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Register a new userThe discovery of a new non-thermal X-ray filament near the Galactic Centre
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Masaaki Sakano
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We report the discovery by XMM-Newton and Chandra of a hard extended X-ray source (XMM J174540-2904.5) associated with a compact non-thermal radio filament (the Sgr A-E `wisp=1LC 359.888-0.086= G359.88-0.07), which is located within ~4 arcmin of the Galactic Centre. The source position is also coincident with the peak of the molecular cloud, M -0.13-0.08 (the `20 km/s cloud). The X-ray spectrum is non-thermal with an energy index of 1.0 (+1.1 -0.9) and column density of 38 (+7 -11) x 10^22 H/cm2. The observed 2--10 keV flux of 4 x 10^-13 erg/s/cm2 converts to an unabsorbed X-ray luminosity of 1 x 10^34 erg/s assuming a distance of 8.0 kpc. The high column density strongly suggests that this source is located in or behind the Galactic Centre Region. Taking account of the broad-band spectrum, as well as the source morphology and the positional coincidence with a molecular cloud, we concluded that both the radio and X-ray emission are the result of synchrotron radiation. This is the first time a filamentary structure in the Galactic Centre Region. has been shown, unequivocally, to have a non-thermal X-ray spectrum.
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A comparison of the XMM-Newton and Chandra Galactic Centre (GC) Surveys has revealed two faint X-ray transients with contrasting properties. The X-ray spectrum of XMM J174544-2913.0 shows a strong iron line with an equivalent width of ~2 keV, whereas that of XMM J174457-2850.3 is characterised by a very hard continuum with photon index ~1.0. The X-ray flux of both sources varied by more than 2 orders of magnitude over a period of months with a peak X-ray luminosity of 5 x 10^34 erg/s. We discuss the nature of these peculiar sources.
One of the most unique phenomena in the Galactic center region is the existence of numerous long and narrow filamentary structures within a few hundred parsecs of Sgr A$^{star}$. While more than one than one hundred radio filaments have been revealed by MeerKAT, about two dozens X-ray filaments have been discovered so far. In this article, we report our analysis on the deep Chandra and NuSTAR observations of a non-thermal X-ray filament, G0.13-0.11, which is located adjacent to the Radio arc. Chandra revealed a unique morphology of G0.13-0.11, which is an elongated (0.1 pc in width and 3.2 pc in length) structure slightly bended towards the Radio arc. A pulsar candidate ($Gamma sim 1.4$) is detected in the middle of the filament, with a tail of diffuse non-thermal X-ray emission on one side of the filament. The filament is detected by NuSTAR up to 79 keV, with the hard X-ray centroid consistent with the pulsar candidate. We found that the X-ray intensity decays along the filament farther away from the pulsar candidate, dropping to half of its peak value at 2.2 pc away. This system is mostly likely a Pulsar Wind Nebula interacting with ambient interstellar magnetic field, where the filaments are kinetic jets from PWN as recently proposed. The nature of this filament adds to complex origin of the X-ray filaments, which serve as powerful tools to probe local and global powerful particle accelerators in the Galactic center.
We present results of a multi-wavelength program to study the faint discrete X-ray source population discovered by Chandra in the Galactic Centre (GC). From IR imaging obtained with the VLT we identify candidate K-band counterparts to 75% of the X-ray sources in our sample. By combining follow-up VLT K-band spectroscopy of a subset of these candidate counterparts with the magnitude limits of our photometric survey, we suggest that only a small percentage of the sources are HMXBs, while the majority are likely to be canonical LMXBs and CVs at the distance of the GC. In addition, we present our discovery of highly structured small-scale (5-15) extinction towards the Galactic Centre. This is the finest-scale extinction study of the Galactic Centre to date. Finally, from these VLT observations we are able to place constraints on the stellar counterpart to the ``bursting pulsar GRO J1744-28.
The recent detection of an X-ray filament associated with the radio filament G0.173-0.42 adds to four other nonthermal radio filaments with X-ray counterparts, amongst the more than 100 elongated radio structures that have been identified as synchrotron-emitting radio filaments in the inner couple of degrees of the Galactic center. The synchrotron mechanism has also been proposed to explain the emission from X-ray filaments. However, the origin of radio filaments and the acceleration sites of energetic particles to produce synchrotron emission in radio and X-rays remain mysterious. Using MeerKAT, VLA, Chandra, WISE and Spitzer, we present structural details of G0.173-0.42 which consists of multiple radio filaments, one of which has an X-ray counterpart. A faint oblique radio filament crosses the radio and X-ray filaments. Based on the morphology, brightening of radio and X-ray intensities, and radio spectral index variation, we argue that a physical interaction is taking place between two magnetized filaments. We consider that the reconnection of the magnetic field lines at the interaction site leads to the acceleration of particles to GeV energies. We also argue against the synchrotron mechanism for the X-ray emission due to the short $sim$30 year lifetime of TeV relativistic particles. Instead, we propose that the inverse Compton scattering mechanism is more likely to explain the X-ray emission by upscattering of seed photons emitted from a 10^6 solar luminosity star located at the northern tip of the X-ray filament.
I describe the IR and X-ray observational campaign we have undertaken for the purpose of determining the nature of the faint discrete X-ray source population discovered by Chandra in the Galactic Center (GC). Data obtained for this project includes a deep Chandra survey of the Galactic Bulge; deep, high resolution IR imaging from VLT/ISAAC, CTIO/ISPI, and the UKIDSS Galactic Plane Survey (GPS); and IR spectroscopy from VLT/ISAAC and IRTF/SpeX. By cross-correlating the GC X-ray imaging from Chandra with our IR surveys, we identify candidate counterparts to the X-ray sources via astrometry. Using a detailed IR extinction map, we are deriving magnitudes and colors for all the candidates. Having thus established a target list, we will use the multi-object IR spectrograph FLAMINGOS-2 on Gemini-South to carry out a spectroscopic survey of the candidate counterparts, to search for emission line signatures which are a hallmark of accreting binaries. By determining the nature of these X-ray sources, this FLAMINGOS-2 Galactic Center Survey will have a dramatic impact on our knowledge of the Galactic accreting binary population.
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