An Ultracool Star's Candidate Planet

We report here the discovery of the first planet around an ultracool dwarf star. It is also the first extrasolar giant planet (EGP) astrometrically discovered around a main-sequence star. The statistical significance of the detection is shown in two ways. First, there is a 2 x 10^-8 probability that the astrometric motion fits a parallax-and-proper-motion-only model. Second, periodogram analysis shows a false alarm probability of 3 x 10^-5 that the discovered period is randomly generated. The planetary mass is M2 = 6.4 (+2.6,-3.1) Jupiter-masses (MJ), and the orbital period is P = 0.744 (+0.013,-0.008) yr in the most likely model. In less likely models, companion masses that are higher than the 13 MJ planetary mass limit are ruled out by past radial velocity measurements unless the system radial velocity is more than twice the current upper limits and the near-periastron orbital phase was never observed. This new planetary system is remarkable, in part, because its star, VB 10, is near the lower mass limit for a star. Our astrometric observations provide a dynamical mass measurement and will in time allow us to confront the theoretical models of formation and evolution of such systems and their members. We thus add to the diversity of planetary systems and to the small number of known M-dwarf planets. Planets such as VB 10b could be the most numerous type of planets because M stars comprise >70% of all stars. To date they have remained hidden since the dominant radial-velocity (RV) planet-discovery technique is relatively insensitive to these dim, red systems.Comment: 1 30 page pdf file, 9 figures, accepted for publication in The Astrophysical Journa

K-alpha X-rays from cosmic ray oxygen

Equilibrium charge fractions are calculated for subrelativistic cosmic ray oxygen ions in the interstellar medium. These are used to determine the expected flux of K-alpha rays arising from atomic processes for a number of different postulated interstellar oxygen spectra. Relating these results to the diffuse X-ray background measured at the appropriate energy level suggests an observable line feature. If the flux of low energy cosmic ray oxygen is sufficiently large, K-alpha X-ray line emission from these nuclei will comprise a significant fraction of the total diffuse flux at approximately 0.6 keV. A satellite borne detector with a resolution greater than 30 percent could observe this feature if the subrelativistic interstellar cosmic ray oxygen spectrum is as large as certain theoretical estimates expressed in the text

X-ray spectra of the Crab pulsar and nebula

The spectrum of the Crab pulsar was measued from 2 to 50 keV as a function of pulse phase and a progressive hardening and subsequent softening of the spectrum across the pulse was found. The fraction of the pulsed flux which exhibits spectral variability is 0.14 and is concentrated solely in the region between the two peaks. A model is suggested in which the pulsed X-ray emission from the Crab pulsar consists of two components: one which has no spectral dependence with pulse phase and which is physically related to the double peaked gama ray pulse and, perhaps, the radio and optical pulses; and another component which exhibits spectral variability with pulse phase is confined to and comprises the interpeak emission, and which is only seen at X-ray energies. These results and studies of the binary X-ray pulsar Hercules X-1 suggest a phenomonological similarity. If the spectrally varying component in the Crab pulsar arises from a hot, magnetized plasma near the neutron star surface then higher energy spectral observations of this phase region might reveal spectral features which can be used to determine the surface field strength

Chandra observation of Cepheus A: The diffuse emission of HH 168 resolved

X-ray emission from massive stellar outflows has been detected in several cases. We present a Chandra observation of HH 168 and show that the soft X-ray emission from a plasma of 0.55 keV within HH 168 is diffuse. The X-ray emission is observed on two different scales: Three individual, yet extended, regions are embedded within a complex of low X-ray surface brightness. Compared to the bow shock the emission is displaced against the outflow direction. We show that there is no significant contribution from young stellar objects (YSOs) and discuss several shock scenarios that can produce the observed signatures. We establish that the X-ray emission of HH 168 is excited by internal shocks in contrast to simple models, which expect the bow shock to be the most X-ray luminous.Comment: 8 pages, 5 figures, accepted for publication in A&

X-rays from the Power Sources of the Cepheus A Star-Forming Region

We report an observation of X-ray emission from the exciting region of Cepheus A with the Chandra/ACIS instrument. What had been an unresolved X-ray source comprising the putative power sources is now resolved into at least 3 point-like sources, each with similar X-ray properties and differing radio and submillimeter properties. The sources are HW9, HW3c, and a new source that is undetected at other wavelengths "h10." They each have inferred X-ray luminosities >= 10^31 erg s^-1 with hard spectra, T >= 10^7 K, and high low-energy absorption equivalent to tens to as much as a hundred magnitudes of visual absorption. The star usually assumed to be the most massive and energetic, HW2, is not detected with an upper limit about 7 times lower than the detections. The X-rays may arise via thermal bremsstrahlung in diffuse emission regions associated with a gyrosynchrotron source for the radio emission, or they could arise from powerful stellar winds. We also analyzed the Spitzer/IRAC mid-IR observation from this star-formation region and present the X-ray results and mid-IR classifications of the nearby stars. HH 168 is not as underluminous in X-rays as previously reported.Comment: Accepted in the ApJ, 30 pages, 11 figures, in one .pdf fil

