arXiv subjects

Saurabh Jha

Publications and source records attributed to Saurabh Jha.

At least 91 records · Page 5Linked to original sources

arXiv2008

First-Year Spectroscopy for the SDSS-II Supernova Survey

This paper presents spectroscopy of supernovae discovered in the first season of the Sloan Digital Sky Survey-II Supernova Survey. This program searches for and measures multi-band light curves of supernovae in the redshift range z = 0.05 - 0.4, complementing existing surveys at lower and higher redshifts. Our goal is to better characterize the supernova population, with a particular focus on SNe Ia, improving their utility as cosmological distance indicators and as probes of dark energy. Our supernova spectroscopy program features rapid-response observations using telescopes of a range of apertures, and provides confirmation of the supernova and host-galaxy types as well as precise redshifts. We describe here the target identification and prioritization, data reduction, redshift measurement, and classification of 129 SNe Ia, 16 spectroscopically probable SNe Ia, 7 SNe Ib/c, and 11 SNe II from the first season. We also describe our efforts to measure and remove the substantial host galaxy contamination existing in the majority of our SN spectra.

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arXiv2006

Improved Distances to Type Ia Supernovae with Multicolor Light Curve Shapes: MLCS2k2

We present an updated version of the Multicolor Light Curve Shape method to measure distances to type Ia supernovae (SN Ia), incorporating new procedures for K-correction and extinction corrections. We also develop a simple model to disentangle intrinsic color variations and reddening by dust, and expand the method to incorporate U-band light curves and to more easily accommodate prior constraints on any of the model parameters. We apply this method to 133 nearby SN Ia, including 95 objects in the Hubble flow (cz >= 2500 km/s), which give an intrinsic dispersion of less than 7% in distance. The Hubble flow sample, which is of critical importance to all cosmological uses of SN Ia, is the largest ever presented with homogeneous distances. We find the Hubble flow supernovae with Hd >= 7400 km/s yield an expansion rate that is 6.5 +/- 1.8% lower than the rate determined from supernovae within that distance, and this can have a large effect on measurements of the dark energy equation of state with SN Ia. Peculiar velocities of SN Ia host galaxies in the rest frame of the Local Group are consistent with the dipole measured in the Cosmic Microwave Background. Direct fits of SN Ia that are significantly reddened by dust in their host galaxies suggest their mean extinction law may be described by R_V ~= 2.7, but optical colors alone provide weak constraints on R_V.

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arXiv2006

New Hubble Space Telescope Discoveries of Type Ia Supernovae at z > 1: Narrowing Constraints on the Early Behavior of Dark Energy

We have discovered 21 new Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to trace the history of cosmic expansion over the last 10 billion years. These objects, which include 13 spectroscopically confirmed SNe Ia at z > 1, were discovered during 14 epochs of reimaging of the GOODS fields North and South over two years with the Advanced Camera for Surveys on HST. Together with a recalibration of our previous HST-discovered SNe Ia, the full sample of 23 SNe Ia at z > 1 provides the highest-redshift sample known. Combined with previous SN Ia datasets, we measured H(z) at discrete, uncorrelated epochs, reducing the uncertainty of H(z>1) from 50% to under 20%, strengthening the evidence for a cosmic jerk--the transition from deceleration in the past to acceleration in the present. The unique leverage of the HST high-redshift SNe Ia provides the first meaningful constraint on the dark energy equation-of-state parameter at z >1. The result remains consistent with a cosmological constant (w(z)=-1), and rules out rapidly evolving dark energy (dw/dz >>1). The defining property of dark energy, its negative pressure, appears to be present at z>1, in the epoch preceding acceleration, with ~98% confidence in our primary fit. Moreover, the z>1 sample-averaged spectral energy distribution is consistent with that of the typical SN Ia over the last 10 Gyr, indicating that any spectral evolution of the properties of SNe Ia with redshift is still below our detection threshold.

