arXiv subjects

Krzysztof Z. Stanek

Publications and source records attributed to Krzysztof Z. Stanek.

26 records · Page 2Linked to original sources

arXiv2008

An Unexpectedly Swift Rise in the Gamma-ray Burst Rate

The association of long gamma-ray bursts with supernovae naturally suggests that the cosmic GRB rate should trace the star formation history. Finding otherwise would provide important clues concerning these rare, curious phenomena. Using a new estimate of Swift GRB energetics to construct a sample of 36 luminous GRBs with redshifts in the range z=0-4, we find evidence of enhanced evolution in the GRB rate, with ~4 times as many GRBs observed at z~4 than expected from star formation measurements. This direct and empirical demonstration of needed additional evolution is a new result. It is consistent with theoretical expectations from metallicity effects, but other causes remain possible, and we consider them systematically.

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arXiv2008

Difference Imaging of Lensed Quasar Candidates in the SDSS Supernova Survey Region

Difference imaging provides a new way to discover gravitationally lensed quasars because few non-lensed sources will show spatially extended, time variable flux. We test the method on lens candidates in the Sloan Digital Sky Survey (SDSS) Supernova Survey region from the SDSS Quasar Lens Search (SQLS) and their surrounding fields. Starting from 20768 sources, including 49 SDSS quasars and 36 candidate lenses/lensed images, we find that 21 sources including 15 SDSS QSOs and 7 candidate lenses/lensed images are non-periodic variable sources. We can measure the spatial structure of the variable flux for 18 of these sources and identify only one as a non-point source. This source does not display the compelling spatial structure of the variable flux of known lensed quasars, so we reject it as a lens candidate. None of the lens candidates from the SQLS survive our cuts. Given our effective survey area of order 0.71 square degrees, this indicates a false positive rate of order one per square degree for themethod. The fraction of quasars not found to be variable and the false positive rate should both fall if we analyze the full, later data releases for the SDSS fields. While application of the method to the SDSS is limited by the resolution, depth, and sampling of the survey, several future surveys such as Pan-STARRS, LSST, and SNAP will avoid these limitations.

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arXiv2007

Disparate MgII Absorption Statistics towards Quasars and Gamma-Ray Bursts : A Possible Explanation

We examine the recent report by Prochter et al. (2006) that gamma-ray burst (GRB) sight lines have a much higher incidence of strong MgII absorption than quasar sight lines. We propose that the discrepancy is due to the different beam sizes of GRBs and quasars, and that the intervening MgII systems are clumpy with the dense part of each cloudlet of a similar size as the quasars, i.e. < 10^16 cm, but bigger than GRBs. We also discuss observational predictions of our proposed model. Most notably, in some cases the intervening MgII absorbers in GRB spectra should be seen varying, and quasars with smaller sizes should show an increased rate of strong MgII absorbers. In fact, our prediction of variable MgII lines in the GRB spectra has been now confirmed by Hao et al. (2007), who observed intervening FeII and MgII lines at z=1.48 to be strongly variable in the multi-epoch spectra of z=4.05 GRB060206.

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arXiv2007

Characterizing Supernova Progenitors via the Metallicities of their Host Galaxies, from Poor Dwarfs to Rich Spirals

We investigate how the different types of supernovae are relatively affected by the metallicity of their host galaxy. We match the SAI Supernova Catalog to the SDSS-DR4 catalog of star-forming galaxies with measured metallicities. These supernova host galaxies span a range of oxygen abundance from 12 + log(O/H) = 7.9 to 9.3 (~ 0.1 to 2.7 solar) and a range in absolute magnitude from MB = -15.2 to -22.2. To reduce the various observational biases, we select a subsample of well-characterized supernovae in the redshift range from 0.01 to 0.04, which leaves us with 58 SN II, 19 Ib/c, and 38 Ia. We find strong evidence that SN Ib/c are occurring in higher-metallicity host galaxies than SN II, while we see no effect for SN Ia relative to SN II. We note some extreme and interesting supernova-host pairs, including the metal-poor (~ 1/4 solar) host of the recent SN Ia 2007bk, where the supernova was found well outside of this dwarf galaxy. To extend the luminosity range of supernova hosts to even fainter galaxies, we also match all the historical supernovae with z < 0.3 to the SDSS-DR6 sky images, resulting in 1225 matches. This allows us to identify some even more extreme cases, such as the recent SN Ic 2007bg, where the likely host of this hypernova-like event has an absolute magnitude MB ~ -12, making it one of the least-luminous supernova hosts ever observed. This low-luminosity host is certain to be very metal poor (~ 1/20 solar), and therefore this supernova is an excellent candidate for association with an off-axis GRB. The two catalogs that we have constructed are available online and will be updated regularly. Finally, we discuss various implications of our findings for understanding supernova progenitors and their host galaxies.

