Context. Globular cluster are believed to boost the rate of compact binary mergers that may launch a certain type of cosmological gamma-ray burst (GRB). Therefore globular clusters appear to be ...potential sites to search for remnants of such GRBs. Aims. The very-high-energy (VHE) gamma-ray source HESS J1747-248 recently discovered in the direction of the Galactic globular cluster Terzan 5 is investigated for being a GRB remnant. Methods. Signatures created by the ultrarelativistic outflow, the subrelativistic ejecta and the ionizing radiation of a short GRB are estimated for an expected age of such a remnant of t ≳ 104 years. Results. The kinetic energy of a short GRB could roughly be adequate for powering the VHE source in a hadronic scenario. The age of the proposed remnant estimated from its extension possibly agrees with the occurrence of such events in the Galaxy. Subrelativistic merger ejecta could shock-heat the ambient medium. Conclusions. Further VHE observations can probe for the presence of a break towards lower energies expected for particle acceleration in ultrarelativistic shocks. Deep X-ray observations would have the potential to examine whether there is any thermal plasma heated by the subrelativistic ejecta. The identification of a GRB remnant in our own Galaxy may also help to explore the effect of such a highly energetic event on the Earth.
Context. Recently, diffuse and extended sources in TeV γ-rays as well as in X-rays have been detected in the direction of the Galactic globular cluster (GC) Terzan 5. Remarkably, this is among the ...brightest GCs detected in the GeV regime. The nature of either the TeV or the diffuse X-ray signal from Terzan 5 is not yet settled. These emissions most likely indicate the presence of several non-thermal radiation processes in addition to those that give rise to the GeV signal. Aims. The aim of this work is to search for diffuse X-ray emission from all GeV-detected GCs where appropriate X-ray observations are available, and to compare the obtained results with the signal detected from Terzan 5. This study will help to determine whether Terzan 5 stands out among other GC or whether a whole population of globular clusters feature similar properties. Methods. After assessing all archival X-ray observations of GeV detected GCs, we analyzed the data of six Chandra observations pointed toward M 62, NGC 6388, NGC 6541, M 28, M 80, and NGC 6139. For each GC we characterized the diffuse X-ray emission using the same analysis techniques as for Terzan 5. To study the emission on the same relative scales we used the half-mass radius as a scale parameter to determine the extent of the potential emission region. Results. None of the six GCs show significant diffuse X-ray emission above the particle and diffuse Galactic X-ray background components. The derived upper limits allow to assess the validity of different models that were discussed in the interpretation of the multi-wavelength data of Terzan 5. A scenario based on synchrotron emission from relativistic leptons provided by the millisecond pulsar population cannot be securely rejected if a comparable magnetic field strength as in Terzan 5 is assumed for every GC. However, such a scenario seems to be unlikely for NGC 6388, and M 62. An inverse-Compton scenario relying on the presence of a putative GRB remnant with the same properties as proposed for Terzan 5 can be ruled out for all six GCs. Finally, the assumption that each GC hosts a source with the same luminosity as in Terzan 5 is ruled out for all GCs but NGC 6139.
Context. We report on new VLA radio observations of the distant cluster RX J1347.5-1145, which is the most luminous in X-rays. Aims. We aim at investigating the possible presence of diffuse and ...extended radio emission in this very peculiar system which shows both a massive cooling flow and merging signatures. Methods. New low resolution (~18 arcsec) VLA radio observations of this cluster are combined with higher resolution (~2 arcsec) data available in the VLA archive. Results. We discover the presence of a diffuse and extended (~500 kpc) radio source centered on the cluster, unrelated to the radio emission of the central AGN. The properties of the radio source, in particular a) its occurrence at the center of a massive cooling flow cluster, b) its total size comparable to that of the cooling region, c) its agreement with the observational trend between radio luminosity and cooling flow power, indicate that RX J1347.5-1145 hosts a radio mini-halo. We suggest that the radio emission of this mini-halo, which is the most distant object of its class discovered up to now, is due to electron re-acceleration triggered by the central cooling flow. However, we also note that the morphology of the diffuse radio emission shows an elongation coincident with the position of a hot subclump detected in X-rays, thus suggesting that additional energy for the electron re-acceleration might be provided by the submerger event.