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arXiv2006

The Velocity Field of the Local Universe from Measurements of Type Ia Supernovae

We present a measurement of the velocity flow of the local universe relative to the CMB rest frame, based on the Jha, Riess & Kirshner (2007) sample of 133 low redshift type Ia supernovae. At a depth of 4500 km/s we find a dipole amplitude of 279+-68 km/s in the direction (l,b) = (285+-18,-10+-15), consistent with earlier measurements and with the assumption that the local velocity field is dominated by the Great Attractor region. At a larger depth of 5900 km/s we find a shift in the dipole direction towards the Shapley concentration. We also present the first measurement of the quadrupole term in the local velocity flow at these depths. Finally, we have performed detailed studies based on N-body simulations of the expected precision with which the lowest multipoles in the velocity field can be measured out to redshifts of order 0.1. Our mock catalogues are in good agreement with current observations, and demonstrate that our results are robust with respect to assumptions about the influence of local environment on the type Ia supernova rate.

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arXiv2006

Late-Time Spectroscopy of SN 2002cx: The Prototype of a New Subclass of Type Ia Supernovae

We present Keck optical spectra of SN 2002cx, the most peculiar known Type Ia supernova (SN Ia), taken 227 and 277 days past maximum light. Astonishingly, the spectra are not dominated by the forbidden emission lines of iron that are a hallmark of thermonuclear supernovae in the nebular phase. Instead, we identify numerous P-Cygni profiles of Fe II at very low expansion velocities of about 700 km/s, which are without precedent in SNe Ia. We also report the tentative identification of low-velocity O I in these spectra, suggesting the presence of unburned material near the center of the exploding white dwarf. SN 2002cx is the prototype of a new subclass of SNe Ia, with spectral characteristics that may be consistent with recent pure deflagration models of Chandrasekhar-mass thermonuclear supernovae. These are distinct from the majority of SNe Ia, for which an alternative explosion mechanism, such as a delayed detonation, may be required.

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arXiv2006

Spectropolarimetry of the Peculiar Type Ia SN 2005hk

We present Keck spectropolarimetry of the unusual type Ia supernova (SN Ia) 2005hk several days before maximum light. An analysis of the high signal-to-noise-ratio total-flux spectrum shows the object's extreme similarity to the peculiar SN 2002cx. SN 2005hk has an optical spectrum dominated by Fe III lines and only weak lines of intermediate-mass elements, unlike a normal SN Ia at this epoch. The photospheric velocity measured from the minima of strong absorption lines is very low for an SN Ia (~6000 km s^{-1}), solidifying the connection to SN 2002cx. The spectrum-synthesis code Synow was used to identify the presence of iron-peak elements, intermediate-mass elements, and possibly unburned carbon at similar velocities in the outer ejecta of SN 2005hk. Many weak spectral features remain unidentified. The spectropolarimetry shows a low level of continuum polarization (~0.4%) after correction for the interstellar component and only a weak Fe III line feature is detected. The level of continuum polarization is normal for an SN Ia, implying that the unusual features of SN 2005hk cannot be readily explained by large asymmetries.

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arXiv2005

Supernova Cosmology and the ESSENCE project

The proper usage of Type Ia supernovae (SNe Ia) as distance indicators has revolutionized cosmology, and added a new dominant component to the energy density of the Universe, dark energy. Following the discovery and confirmation era, the currently ongoing SNe Ia surveys aim to determine the properties of the dark energy. ESSENCE is a five year ground-based supernova survey aimed at finding and characterizing 200 SNe Ia in the redshift domain z=[0.2-0.8]. The goal of the project is to put constraints on the equation of state parameter, w, of the dark energy with an accuracy of <10%. This paper presents these ongoing efforts in the context of the current developments in observational cosmology.

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arXiv2005

The Calibration of the Swift/UVOT Optical Observations: A Recipe for Photometry

Swift/UVOT has the capability to provide critical insight into the physics of the early afterglows of gamma-ray bursts (GRBs). But without precise calibration of the UVOT to standard photometric systems, it is impossible to leverage late- time, ground-based follow-up data to the early-time UVOT observations. In this paper, we present a calibration of the Swift/UVOT photometry to the standard Johnson UBV system for the UVOT UBV filters,and a step-by-step photometry recipe for analyzing these data. We base our analysis on aperture photometry performed on the ground-based and UVOT observations of the local standard stars in the fields of supernovae (SNe) 2005am and 2005cf, and a number of Landolt standard stars.We find that the optimal photometry aperture radius for UVOT data is small (2".5 for unbinned data,3".0 for 2X2 binned data),and show that the coincidence- loss (C-loss) correction is important even for relatively faint magnitudes (mag 16 to 19). Based on a theoretically motivated model,we fit the C-loss correction with two parameters, the photometric zero point (ZP) and the saturation magnitude (m_inf), and derive tight constraints for both parameters [sigma(ZP) = 0.01 mag and sigma(m_inf) = 0.02 mag)].We find that the color term correction is not necessary for the UVOT B and V filters,but is necessary for the U filter for blue objects [(U - V) < 0.4 mag]. We also apply our calibration results to the UVOT observations of GRB 050603. There is a scatter of ~0.04--0.08 mag in our final UVOT photometry, the cause of which is unclear, but may be partly due to the spatial variation in the pixel sensitivity of the UVOT detector.