A review of x-ray spectral evolution in Crab pulsar

Pravdo et al (1997) claimed that the phase resolved x-ray spectrum in Crab pulsar (PSR B0531+21) shows a spectral hardening at the leading edge of the first peak of its integrated profile; this was a new and unexpected result. This article reanalyzes their data, as well as some other related data, and argues that the spectrum is as likely to be unvarying (i.e., neither hardening nor softening).Comment: 12 pages, 4 figures, accepted for publication in A&

SEASAT synthetic-aperture radar data user's manual

The SEASAT Synthetic-Aperture Radar (SAR) system, the data processors, the extent of the image data set, and the means by which a user obtains this data are described and the data quality is evaluated. The user is alerted to some potential problems with the existing volume of SEASAT SAR image data, and allows him to modify his use of that data accordingly. Secondly, the manual focuses on the ultimate focuses on the ultimate capabilities of the raw data set and evaluates the potential of this data for processing into accurately located, amplitude-calibrated imagery of high resolution. This allows the user to decide whether his needs require special-purpose data processing of the SAR raw data

High-precision astrometry on the VLT/FORS1 at time scales of few days

We investigate the accuracy of astrometric measurements with the VLT/FORS1 camera and consider potential applications. The study is based on two-epoch (2000 and 2002/2003) frame series of observations of a selected Galactic Bulge sky region that were obtained with FORS1 during four consecutive nights each. Reductions were carried out with a novel technique that eliminates atmospheric image motion and does not require a distinction between targets and reference objects. The positional astrometric precision was found to be limited only by the accuracy of the determination of the star photocentre, which is typically 200-300 microarcsec per single measurement for bright unsaturated stars B=18-19. Several statistical tests have shown that at time-scales of 1-4 nights the residual noise in measured positions is essentially a white noise with no systematic instrumental signature and no significant deviation from a Gaussian distribution. Some evidence of a good astrometric quality of the VLT for frames separated by two years has also been found. Our data show that the VLT with FORS1/2 cameras can be effectively used for astrometric observations of planetary microlensing events and other applications where a high accuracy is required, that is expected to reach 30-40 microarcsec for a series of 50 frames (one hours with R filter).Comment: 11 pages, 9 figures, accepted for publication in A&

X-ray and HeI 1.0830 mu emission from protostellar jets

Context. The high energies of protostellar jets, implied by recent observations of X-rays from such flows, came very much as a surprise. Inferred shock velocities are considerably higher than what was previously known, hence putting even larger energy demands on the driving sources of the jets. The statistics of X-ray emitting jets are still poor, yet a few cases exist which seem to imply a correlation between the presence of HeI 1.0830 mu emission and X-ray radiation in a given source. Aims. This tentative correlation needs confirmation and explanation. If the jet regions of HeI 1.0830 mu emission are closely associated with those producing X-rays, high resolution infared spectroscopy can be used to observationally study the velocity fields in the hot plasma regions of the jets. This would provide the necessary evidence to test and further develop theoretical models of intermediately fast (> 500 - 1500 km/s) interstellar shock waves. Methods. The HH 154 jet flow from the embedded protostellar binary L 1551 IRS 5 provides a case study, since adequate IR and X-ray spectroscopic data are in existence. The thermal X-ray spectrum is fed into a photoionization code to compute, in particular, the line emission of HeI and HI and to account for the observed unusual line intensity ratios. Results. The advanced model is capable of accounting for most observables, but shows also major weaknesses. It seems not unlikely that these could, in principle, be overcome by a time dependent hydrodynamical calculation with self-consistent cooling. However, such sophisticated model development is decisively beyond the scope of the present work. Conclusions. Continued X-ray observations, coordinated with simultaneous high resolution infrared spectroscopy, are highly desirable.Comment: 5 pages, 4 Postscript figures, 3 in color. accepted for publication in Astronomy and Astrophysic

Discovery of a Peculiar Dip from GX 301-2

We present temporal and spectral properties of a unique X-ray dip in GX 301-2 as seen with Rossi X-ray Timing Explorer in May 2010. The X-ray pulsation from the source gradually declined prior to the dip, disappears for one spin cycle during the dip and is abruptly restored in the spin cycle immediately after the dip. Moreover, the phase-integrated spectrum of the source becomes softer before and during the dip and it quickly hardens again following the dip. Our findings indicate the fact that the mechanism for pulsations gradually turned off briefly and underlying dim and softer emission likely from the accretion column became observable in the brief absence of high level emission due to wind accretion.Comment: Accepted for publication in A&A Letter