We investigate the impact of galactic mass loss triggered by ram-pressure stripping of cluster galaxies on the evolution of the intra-cluster medium (ICM). We use combined N-body and hydrodynamic ...simulations together with a phenomenological galaxy formation model and a prescription of the effect of ram-pressure stripping on the galaxies. We analyze the effect of galaxy – ICM interaction for different model clusters with different masses and different merger histories. Our simulations show that ram-pressure stripping can account for ~10% of the overall observed level of enrichment in the ICM within a radius of 1.3 Mpc. The efficiency of metal ejection of cluster galaxies depends in the first few Gyr of the simulation mainly on the cluster mass and is significantly increased during major merger events. Additionally we show that ram-pressure stripping is most efficient in the center of the galaxy cluster and the level of enrichment drops quite fast at larger radii. We present emission-weighted metallicity maps of the ICM which can be compared with X-ray observations. The resulting distribution of metals in the ICM shows a complex pattern with stripes and plumes of metal-rich material. The metallicity maps can be used to trace the present and past interactions between the ICM and cluster galaxies.
Aims. Terzan 5 , a globular cluster (GC) prominent in mass and population of compact objects, is searched for diffuse X-ray emission, as proposed by several models. Methods. We analyzed the data of ...an archival Chandra observation of Terzan 5 to search for extended diffuse X-ray emission outside the half-mass radius of the GC. We removed detected point sources from the data to extract spectra from diffuse regions around Terzan 5. The Galactic background emission was modeled by a 2-temperature thermal component, which is typical for Galactic diffuse emission. Results. We detected significant diffuse excess emission above the particle background level from the whole field-of-view. The surface brightness appears to be peaked at the GC center and decreases smoothly outwards. After the subtraction of particle and Galactic background, the excess spectrum of the diffuse emission between the half-mass radius and 3' can be described by a power-law model with photon index Γ = 0.9 ± 0.5 and a surface flux of Fx = (1.17±0.16) × 10-7 erg cm-2 s-1 sr-1 in the 1–7 keV band. We estimated the contribution from unresolved point sources to the observed excess to be negligible. The observations suggest that a purely thermal origin of the emission is less likely than a non-thermal scenario. However, from simple modeling we cannot identify a clearly preferred scenario.
The unidentified very high energy (VHE, E > 100 GeV) gamma-ray source HESS J1507-622 seems to not fit into standard models for sources related to young supernova remnants, pulsar wind nebulae, or ...young stellar populations in general. This is due to its intrinsically extended, but yet compact morphology, coupled with a relative large offset ( similar to 3.5 degree ) from the Galactic plane. Therefore, it has been suggested that this object may be the first representative of a new distinct class of extended off-plane gamma-ray sources. The distance to HESS J1507-622 is the key parameter to constrain the source's most important properties, such as age and energetics of the relativistic particle population. In this paper, we report on results of follow-up observations of the potential X-ray counterpart with Suzaku. We present detailed measurements of its spectral parameters and find a high-absorbing hydrogen column density, compatible with the total amount of Galactic gas in this direction. In comparisons to measurements and models of the Galactic 3D gas distribution, we show that the potential X-ray counterpart of HESS J1507-622 may be located at the far end of the Galaxy. If the gamma-ray source is indeed physically connected to this extended X-ray source, this in turn would place the object outside of the usual distribution of Galactic VHE gamma-ray emitters.
Context. The very-high energy (VHE, E> 100 GeV) gamma-ray sky shows diverse Galactic and extragalactic source populations. For some sources the astrophysical object class could not be identified so ...far. Aims. The nature (Galactic or extragalactic) of the VHE gamma-ray source HESS J1943+213 is explored. We specifically investigate the proposed near-infrared counterpart 2MASS J19435624+2118233 of HESS J1943+213 and investigate the implications of a physical association. Methods. We present K-band imaging from the 3.5 m CAHA telescope of 2MASS J19435624+2118233. Furthermore, 5 years of Fermi-LAT data were analyzed to search for a high-energy (HE, 100 MeV <E< 100 GeV) counterpart. Results. The CAHA observations revealed that the near-infrared counterpart is extended with an intrinsic half light radius of 2′′–2.5′′. These observations also show a smooth, centrally concentrated light profile that is typical of a galaxy, and thus point toward an extragalactic scenario for the VHE gamma-ray source, assuming that the near-infrared source is the counterpart of HESS J1943+213. A high-Sérsic index profile provides a better fit than an exponential profile, indicating that the surface brightness profile of 2MASS J19435624+2118233 follows that of a typical, massive elliptical galaxy more closely than that of a disk galaxy. With Fermi-LAT a HE counterpart is found with a power-law spectrum above 1 GeV, with a normalization of (3.0 ± 0.8stat ± 0.6sys) × 10-15 cm-2 s-1 MeV-1 at the decorrelation energy Edec = 15.1 GeV and a spectral index of Γ = 1.59 ± 0.19stat ± 0.13sys. This gamma-ray spectrum shows a rather sharp break between the HE and VHE regimes of ΔΓ = 1.47 ± 0.36. Conclusions. The infrared and HE data strongly favor an extragalactic origin of HESS J1943+213, where the infrared counterpart traces the host galaxy of an extreme blazar and where the rather sharp spectral break between the HE and VHE regime indicates attenuation on extragalactic background light. The source is most likely located at a redshift between 0.03 and 0.45 according to extension and EBL attenuation arguments.
