Person: Kashyap, Vinay
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Kashyap
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Vinay
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Kashyap, Vinay
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Publication A Full Year's Chandra Exposure on Sloan Digital Sky Survey Quasars From the Chandra Multiwavelength Project(IOP Publishing, 2008) Green, Paul J.; Aldcroft, Thomas; Richards, G. T.; Barkhouse, W. A.; Constantin, Ana-Maria; Haggard, D.; Karovska, Margarita; Kim, D.-W.; Kim, M.; Viklinin, Alexey; Anderson, S. F.; Mossman, Amy; Kashyap, Vinay; Myers, A. C.; Silverman, J. D.; Wilkes, Belinda; Tananbaum, HarveyWe study the spectral energy distributions and evolution of a large sample of optically selected quasars from the Sloan Digital Sky Survey that were observed in 323 Chandra images analyzed by the Chandra Multiwavelength Project. Our highest-confidence matched sample includes 1135 X-ray detected quasars in the redshift range 0.2 3, substantially expanding the known sample. We find no evidence for evolution out to z ∼ 5 for either the X-ray photon index Γ or for the ratio of optical/UV to X-ray flux αox. About 10% of detected QSOs show best-fit intrinsic absorbing columns greater than 1022 cm−2, but the fraction might reach ∼1/3 if most nondetections are absorbed. We confirm a significant correlation between αox and optical luminosity, but it flattens or disappears for fainter (MB −23) active galactic nucleus (AGN) alone. We report significant hardening of Γ both toward higher X-ray luminosity, and for relatively X-ray loud quasars. These trends may represent a relative increase in nonthermal X-ray emission, and our findings thereby strengthen analogies between Galactic black hole binaries and AGN. For uniformly selected subsamples of narrow-line Seyfert 1s and narrow absorption line QSOs, we find no evidence for unusual distributions of either αox or Γ.Publication Chandra Multiwavelength Project. I. First X‐Ray Source Catalog(IOP Publishing, 2004) Kim, D.‐W.; Cameron, R. A.; Drake, Jeremy; Evans, Nancy; Freeman, P.; Gaetz, Terrance; Ghosh, H.; Green, P. J.; Harnden, F. R. Jr.; Karovska, Margarita; Kashyap, Vinay; Maksym, Peter; Ratzlaff, Peter; Schlegel, E. M.; Silverman, J. D.; Tananbaum, Harvey; Viklinin, Alexey; Wilkes, Belinda; Grimes, J. P.The Chandra Multiwavelength Project (ChaMP) is a wide-area (~14 deg2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed active galactic nuclei, high-z clusters of galaxies) at flux levels (fX ~ 10-15 to 10-14 ergs s-1 cm-2) intermediate between the Chandra deep surveys and previous missions. We present the first ChaMP catalog, which consists of 991 near on-axis, bright X-ray sources obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. To assess source reliability and positional uncertainty, we perform a series of simulations and also use Chandra data to complement the simulation study. The false source detection rate is found to be as good as or better than expected for a given limiting threshold. On the other hand, the chance of missing a real source is rather complex, depending on the source counts, off-axis distance (or PSF), and background rate. The positional error (95% confidence level) is usually less than 1'' for a bright source, regardless of its off-axis distance, while it can be as large as 4'' for a weak source (~20 counts) at a large off-axis distance (Doff-axis > 8'). We have also developed new methods to find spatially extended or temporary variable sources, and those sources are listed in the catalog.Publication The Chandra Multiwavelength Project: Optical Follow‐up of Serendipitous Chandra Sources(IOP Publishing, 2004) Green, P. J.; Silverman, J. D.; Cameron, R. A.; Kim, D.‐W.; Wilkes, Belinda; Barkhouse, W. A.; LaCluyze, A.; Morris, D.; Mossman, Amy; Ghosh, H.; Grimes, J. P.; Jannuzi, B. T.; Tananbaum, Harvey; Aldcroft, Thomas; Baldwin, J. A.; Chaffee, F. H.; Dey, A.; Dosaj, A.; Evans, Nancy; Fan, X.; Foltz, C.; Gaetz, Terrance; Hooper, E. J.; Kashyap, Vinay; Mathur, S.; McGarry, M. B.; Romero‐Colmenero, E.; Smith, M. G.; Smith, P. S.; Smith, R. C.; Torres, Guillermo; Viklinin, Alexey; Wik, D. R.We present follow-up optical g', r', and i' imaging and spectroscopy of serendipitous X-ray sources detected in six archival Chandra images included in the Chandra Multiwavelength Project (ChaMP). Of the 486 X-ray sources detected between 3 × 10-16 and 2 × 10-13 (with a median flux of 3 × 10-15) ergs cm-2 s-1, we find optical counterparts for 377 (78%), or 335 (68%) counting only unique counterparts. We present spectroscopic classifications for 125 objects, representing 75% of sources with r* < 21 optical counterparts (63% to r* = 22). Of all classified objects, 63 (50%) are broad-line active galactic nuclei (AGNs), which tend to be blue in (g*-r*) colors. X-ray information efficiently segregates these quasars from stars, which otherwise strongly overlap in these SDSS colors until z > 3.5. We identify 28 sources (22%) as galaxies that show narrow emission lines, while 22 (18%) are absorption line galaxies. Eight galaxies lacking broad-line emission have X-ray luminosities that require they host an AGN (logLX > 43). Half of these have hard X-ray emission suggesting that high gas columns obscure both the X-ray continuum and the broad emission line regions. We find objects in our sample that show signs of X-ray or optical absorption, or both, but with no strong evidence that these properties are coupled. ChaMP's deep X-ray and optical imaging enable multiband selection of small and/or high-redshift groups and clusters. In these six fields we have discovered three new clusters of galaxies, two with z > 0.4, and one with photometric evidence for a similar redshift.Publication Ten Simple Rules for the Care and Feeding of Scientific Data(Public Library of Science (PLoS), 2014) Goodman, Alyssa; Pepe, Alberto; Blocker, Alexander Weaver; Borgman, Christine L.; Cranmer, Kyle; Crosas, Merce; Di Stefano, Rosanne; Gil, Yolanda; Groth, Paul; Hedstrom, Peg; Hogg, David W.; Kashyap, Vinay; Mahabal, Ashish; Siemiginowska, Aneta; Slavkovic, AleksandraPublication Chandra Multiwavelength Project. II. First Results of X‐Ray Source Properties(IOP Publishing, 2004) Kim, D.