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arXiv2005

UBVRI Light Curves of 44 Type Ia Supernovae

We present UBVRI photometry of 44 type-Ia supernovae (SN Ia) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SN Ia to date, nearly doubling the number of well-observed, nearby SN Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SN Ia observed at high redshift. The decline rate of SN Ia U-band light curves correlates well with the decline rate in other bands, as does the U-B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ~40% intrinsic scatter compared to B-band.

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arXiv2005

Identification of the Red Supergiant Progenitor of Supernova 2005cs: Do the Progenitors of Type II-P Supernovae Have Low Mass?

The stars that end their lives as supernovae (SNe) have been directly observed in only a handful of cases, due mainly to the extreme difficulty in identifying them in images obtained prior to the SN explosions. Here we report the identification of the progenitor for the recent Type II-plateau (core-collapse) SN 2005cs in pre-explosion archival images of the Whirlpool Galaxy (M51) obtained with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). From high-quality ground-based images of the SN from the Canada-France-Hawaii Telescope, we precisely determine the position of the SN and are able to isolate the SN progenitor to within 0".04 in the HST/ACS optical images. We further pinpoint the SN location to within 0".005 from HST/ACS ultraviolet images of the SN, confirming our progenitor identification. From photometry of the SN progenitor obtained with the pre-SN ACS images, and also limits to its brightness in pre-SN HST/NICMOS images, we infer that the progenitor is a red supergiant star of spectral type K0--M3, with initial mass 7--9 Msun. We also discuss the implications of the SN 2005cs progenitor identification and its mass estimate. There is an emerging trend that the most common Type II-plateau SNe originate from low-mass supergiants 8--15 Msun.

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arXiv2005

Cepheid Calibrations from the Hubble Space Telescope of the Luminosity of Two Recent Type Ia Supernovae and a Re-determination of the Hubble Constant

We report observations of two nearby Type Ia supernovae (SNe Ia) for which observations of Cepheid variables in the host galaxies have been obtained with the Hubble Space Telescope: SN 1994ae in NGC 3370 and SN 1998aq in NGC 3982. For NCG 3370, we used the Advanced Camera for Surveys to observe 64 Cepheids that yield a distance of 29 Mpc, the farthest direct measurement of Cepheids. We have measured emission lines from H II regions in both host galaxies which provide metallicity-dependent corrections to their period-luminosity relations. These two SNe Ia double the sample of ``ideal'' luminosity calibrators: objects with well-observed and well-calibrated light curves of typical shape and with low reddening. By comparing them to all similarly well-measured SNe Ia in the Hubble flow, we find that H_0 = 73 +/- 4 (statistical) +/- 5 (systematic). A detailed analysis demonstrates that most of the past disagreement over the value of H_0 as determined from SNe Ia is abated by the replacement of past, problematic data by more accurate and precise, modern data.

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arXiv2005

A transiting extrasolar giant planet around the star OGLE-TR-10

We report a transiting extrasolar giant planet around the star OGLE-TR-10 (orbital period = 3.1 days), which was uncovered as a candidate by the OGLE team in their photometric survey towards the Galactic center (Udalski et al, 2002a). We observed OGLE-TR-10 spectroscopically over a period of two years (2002-2004) using the HIRES instrument with an iodine cell on the Keck I telescope, and measured small radial velocity variations that are consistent with the presence of a planetary companion. This confirms the earlier identification of OGLE-TR-10b by our team and also recently by Bouchy et al (2004a) as a possible planet. Additionally, in this paper we are able to rule out a blend scenario as an alternative explanation. From an analysis combining all available radial velocity measurements with the OGLE light curve we find that OGLE-TR-10b has a mass of 0.57+/-0.12 MJup and a radius of 1.24+/-0.09 RJup. These parameters bear close resemblance to those of the first known transiting extrasolar planet, HD 209458b.

