A Chandra X-ray detection of the L dwarf binary Kelu-1: Simultaneous Chandra and Very Large Array observations

Magnetic activity in ultracool dwarfs, as measured in X-rays and H$\alpha$, shows a steep decline after spectral type M7-M8. So far, no L dwarf has been detected in X-rays. In contrast, L dwarfs may have higher radio activity than M dwarfs. We observe L and T dwarfs simultaneously in X-rays and radio to determine their level of magnetic activity in the context of the general decline of magnetic activity with cooler effective temperatures. The field L dwarf binary Kelu-1 was observed simultaneously with Chandra and the Very Large Array. Kelu-1AB was detected in X-rays with $L_{\rm X} = 2.9_{-1.3}^{+1.8} \times 10^{25}$ erg/s, while it remained undetected in the radio down to a $3 \sigma$ limit of $L_{\rm R} \leq 1.4 \times 10^{13}$ erg/s/Hz. We argue that, whereas the X-ray and H$\alpha$ emissions decline in ultracool dwarfs with decreasing effective temperature, the radio luminosity stays (more or less) constant across M and early-L dwarfs. The radio surface flux or the luminosity may better trace magnetic activity in ultracool dwarfs than the ratio of the luminosity to the bolometric luminosity. Deeper radio observations (and at short frequencies) are required to determine if and when the cut-off in radio activity occurs in L and T dwarfs, and what kind of emission mechanism takes place in ultracool dwarfs.Comment: Accepted for publication as a Letter in Astronomy & Astrophysic

The Extended Distribution of Baryons Around Galaxies

We summarize and reanalyze observations bearing upon missing galactic baryons, where we propose a consistent picture for halo gas in L >~ L* galaxies. The hot X-ray emitting halos are detected to 50-70 kpc, where typically, M_hot(<50 kpc) ~ 5E9 Msun, and with density n \propto r^-3/2. When extrapolated to R200, the gas mass is comparable to the stellar mass, but about half of the baryons are still missing from the hot phase. If extrapolated to 1.9-3 R200, the baryon to dark matter ratio approaches the cosmic value. Significantly flatter density profiles are unlikely for R < 50 kpc and they are disfavored but not ruled out for R > 50 kpc. For the Milky Way, the hot halo metallicity lies in the range 0.3-1 solar for R < 50 kpc. Planck measurements of the thermal Sunyaev-Zeldovich effect toward stacked luminous galaxies (primarily early-type) indicate that most of their baryons are hot, near the virial temperature, and extend beyond R200. This stacked SZ signal is nearly an order of magnitude larger than that inferred from the X-ray observations of individual (mostly spiral) galaxies with M_* > 10^11.3 Msun. This difference suggests that the hot halo properties are distinct for early and late type galaxies, possibly due to different evolutionary histories. For the cooler gas detected in UV absorption line studies, we argue that there are two absorption populations: extended halos; and disks extending to ~50 kpc, containing most of this gas, and with masses a few times lower than the stellar masses. Such extended disks are also seen in 21 cm HI observations and in simulations.Comment: 22 pages, 20 figures, 2 tables, submitted to Ap

The 2015 decay of the black hole X-ray binary V404 Cygni: Robust disk-jet coupling and a sharp transition into quiescence

We present simultaneous X-ray and radio observations of the black hole X-ray binary V404 Cygni at the end of its 2015 outburst. From 2015 July 11-August 5, we monitored V404 Cygni with Chandra, Swift, and NuSTAR in the X-ray, and with the Karl G. Jansky Very Large Array and the Very Long Baseline Array in the radio, spanning a range of luminosities that were poorly covered during its previous outburst in 1989 (our 2015 campaign covers ). During our 2015 campaign, the X-ray spectrum evolved rapidly from a hard photon index of G ˜ 1.6 (at LX ˜ 10 erg s-34 1) to a softer G ˜ 2 (at L ˜ 3 × 10 erg s- X 33 1). We argue that V404 Cygni reaching marks the beginning of the quiescent spectral state, which occurs at a factor of ˜3-4 higher X-ray luminosity than the average pre-outburst luminosity of . V404 Cygni falls along the same radio/X-ray luminosity correlation that it followed during its previous outburst in 1989, implying a robust disk-jet coupling. We exclude the possibility that a synchrotron-cooled jet dominates the X-ray emission in quiescence, leaving synchrotron self-Compton from either a hot accretion flow or from a radiatively cooled jet as the most likely sources of X-ray radiation, and/or particle acceleration along the jet becoming less efficient in quiescence. Finally, we present the first indications of correlated radio and X-ray variability on minute timescales in quiescence, tentatively measuring the radio emission to lag the X-ray by 15 ± 4minute, suggestive of X-ray variations propagating down a jet with a length of &lt;3.0 au. Copyright © 2017 The American Astronomical Society. All rights reserved