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arXiv2005

On the Rotation Period of (90377) Sedna

We present precise, ~1%, r-band relative photometry of the unusual solar system object (90377) Sedna. Our data consist of 143 data points taken over eight nights in October 2004 and January 2005. The RMS variability over the longest contiguous stretch of five nights of data spanning nine days is only 1.3%. This subset of data alone constrain the amplitude of any long-period variations with period P to be A<1% (P/20 days)^2. Over the course of any given 5-hour segment, the data exhibits significant linear trends not seen in a comparison star of similar magnitude, and in a few cases these segments show clear evidence for curvature at the level of a few millimagnitudes per hour^2. These properties imply that the rotation period of Sedna is O(10 hours), cannot be < 5 hours, and cannot be >10 days, unless the intrinsic light curve has significant and comparable power on multiple timescales, which is unlikely. A sinusoidal fit yields a period of P=(10.273 +/- 0.002) hours and semi-amplitude of A=(1.1 +/- 0.1)%. There are additional acceptable fits with flanking periods separated by ~3 minutes, as well as another class of fits with P ~ 18 hours, although these later fits appear less viable based on visual inspection. Our results indicate that the period of Sedna is likely consistent with typical rotation periods of solar system objects, thus obviating the need for a massive companion to slow its rotation.

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arXiv2004

The Most Massive Stars in the Local Group: Measuring Accurate Masses of Stars in Eclipsing Binaries

Accurate masses and, in general, all fundamental parameters of distant stars can only be measured in eclipsing binaries. Several massive star candidates with masses near 200 Mo exist, however they have large uncertainties associated with them. The most massive binary ever measured accurately is WR 20a, for which we present the light curve. Measuring the period and inclination, we derive masses greater than 80 Mo for each component. Massive binaries are bound to exist in Local Group galaxies, such as M31 and M33. These can be selected from their light curves, obtained by variability studies, such as the DIRECT project. We present photometry and spectroscopy of the detached system M33A, for which we are obtaining a direct distance determination. The DIRECT project has detected several candidate massive binaries which are brighter but non-detached systems, perhaps similar to WR 20a. We plan to obtain spectra for them and measure their masses.

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arXiv2004

KH 15D: Gradual Occultation of a Pre-Main-Sequence Binary

We propose that the extraordinary ``winking star'' KH 15D is an eccentric pre-main-sequence binary that is gradually being occulted by an opaque screen. This model accounts for the periodicity, depth, duration, and rate of growth of the modern eclipses; the historical light curve from photographic plates; and the existing radial velocity measurements. It also explains the re-brightening events that were previously observed during eclipses, and the subsequent disappearance of these events. We predict the future evolution of the system and its full radial velocity curve. Given the small velocity of the occulting screen relative to the center of mass of the binary, the screen is probably associated with the binary, and may be the edge of a precessing circumbinary disk.

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arXiv2001

The Redshift of the Optical Transient Associated with GRB 010222

The gamma-ray burst (GRB) 010222 is the brightest GRB detected to date by the BeppoSAX satellite. Prompt identification of the associated optical transient (OT) allowed for spectroscopy with the Tillinghast 1.5m telescope at F. L. Whipple Observatory while the source was still relatively bright (R ~ 18.6 mag), within five hours of the burst. The OT shows a blue continuum with many superimposed absorption features corresponding to metal lines at z = 1.477, 1.157, and possibly also at 0.928. The redshift of GRB 010222 is therefore unambiguously placed at z >= 1.477. The high number of Mg II absorbers and especially the large equivalent widths of the Mg II, Mg I, and Fe II absorption lines in the z = 1.477 system further argue either for a very small impact parameter or that the z = 1.477 system is the GRB host galaxy itself. The spectral index of the OT is relatively steep, beta = 0.89 +/- 0.03, and this cannot be caused by dust with a standard Galactic extinction law in the z = 1.477 absorption system. This spectroscopic identification of the redshift of GRB 010222 shows that prompt and well-coordinated followup of bright GRBs can be successful even with telescopes of modest aperture.

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