement with the Naval‐Research‐Laboratory's‐Mass‐Spectrometer‐Incoherent‐Scatter‐Radar‐Extended (NRLMSISE‐00) model, except for the altitude range of 70–110 km, where the measured density is ∼50% smaller than the model. Such a deviation is consistent with the recent measurement with the SABER aboard the TIMED satellite (Cheng et al., 2020, https://doi.org/10.3390/atmos11040341). Given that the NRLMSISE‐00 model was constructed some time ago, the density decline could be due to the radiative cooling/contracting of the upper atmosphere as a result of greenhouse warming in the troposphere. However, we cannot rule out a possibility that the NRL model is simply imperfect in this region. We also present future prospects for the upcoming Japan‐US X‐ray astronomy satellite, X‐Ray Imaging and Spectroscopy Mission (XRISM), which will allow us to measure atmospheric composition with unprecedented spectral resolution of ΔE ∼ 5 eV in 0.3–12 keV.
Key Points
Combined O and N densities at altitudes 70–200 km are measured with X‐ray astronomy satellites Suzaku and Hitomi
The vertical density profile agrees with a prediction of the NRL model, except for altitudes 70–110 km showing significant density deficit
The density deficit could be due to either long‐term radiative cooling of the upper atmosphere or imperfect modeling
We present long‐term density trends of the Earth's upper atmosphere at altitudes between 71 and 116 km, based on atmospheric occultations of the Crab Nebula observed with X‐ray astronomy satellites, ...ASCA, RXTE, Suzaku, NuSTAR, and Hitomi. The combination of the five satellites provides a time period of 28 years from 1994 to 2022. To suppress seasonal and latitudinal variations, we concentrate on the data taken in autumn (49 < doy < 111) and spring (235 < doy < 297) in the northern hemisphere with latitudes of 0°–40°. With this constraint, local times are automatically limited either around noon or midnight. We obtain four sets (two seasons × two local times) of density trends at each altitude layer. We take into account variations due to a linear trend and the 11‐year solar cycle using linear regression techniques. Because we do not see significant differences among the four trends, we combine them to provide a single vertical profile of trend slopes. We find a negative density trend of roughly −5%/decade at every altitude. This is in reasonable agreement with inferences from settling rate of the upper atmosphere. In the 100–110‐km altitude, we found an exceptionally high density decline of about −12%/decade. This peak may be the first observational evidence for strong cooling due to water vapor and ozone near 110 km, which was first identified in a numerical simulation by Akmaev et al. (2006, https://doi.org/10.1016/j.jastp.2006.03.008). Further observations and numerical simulations with suitable input parameters are needed to establish this feature.
Plain Language Summary
Numerical simulations have shown that, while an increase of greenhouse gases such as CO2 in the atmosphere causes heating of the troposphere (near surface), it causes cooling of the middle and upper atmosphere, which is the so‐called “greenhouse cooling.” The greenhouse cooling should result in atmospheric contraction and consequently a temporal density decrease at a fixed height. However, observational evidence for the density decrease has been scarce in the mesosphere and lower thermosphere (MLT: 80–110 km), owing to difficulty in measuring the density in this region. Here, we present the first direct measurements of long‐term variations for combined N and O atom number density in the MLT, based on atmospheric occultations of the Crab Nebula observed with X‐ray astronomy satellites. The combination of five X‐ray astronomy satellites, ASCA, RXTE, Suzaku, NuSTAR, and Hitomi, allows us to explore density trends for a long period from 1994 to 2022. We take into account variations due to a temporal linear trend and the 11‐year solar cycle, using linear regression techniques. As a result, we find a negative density trend of roughly −5%/decade at every altitude, with a local minimum of −12%/decade near 105 km. This is in reasonable agreement with the state‐of‐the‐art numerical simulations.