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arXiv2004

Testing blend scenarios for extrasolar transiting planet candidates. II. -- OGLE-TR-56

We re-examine the photometric and spectroscopic evidence available for the star OGLE-TR-56, recently discovered to harbor a giant planet presenting transits and orbiting with a period of 1.21 days. We investigate the possibility that the observational signatures reported might be the result of "blending" with the light of an eclipsing binary along the same line of sight. Using techniques developed earlier we perform fits to the light curve under a variety of blend configurations, subject to all observational constraints, and we infer further properties of these possible blends. We then carry out realistic end-to-end simulations based on those properties in order to quantify the spectral line asymmetries and radial velocity variations expected from such scenarios. We confront these calculations with the observations. The results from these models are clearly inconsistent with the measured radial velocity and bisector span variations, ruling out blends and confirming the planetary nature of the companion. The example of OGLE-TR-56 serves to illustrate the sort of tests that can and should be performed on transiting planet candidates to eliminate the possibility of "false positives", and in particular, line-of-sight contamination from eclipsing binaries.

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arXiv2004

Hubble Space Telescope and Ground-Based Observations of SN 1993J and SN 1998S: CNO Processing in the Progenitors

Ground-based and Hubble Space Telescope observations are presented for SN 1993J and SN 1998S. SN 1998S shows strong, relatively narrow circumstellar emission lines of N III-V and C III-IV, as well as broad lines from the ejecta. Both the broad ultraviolet and optical lines in SN 1998S indicate an expansion velocity of ~7,000 km/s. The broad emission components of Ly-alpha and Mg II are strongly asymmetrical after day 72 past the explosion, and differ in shape from H-alpha. Different models based on dust extinction from dust in the ejecta or shock region, in combination with H-alpha from a circumstellar torus, are discussed. It is concluded, however, that the double-peaked line profiles are more likely to arise as a result of optical depth effects in the narrow, cool, dense shell behind the reverse shock, than in a torus-like region. The ultraviolet lines of SN 1993J are broad, with a box-like shape, coming from the ejecta and a cool dense shell. The shapes of the lines are well fitted with a shell with inner velocity ~7,000 km/s and outer velocity ~10,000 km/s. For both SN 1993J and SN 1998S a strong nitrogen enrichment is found, with N/C~12.4 in SN 1993J and N/C~6.0 in SN 1998S. From a compilation of all supernovae with determined CNO ratios, we discuss the implications of these observations for the structure of the progenitors of Type II supernovae.

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arXiv2004

The Luminosity of SN 1999by in NGC 2841 and the Nature of `Peculiar' Type Ia Supernovae

We present UBVRIJHK photometry and optical spectroscopy of the so-called 'peculiar' Type Ia supernova 1999by in NGC 2841. The observations began one week before visual maximum light which is well-defined by daily observations. The light curves and spectra are similar to those of the prototypical subluminous event SN 1991bg. We find that maximum light in B occurred on 1999 May 10.3 UT (JD 2,451,308.8 +/- 0.3) with B=13.66 +/- 0.02 mag and a color of B_max-V_max=0.51 +/- 0.03 mag. The late-time color implies minimal dust extinction from the host galaxy. Our photometry, when combined with the recent Cepheid distance to NGC 2841 (Macri et al. 2001), gives a peak absolute magnitude of M_B=-17.15 +/- 0.23 mag, making SN 1999by one of the least luminous Type Ia events ever observed. We estimate a decline rate parameter of dm15(B)=1.90 mag, versus 1.93 for SN 1991bg, where 1.10 is typical for so-called 'normal' events. We compare SN 1999by with other subluminous events and find that the B_max-V_max color correlates strongly with the decline rate and may be a more sensitive indicator of luminosity than the fading rate for these objects. We find a good correlation between luminosity and the depth of the spectral feature at 580 nm, which had been attributed solely to Si II. We show that in cooler photospheres the 580 nm feature is dominated by Ti II, which provides a simple physical explanation for the correlation. Using only subluminous Type Ia supernovae we derive a Hubble parameter of H_0=75 +12 -11 km/s Mpc, consistent with values found from brighter events.

