Abstract
BL Lac objects emitting in the very high energy (VHE) regime are unique tools to peer into the properties of the extragalactic background light (EBL). However, due to the typical absence of ...features in their spectra, the determination of their redshifts has proven challenging. In this work, we exploit the superb spatial resolution delivered by the new Advanced Rayleigh guided Ground layer adaptive Optics System (ARGOS) at the Large Binocular Telescope to detect the host galaxy of HESS J1943+213, a VHE emitting BL Lac shining through the Galaxy. Deep H-band imaging collected during the ARGOS commissioning allowed us to separate the contribution of the nuclear emission and to unveil the properties of the host galaxy with unprecedented detail. The host galaxy is well fitted by a Sérsic profile with index of n ∼ 2 and total magnitude of HHost ∼ 16.15 mag. Under the assumption that BL Lac host galaxies are standard candles, we infer a redshift of z ∼ 0.21. In the framework of the current model for the EBL, this value is in agreement with the observed dimming of the VHE spectrum due to the annihilation of energetic photons on the EBL
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.
We carried out optical observations of the field of the X–ray pulsator RX J0806.3+1527. A blue $V =$ 21.1 star was found to be the only object consistent with the X–ray position. VLT FORS spectra ...revealed a blue continuum with no intrinsic absorption lines. Broad ($v \sim$ 1500 km s-1), low equivalent width (~–1 $\div$ –6 Å) emission lines from the HeII Pickering series were clearly detected. B, V and R time–resolved photometry revealed the presence of ~15% pulsations at the ~321 s X–ray period, confirming the identification. These findings, together with the period stability and absence of any additional modulation in the 1 min$ - $5 hr period range, argue in favour of the orbital interpretation of the 321 s pulsations. The most likely scenario is thus that RX J0806.3+1527 is a double degenerate system of the AM CVn class. This would make RX J0806.3+1527 the shortest orbital period binary currently known and one of the best candidates for gravitational wave detection.
H α emission line spectroscopy in NGC 330 Hummel, W.; Gässler, W.; Muschielok, B. ...
Astronomy and astrophysics (Berlin),
06/2001, Letnik:
371, Številka:
3
Journal Article
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