‐W.; Wilkes, Belinda; Green, P. J.; Cameron, R. A.; Drake, Jeremy; Evans, Nancy; Freeman, P.; Gaetz, Terrance; Ghosh, H.; Harnden, F. R. Jr.; Karovska, Margarita; Kashyap, Vinay; Maksym, Peter; Ratzlaff, Peter; Schlegel, E. M.; Silverman, J. D.; Tananbaum, Harvey; Viklinin, AlexeyThe Chandra Multiwavelength Project (ChaMP) is a wide-area (~14 deg2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed active galactic nuclei [AGNs] and high-z clusters of galaxies) at flux levels (fX ~ 10-15 to 10-14 ergs s-1 cm-2) intermediate between the Chandra Deep Field surveys and previous missions. We present the first results of X-ray source properties obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. Utilizing only near-on-axis X-ray-bright sources (to avoid problems caused by incompleteness and the Eddington bias), we derive the log N- log S relation in soft (0.5-2 keV) and hard (2-8 keV) energy bands. The ChaMP data are consistent with previous results of ROSAT, ASCA, and Chandra Deep Field surveys. In particular, our data nicely fill in the flux gap in the hard band between the Chandra Deep Field data and the previous ASCA data. We check whether there is any systematic difference in the source density between cluster and noncluster fields and also search for field-to-field variation, both of which have been previously reported. We found no significant field-to-field cosmic variation in either test within the statistics (~1 σ) across the flux levels included in our sample. In the X-ray color-color plot, most sources fall in the location characterized by photon index = 1.5-2 and NH = a few × 1020 cm2, suggesting that they are typical broadline AGNs. There also exist a considerable number of sources with peculiar X-ray colors (e.g., highly absorbed, very hard, very soft). We confirm a trend that on average the X-ray color hardens as the count rate decreases. Since the hardening is confined to the softest energy band (0.3-0.9 keV), we conclude that it is most likely due to absorption. We cross-correlate the X-ray sources with other catalogs and describe their properties in terms of optical color, X-ray-to-optical luminosity ratio, and X-ray colors.Publication H-means image segmentation to identify solar thermal features(Institute of Electrical and Electronics Engineers, 2012) Stein, Nathan; Stein, Nathan; Kashyap, Vinay; Meng, Xiao-li; van Dyk, DavidProperly segmenting multiband images of the Sun by their thermal properties will help determine the thermal structure of the solar corona. However, off-the-shelf segmentation algorithms are typically inappropriate because temperature information is captured by the relative intensities in different passbands, while the absolute levels are not relevant. Input features are therefore pixel-wise proportions of photons observed in each band. To segment solar images based on these proportions, we use a modification of k-means clustering that we call the H-means algorithm because it uses the Hellinger distance to compare probability vectors. H-means has a closed-form expression for cluster centroids, so computation is as fast as k-means. Tempering the input probability vectors reveals a broader class of H-means algorithms which include spherical k-means clustering. More generally, H-means can be used anytime the input feature is a probabilistic distribution, and hence is useful beyond image segmentation applications.Publication A Possible Planet Candidate in an External Galaxy Detected Through X-Ray Transit(Springer Science and Business Media LLC, 2021-10-25) Di Stefano, Rosanne; Berndtsson, Julia; Urquhart, Ryan; Soria, Roberto; Kashyap, Vinay; Carmichael, Theron; Imara, NiaMany lines of reasoning suggest that external galaxies should host planetary systems but detecting them by methods typically used in our own Galaxy is not possible. An alternative approach is to study the temporal behavior of X-rays emitted by bright extragalactic X-ray sources, where an orbiting planet would temporarily block the X-rays and cause a brief eclipse. We report on such a potential event in the X-ray binary M51-ULS-1 in the galaxy M51. We examined a range of explanations for the observed X-ray dip, including a variety of transiting objects and enhancements in the density of gas and dust. The latter are ruled out by the absence of changes in X-ray colors, save any with sharp density gradients that cannot be probed with our data. Instead, the data are well fit by a planet transit model in which the eclipser is most likely to be the size of Saturn. We also find that the locations of possible orbits are consistent with the survival of a planet bound to a mass-transfer binary.