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Register a new userHigh Resolution Chandra HETG spectroscopy of V404 Cygni in Outburst
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As one of the best-characterized stellar-mass black holes, with good measurements of its mass, distance and inclination, V404 Cyg is the ideal candidate to study Eddington-limited accretion episodes. After a long quiescent period, V404 Cyg underwent a new outburst in June 2015. We obtained two Chandra HETG exposures of 20 ksec and 25 ksec. Many strong emission lines are observed; the ratio of Si He-like triplet lines gives an estimate for the formation region distance of $4times10^{11}$ cm, while the higher ionization Fe XXV He-like triplet gives an estimate of $7times10^9$ cm. A narrow Fe K$alpha$ line is detected with an equivalent width greater than 1 keV in many epochs, signaling that we do not directly observe the central engine. Obscuration of the central engine and strong narrow emission lines signal that the outer disk may be illuminated, and its structure may help to drive the strong variability observed in V404 Cyg. In the highest flux phases, strong P-Cygni profiles consistent with a strong disk wind are observed. The kinetic power of this wind may be extremely high.
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We report our measurements of the bulk radial velocity from a sample of small, metal-rich ejecta knots in Keplers Supernova Remnant (SNR). We measure the Doppler shift of the He-like Si K$alpha$ line center energy in the spectra of these knots based on our $Chandra$ High-Energy Transmission Grating Spectrometer (HETGS) observation to estimate their radial velocities. We estimate high radial velocities of up to $sim$ 8,000 km s$^{-1}$ for some of these ejecta knots. We also measure proper motions for our sample based on the archival $Chandra$ Advanced CCD Imaging Spectrometer (ACIS) data taken in 2000, 2006, and 2014. Our measured radial velocities and proper motions indicate that some of these ejecta knots are almost freely-expanding after $sim$ 400 years since the explosion. The fastest moving knots show proper motions up to $sim$ 0.2 arcseconds per year. Assuming that these high velocity ejecta knots are traveling ahead of the forward shock of the SNR, we estimate the distance to Keplers SNR $d$ $sim$ 4.4 to 7.5 kpc. We find that the ejecta knots in our sample have an average space velocity of $ v_{s} sim$ 4,600 km s$^{-1}$ (at a distance of 6 kpc). We note that 8 out of the 15 ejecta knots from our sample show a statistically significant (at the 90$%$ confidence level) redshifted spectrum, compared to only two with a blueshifted spectrum. This may suggest an asymmetry in the ejecta distribution in Keplers SNR along the line of sight, however a larger sample size is required to confirm this result.
The black-hole binary, V404 Cygni, went into outburst in June 2015, after 26 years of X-ray quiescence. We observed the outburst with the Neil Gehrels Swift observatory. We present optical/UV observations taken with the Swift Ultra-violet Optical Telescope, and compare them with the X-ray observations obtained with the Swift X-ray Telescope. We find that dust extinction affecting the optical/UV, does not correlate with absorption due to neutral hydrogen that affects the X-ray emission. We suggest there is a small inhomogeneous high density absorber containing a negligible amount of dust, close to the black hole. Overall, temporal variations in the optical/UV appear to trace those in the X-rays. During some epochs we observe an optical time-lag of (15 - 35)s. For both the optical/UV and X-rays, the amplitude of the variations correlates with flux, but this correlation is less significant in the optical/UV. The variability in the light curves may be produced by a complex combination of processes. Some of the X-ray variability may be due to the presence of a local, inhomogeneous and dust-free absorber, while variability visible in both the X-ray and optical/UV may instead be driven by the accretion flow: the X-rays are produced in the inner accretion disc, some of which are reprocessed to the optical/UV; and/or the X-ray and optical/UV emission is produced within the jet.
The microquasar V404 Cygni underwent a series of outbursts in 2015, June 15-31, during which its flux in hard X-rays (20-40 keV) reached about 40 times the Crab Nebula flux. Because of the exceptional interest of the flaring activity from this source, observations at several wavelengths were conducted. The MAGIC telescopes, triggered by the INTEGRAL alerts, followed-up the flaring source for several nights during the period June 18-27, for more than 10 hours. One hour of observation was conducted simultaneously to a giant 22 GHz radio flare and a hint of signal at GeV energies seen by Fermi-LAT. The MAGIC observations did not show significant emission in any of the analysed time intervals. The derived flux upper limit, in the energy range 200--1250 GeV, is 4.8$times 10^{-12}$ ph cm$^{-2}$ s$^{-1}$. We estimate the gamma-ray opacity during the flaring period, which along with our non-detection, points to an inefficient acceleration in the V404,Cyg jets if VHE emitter is located further than $1times 10^{10}$ cm from the compact object.
The HETG can be used to obtain spatially resolved spectra of moderately extended sources. We present preliminary results for two well studied, oxygen rich supernova remnants in the Magellanic clouds, E0102-72 and N132D. The dispersed spectrum of E0102-72 shows images of the remnant in the light of individual emission lines from H-like and He-like ions of O, Mg, Ne and He-like Si with no evidence of Fe. The diameters of the images for various ions, measured in the cross-dispersion direction, increase monotonically with the ionization age for the given ion. This shows in detail the progression of the reverse shock through the expanding stellar ejecta. We see clear evidence for asymmetric Doppler shifts across E0102-72 of ~2000 km/s. These can be modelled approximately by a partially-filled, expanding shell inclined to the line of sight. The dispersed spectrum of N132D is more affected by spatial/spectral overlap but also shows monochromatic images in several strong lines. Preliminary spectra have been extracted for several bright knots. Some regions of oxygen-rich material, presumably stellar ejecta, are clearly identified. Additional details on E0102-72 are presented by Flanagan et al. and Davis et al. in these proceedings, and further analysis is in progress.
We report on Fermi/Large Area Telescope observations of the accreting black hole low-mass X-ray binary V404 Cygni during its outburst in June-July 2015. Detailed analyses reveal a possible excess of $gamma$-ray emission on 26 June 2015, with a very soft spectrum above $100$ MeV, at a position consistent with the direction of V404 Cyg (within the $95%$ confidence region and a chance probability of $4 times 10^{-4}$). This emission cannot be associated with any previously-known Fermi source. Its temporal coincidence with the brightest radio and hard X-ray flare in the lightcurve of V404 Cyg, at the end of the main active phase of its outburst, strengthens the association with V404 Cyg. If the $gamma$-ray emission is associated with V404 Cyg, the simultaneous detection of $511,$keV annihilation emission by INTEGRAL requires that the high-energy $gamma$ rays originate away from the corona, possibly in a Blandford-Znajek jet. The data give support to models involving a magnetically-arrested disk where a bright $gamma$-ray jet can re-form after the occurrence of a major transient ejection seen in the radio.
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