Investigation of dust bands from blue ice fields in the Lewis Cliff (Beardmore) area, Antarctica: A progress report

Blue ice fields in Antarctica are well known for their high areal meteorite concentrations. The exact type of accumulation model and the age of the ice is still not well known. Dust bands on blue ice fields may help to clarify some of these problems. Dust, which has been isolated from dust band samples from blue ice areas in the Lewis Cliff/Walcott Neve area (Beardmore region), Antarctica, was studied to determine petrographic characteristics and chemical compositions. One sample has an average grain size of around 0.5mm, and is rather different from the others in its abundances of trace elements. The REE pattern and some other trace element ratios of that sample suggest it is a sediment from the local Beacon Supergroup, which has been scooped up from the ground by ice movement. The other five samples which were investigated have very small grain sizes (20μm), and abundant glass shards. Major element data on the glass shards (and some feldspar crystals, which are also present in the dust band samples) allow the conclusion that they have originated from an alkaline volcano. The chemical composition of the glasses is highly variable, some showing basanitic composition, some showing trachytic or peralkaline K-trachytic composition. The silica vs. sum of alkalis plot shows that the Lewis Cliff samples are different from dust collected at the Allan Hills, but that there is a close similarity with volcanic material from The Pleiades, Northern Victoria Land. The trace element chemistry of all volcanic samples show the characteristic volcanic trace elements, like Ta, W, Sb, Th, and the REE, enriched by a considerable factor. The REE patterns exhibit a prominent negative Eu anomaly, which may be explained by mixing basanites (no Eu anomaly, but steep REE patterns) with K-trachytes and peralkaline K-trachytes (very pronounced negative Eu anomaly). The same components are obvious in major element analyses of individual glass shards, thus each dust band is a mixture of at least three different source materials (which, however, originated from the same volcano in a single eruption). The Pleiades seem to be a likely source for the volcanic debris found in the dust bands at Lewis Cliff

NuSTAR and Chandra observations of new X-ray transients in the central parsec of the Galaxy

We report NuSTAR and Chandra observations of two X-ray transients, SWIFT J174540.7$-$290015 (T15) and SWIFT J174540.2$-$290037 (T37), which were discovered by the Neil Gehrels Swift Observatory in 2016 within $r\sim1$ pc of Sgr A*. NuSTAR detected bright X-ray outbursts from T15 and T37, likely in the soft and hard states, with 3-79~keV luminosities of $8\times10^{36}$ and $3\times10^{37}$ erg/s, respectively. No X-ray outbursts have previously been detected from the two transients and our Chandra ACIS analysis puts an upper limit of $L_X \lesssim 2 \times10^{31}$ erg/s on their quiescent 2-8 keV luminosities. No pulsations, significant QPOs, or type I X-ray bursts were detected in the NuSTAR data. While T15 exhibited no significant red noise, the T37 power density spectra are well characterized by three Lorentzian components. The declining variability of T37 above $\nu \sim 10$ Hz is typical of black hole (BH) transients in the hard state. NuSTAR spectra of both transients exhibit a thermal disk blackbody, X-ray reflection with broadened Fe atomic features, and a continuum component well described by Comptonization models. Their X-ray reflection spectra are most consistent with high BH spin ($a_{*} \gtrsim 0.9$) and large disk density ($n_e\sim10^{21}$ cm$^{-3}$). Based on the best-fit ionization parameters and disk densities, we found that X-ray reflection occurred near the inner disk radius, which was derived from the relativistic broadening and thermal disk component. These X-ray characteristics suggest the outbursting BH-LMXB scenario for both transients and yield the first BH spin measurements from X-ray transients in the central 100 pc region.Comment: 15 pages, 7 figures, accepted for publication in Ap