Key Points
Time series of combined O and N densities are measured in the MLT, based on atmospheric occultations of the Crab Nebula using X‐ray astronomy satellites
The density is decreasing everywhere, with a local minimum of −12%/decade near 105 km
The local minimum in density trends may be due to strong cooling by water vapor and ozone, as was first predicted by Akmaev et al. (2006, https://doi.org/10.1016/j.jastp.2006.03.008)
A far-infrared counterpart to the west hot spot of the radio galaxy Pictor A is discovered with the Spectral and Photometric Imaging REceiver (SPIRE) on board Herschel. The color-corrected flux ...density of the source is measured as 70.0 9.9 mJy at the wavelength of 350 m. A close investigation into its radio-to-optical spectrum indicates that the mid-infrared excess over the radio synchrotron component, detected with Wide-field Infrared Survey Explorer and Spitzer, significantly contributes to the far-infrared band. Thanks to the SPIRE data, it is revealed that the spectrum of the excess is described by a broken power-law model subjected to a high-energy cutoff. By applying the radiative cooling break under continuous energy injection (Δ = 0.5), the broken power-law model supports an idea that the excess originates in 10 pc scale substructures within the hot spot. From the break frequency, Hz, the magnetic field was estimated as B 1-4 mG. This is higher than the minimum-energy magnetic field of the substructures by a factor of 3-10. Even if the origin of the excess is larger than ∼100 pc, the magnetic field stronger than the minimum-energy field is confirmed. It is proposed that regions with a magnetic field locally boosted via plasma turbulence are observed as the substructures. The derived energy index below the break, ∼ 0.22 (conservatively <0.42), is difficult to be attributed to the strong-shock acceleration ( = 0.5). Stochastic acceleration and magnetic reconnection are considered as a plausible alternative mechanism.
The metal abundance of the hot plasma that permeates galaxy clusters represents the accumulation of heavy elements produced by billions of supernovae. Therefore, X-ray spectroscopy of the ...intracluster medium provides an opportunity to investigate the nature of supernova explosions integrated over cosmic time. In particular, the abundance of the iron-peak elements (chromium, manganese, iron and nickel) is key to understanding how the progenitors of typical type Ia supernovae evolve and explode. Recent X-ray studies of the intracluster medium found that the abundance ratios of these elements differ substantially from those seen in the Sun, suggesting differences between the nature of type Ia supernovae in the clusters and in the Milky Way. However, because the K-shell transition lines of chromium and manganese are weak and those of iron and nickel are very close in photon energy, high-resolution spectroscopy is required for an accurate determination of the abundances of these elements. Here we report observations of the Perseus cluster, with statistically significant detections of the resonance emission from chromium, manganese and nickel. Our measurements, combined with the latest atomic models, reveal that these elements have near-solar abundance ratios with respect to iron, in contrast to previous claims. Comparison between our results and modern nucleosynthesis calculations disfavours the hypothesis that type Ia supernova progenitors are exclusively white dwarfs with masses well below the Chandrasekhar limit (about 1.4 times the mass of the Sun). The observed abundance pattern of the iron-peak elements can be explained by taking into account a combination of near- and sub-Chandrasekhar-mass type Ia supernova systems, adding to the mounting evidence that both progenitor types make a substantial contribution to cosmic chemical enrichment.
We present the first radio polarimetric observations of a fast-rising blue optical transient, AT2018cow. Two epochs of polarimetry with additional coincident photometry were performed with the ...Atacama Large Millimeter/submillimeter Array. The overall photometric results based on simultaneous observations in the 100 and 230 GHz bands are consistent with the nonthermal radiation model reported by Ho et al. and indicate that the spectral peaks (∼110 GHz at the first epoch and ∼67 GHz at the second epoch) represent the synchrotron self-absorption frequency. The non-detection of linear polarization with <0.15% in the 230 GHz band at the phase when the effect of synchrotron self-absorption was quite small in the band may be explained by internal Faraday depolarization with high circumburst density and strong magnetic field. This result supports the stellar explosion scenario rather than the tidal disruption model. The maximum energy of accelerating particles at the shocks of AT2018cow-like objects is also discussed.
Abstract
Extending the earlier measurements reported in Hitomi collaboration (2016, Nature, 535, 117), we examine the atmospheric gas motions within the central 100 kpc of the Perseus cluster using ...observations obtained with the Hitomi satellite. After correcting for the point spread function of the telescope and using optically thin emission lines, we find that the line-of-sight velocity dispersion of the hot gas is remarkably low and mostly uniform. The velocity dispersion reaches a maxima of approximately 200 km s−1 toward the central active galactic nucleus (AGN) and toward the AGN inflated northwestern “ghost” bubble. Elsewhere within the observed region, the velocity dispersion appears constant around 100 km s−1. We also detect a velocity gradient with a 100 km s−1 amplitude across the cluster core, consistent with large-scale sloshing of the core gas. If the observed gas motions are isotropic, the kinetic pressure support is less than 10% of the thermal pressure support in the cluster core. The well-resolved, optically thin emission lines have Gaussian shapes, indicating that the turbulent driving scale is likely below 100 kpc, which is consistent with the size of the AGN jet inflated bubbles. We also report the first measurement of the ion temperature in the intracluster medium, which we find to be consistent with the electron temperature. In addition, we present a new measurement of the redshift of the brightest cluster galaxy NGC 1275.
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