The optical spectra of X-shaped radio galaxies

X-shaped radio galaxies are defined by their peculiar large-scale radio morphology. In addition to the classical double-lobed structure they have a pair of low-luminosity wings that straddles the nucleus at almost right angles to the active lobes, thus giving the impression of an 'X'. In this paper we study for the first time the optical spectral properties of this object class using a large sample (~50 sources). We find that the X-shaped radio population is composed roughly equally of sources with weak and strong emission line spectra, which makes them, in combination with the well-known fact that they preferentially have radio powers intermediate between those of Fanaroff-Riley type I (FR I) and type II (FR II) radio galaxies, the archetypal transition population. We do not find evidence in support of the proposition that the X-shape is the result of a recent merger: X-shaped radio sources do not have unusually broad emission lines, their nuclear environments are in general not dusty, and their host galaxies do not show signs of enhanced star formation. Instead, we observe that the nuclear regions of X-shaped radio sources have relatively high temperatures. This finding favours models, which propose that the X-shape is the result of an overpressured environment.Comment: 12 pages, 8 figures, accepted by MNRA

A Luminous Be+White Dwarf Supersoft Source in the Wing of the SMC: MAXI J0158-744

We present a multi-wavelength analysis of the very fast X-ray transient MAXI J0158-744, which was detected by MAXI/GSC on 2011 November 11. The subsequent exponential decline of the X-ray flux was followed with Swift observations, all of which revealed spectra with low temperatures (~100eV) indicating that MAXI J0158-744 is a new Supersoft Source (SSS). The Swift X-ray spectra near maximum show features around 0.8 keV that we interpret as possible absorption from OVIII, and emission from O, Fe, and Ne lines. We obtained SAAO and ESO optical spectra of the counterpart early in the outburst and several weeks later. The early spectrum is dominated by strong Balmer and HeI emission, together with weaker HeII emission. The later spectrum reveals absorption features that indicate a B1/2IIIe spectral type, and all spectral features are at velocities consistent with the Small Magellanic Cloud. At this distance, it is a luminous SSS (>10^37 erg/s) but whose brief peak luminosity of >10^39 erg/s in the 2-4 keV band makes it the brightest SSS yet seen at "hard" X-rays. We propose that MAXI J0158-744 is a Be-WD binary, and the first example to possibly enter ULX territory. The brief hard X-ray flash could possibly be a result of the interaction of the ejected nova shell with the B star wind in which the white dwarf (WD) is embedded. This makes MAXI J0158-744 only the third Be/WD system in the Magellanic Clouds, but it is by far the most luminous. The properties of MAXI J0158-744 give weight to previous suggestions that SSS in nearby galaxies are associated with early-type stellar systems.Comment: 15 pages, 8 figures; ApJ accepte

Exploring the dust content of galactic haloes with Herschel - IV. NGC 3079

We present the results from an analysis of deep Herschel far-infrared (far-IR) observations of the edge-on disc galaxy NGC 3079. The point spread function-cleaned Photodetector Array Camera and Spectrometer (PACS) images at 100 and 160 μm display a 25 × 25 kpc2 X-shape structure centred on the nucleus that is similar in extent and orientation to that seen in H α, X-rays, and the far-ultraviolet. One of the dusty filaments making up this structure is detected in the Spectral and Photometric Imaging Receiver 250 μm map out to ∼25 kpc from the nucleus. The match between the far-IR filaments and those detected at other wavelengths suggests that the dusty material has been lifted out of the disc by the same large-scale galactic wind that has produced the other structures in this object. A closer look at the central 10 × 10 kpc2 region provides additional support for this scenario. The dust temperatures traced by the 100-160 μm flux ratios in this region are enhanced within a biconical region centred on the active galactic nucleus, aligned along the minor axis of the galaxy, and coincident with the well-known double-lobed cm-wave radio structure and H α-X-ray nuclear superbubbles. PACS imaging spectroscopy of the inner 6 kpc region reveals broad [C ii] 158 μm emission line profiles and OH 79 μm absorption features along the minor axis of the galaxy with widths well in excess of those expected from beam smearing of the disc rotational motion. This provides compelling evidence that the cool material traced by the [C ii] and OH features directly interacts with the nuclear ionized and relativistic outflows traced by the H α, X-ray, and radio emission