Context. Several extended sources of very-high-energy (VHE; E > 100 GeV) gamma rays have been found that lack counterparts belonging to an established class of VHE gamma-ray emitters. Aims. The ...nature of the first unidentified VHE gamma-ray source with significant angular offset from the Galactic plane of 3.5°, HESS J1507–622, is explored. Methods.Fermi-LAT data in the high-energy (HE, 100 MeV < E < 100 GeV) gamma-ray range collected over 34 month are used to describe the spectral energy distribution (SED) of the source. Additionally, implications of the off-plane location of the source for a leptonic and hadronic gamma-ray emission model are investigated. Results. HESS J1507–622 is detected in the Fermi energy range and its spectrum is best described by a power law in energy with Γ = 1.7 ± 0.1stat ± 0.2sys and integral flux between (0.3−300) GeV of F = (2.0 ± 0.5stat ± 1.0sys) × 10-9 cm-2 s-1. The SED constructed from the Fermi and H.E.S.S. data for this source does not support a smooth power-law continuation from the VHE to the HE gamma-ray range. With the available data it is not possible to discriminate between a hadronic and a leptonic scenario for HESS J1507–622. The location and compactness of the source indicate a considerable physical offset from the Galactic plane for this object. In case of a multiple-kpc distance, this challenges a pulsar wind nebula (PWN) origin for HESS J1507–622 since the time of travel for a pulsar born in the Galactic disk to reach such a location would exceed the inverse Compton (IC) cooling time of electrons that are energetic enough to produce VHE gamma-rays. However, an origin of this gamma-ray source connected to a pulsar that was born off the Galactic plane in the explosion of a hypervelocity star cannot be excluded. Conclusions. The nature of HESS J1507–622 is still unknown to date, and a PWN scenario cannot be ruled out in general. On the contrary HESS J1507–622 could be the first discovered representative of a population of spatially extended VHE gamma-ray emitters with HE gamma-ray counterpart that are located at considerable offsets from the Galactic plane. Future surveys in the VHE gamma-ray range are necessary to probe the presence or absence of such a source population.
Aims.We assess the importance of AGN outflows with respect to the metal enrichment of the intracluster medium (ICM) in galaxy clusters. Methods.We use combined N-body and hydrodynamic simulations, ...along with a semi-numerical galaxy formation and evolution model. Using assumptions based on observations, we attribute outflows of metal-rich gas initiated by AGN activity to a certain fraction of our model galaxies. The gas is added to the model ICM, where the evolution of the metallicity distribution is calculated by the hydrodynamic simulations. For the parameters describing the AGN content of clusters and their outflow properties, we use the most observationally favorable values. Results.We find that AGNs have the potential to contribute significantly to the metal content of the ICM or even explain the complete abundance, which is typically ~$0.5~Z_{\odot}$ in core regions. Furthermore, the metals end up being inhomogeneously distributed, in accordance with observations.
We investigate the efficiency of galactic mass loss, triggered by ram-pressure stripping and galactic winds of cluster galaxies, on the chemical enrichment of the intra-cluster medium (ICM). We ...combine N-body and hydrodynamic simulations with a semi-numerical galaxy formation model. By including simultaneously different enrichment processes, namely ram-pressure stripping and galactic winds, in galaxy-cluster simulations, we are able to reproduce the observed metal distribution in the ICM. We find that the mass loss by galactic winds in the redshift regime $z>2$ is ~10% to 20% of the total galactic wind mass loss, whereas the mass loss by ram-pressure stripping in the same epoch is up to 5% of the total ram-pressure stripping mass loss over the whole simulation time. In the cluster formation epochs $z<2$ ram-pressure stripping becomes more dominant than galactic winds. We discuss the non-correlation between the evolution of the mean metallicity of galaxy clusters and the galactic mass losses. For comparison with observations we present two dimensional maps of the ICM quantities and radial metallicity profiles. The shape of the observed profiles is well reproduced by the simulations in the case of merging systems. In the case of cool-core clusters the slope of the observed profiles are reproduced by the simulation at radii below ~300 kpc, whereas at larger radii the observed profiles are shallower. We confirm the inhomogeneous metal distribution in the ICM found in observations. To study the robustness of our results, we investigate two different descriptions for the enrichment process interaction.
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