A NuSTAR view of GRS 1716−249 in the hard and intermediate states

ABSTRACT We present a detailed analysis of the spectral properties of the black hole transient GRS 1716−249, based on the archival Swift and NuSTAR observations taken during the outburst of this source in 2016–2017. The first six NuSTAR observations show that the source is in a canonical hard state, where the spectrum is dominated by a power-law continuum. The seventh NuSTAR observation is taken during the intermediate state where both a disc thermal component and a power-law continuum are shown. All of our observations show a broad emission-line feature in the iron band and a Compton hump above 10 keV. We model the broad-band spectra using a high-density disc reflection model, where the soft X-ray emission in the hard state is interpreted as part of the disc reflection component. This model enables us to constrain the disc density parameter of GRS 1716−249 in the range of 1019–1020 cm−3. We only obtain an upper limit of the inner disc radius using high-density disc reflection spectroscopy and the results indicate either a non-truncated disc or a slightly truncated disc with Rin ≲ 20 rg.

Spectroscopic follow-up of the quadruply lensed quasar WGD2038-4008/GRAL2038-4008

We present independent optical spectroscopic follow-up of WGD2038-4008/GRAL2038-4008, a background quasar strongly lensed by a foreground elliptical galaxy into four images, recently discovered independently by Agnello et al. [2018, MNRAS, 479, 4345] and Krone-Martins et al. [2018, A&A, 616, L11] thanks to the exquisite spatial resolution of Gaia. The quasar images are bright (i ∼ 19 mag), thus enabling us to reach S/N > 20 for the continuum within 30 min of exposure time with the Andalucia Faint Object Spectrograph and Camera spectrograph mounted on the 2.56-m Nordic Optical Telescope. The flexible scheduling and high sensitivity delivered by Andalucia Faint Object Spectrograph and Camera provide timely redshifts and reveal the nature of the quasar images; both are essential for lensing modelling and cosmography. Our analysis shows a strong emission feature in a data gap in Agnello et al. [2018, MNRAS, 479, 4345], which can be attributed to as an interloper emission line feature from the foreground lensing galaxy, or hinting to a higher redshift of the background quasar. We discuss these two scenarios and outline possible tests to verify these scenarios.

ROSAT Deep Surveys

A series of 20 deep pointed observations with the ROSAT PSPC is discussed. 530 X-ray sources with 0.5-2 keV fluxes down to 3·10-15erg cm-2 s-1 have been discovered in 5.9 deg2. The N(>S) relation of the sources selected in the 0.5-2 keV band shows a density in excess of 200 deg-2 at the faintest fluxes and a flattening below 2·10-14erg cm-2 s-1. The average spectrum of those sources is a power law with energy index 1.2 ±0.1. The absorption column densities are consistent with the galactic HI columns. More than 50% of the 1-2 keV background has been resolved into discrete sources in the deepest field. The total background spectrum shows an emission line feature around 0.65 keV, most probably due to OVII-OVIII from a 2·106K plasma. Above ~ 1 keV the background is dominated by a power law spectrum with a normalization of 13.4 ± 0.2 keV cm-2 s-1 sr-1 keV-1 and a slope 1.2 ± 0.1, i.e. considerably steeper than the extrapolation from higher energies.

The Draco Nebula, a Molecular Cloud Associated with a High Velocity Cloud?

Extended and very faint bright nebulae are found in high (b≳ 30°) galactic latitudes at the Palomar Observatory Sky Survey (POSS) (Lynds, 1965) and even more pronounced at a very sensitive photographic survey of the galactic polar caps by Sandage (1976). Such a nebula, located in the constellation Draco and called “Draco Nebula” or “Dracula”, was found to be in detailed positional coincidence with a 21-cm emission line feature at a LSR velocity of VLSR ≈ -22 km s-1 by Goerigk et al. (1983).