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arXiv2004

Testing blend scenarios for extrasolar transiting planet candidates. I. -- OGLE-TR-33: a false positive

We report high-resolution spectroscopic follow-up observations of the faint transiting planet candidate OGLE-TR-33 (V = 14.7), located in the direction of the Galactic center. Small changes in the radial velocity of the star were detected that suggested initially the presence of a large planet or brown dwarf in orbit. However, further analysis revealed spectral line asymmetries that change in phase with the 1.95-day period, casting doubt on those measurements. These asymmetries make it more likely that the transit-like events in the light curve are the result of contamination from the light of an eclipsing binary along the same line of sight (referred to as a "blend"). We performed detailed simulations in which we generated synthetic light curves resulting from such blend scenarios and fitted them to the measured light curve. Guided by these fits and the inferred properties of the stars, we uncovered a second set of lines in our spectra that correspond to the primary of the eclipsing binary and explain the asymmetries. Using all the constraints from spectroscopy we were then able to construct a model that satisfies all the observations, and to characterize the three stars based on model isochrones. OGLE-TR-33 is fully consistent with being a hierarchical triple system composed of a slightly evolved F6 star (the brighter object) near the end of its main-sequence phase, and an eclipsing binary with a K7-M0 star orbiting an F4 star. The application to OGLE-TR-33 of the formalism developed to fit light curves of transit candidates illustrates the power of such simulations for predicting additional properties of the blend and for guiding further observations that may serve to confirm that scenario, thereby ruling out a planet. Tests such as this can be very important for validating faint candidates.

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arXiv2004

A transiting extrasolar giant planet around the star OGLE-TR-113

We report the independent discovery of a new extrasolar transiting planet around OGLE-TR-113, a candidate star from the Optical Gravitational Lensing Experiment. Small radial-velocity variations have been detected based on observations conducted with the MIKE spectrograph on the Magellan I (Baade) telescope at the Las Campanas Observatory (Chile) during 2003. We have also carried out a light-curve analysis incorporating new photometry and realistic physical parameters for the star. OGLE-TR-113b has an orbital period of only 1.43 days, a mass of 1.08 +/- 0.28 M_Jup, and a radius of 1.09 +/- 0.10 R_Jup. Similar parameters have been obtained very recently in an independent study by Bouchy et al., from observations taken a year later. The orbital period of OGLE-TR-113b, and also that of the previously announced planet OGLE-TR-56b (P_orb = 1.21 days) -- the first two found photometrically -- are much shorter than the apparent cutoff of close-in giant planets at 3-4-day periods found from high-precision radial velocities surveys. Along with a third case reported by Bouchy et al. (OGLE-TR-132b, P_orb = 1.69 days), these objects appear to form a new class of ``very hot Jupiters'' that pose very interesting questions for theoretical study.

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arXiv2004

Type Ia Supernova Discoveries at z>1 From the Hubble Space Telescope: Evidence for Past Deceleration and Constraints on Dark Energy Evolution

We have discovered 16 Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to provide the first conclusive evidence for cosmic deceleration that preceded the current epoch of cosmic acceleration. These objects, discovered during the course of the GOODS ACS Treasury program, include 6 of the 7 highest-redshift SNe Ia known, all at z>1.25, and populate the Hubble diagram in unexplored territory. The luminosity distances to these and 170 previous SNe Ia are provided. A purely kinematic interpretation of the SN Ia sample provides evidence at the > 99% confidence level for a transition from deceleration to acceleration or similarly, strong evidence for a cosmic jerk. Using a simple model of the expansion history, the transition between the two epochs is constrained to be at z=0.46 +/- 0.13. The data are consistent with the cosmic concordance model of Omega_M ~ 0.3, Omega_Lambda~0.7 (chi^2_dof=1.06), and are inconsistent with a simple model of evolution or dust as an alternative to dark energy. For a flat Universe with a cosmological constant. When combined with external flat-Universe constraints we find w=-1.02 + 0.13 - 0.19 (and $<-0.76 at the 95% confidence level) for an assumed static equation of state of dark energy, P = wρc^2. Joint constraints on both the recent equation of state of dark energy, $w_0$, and its time evolution, dw/dz, are a factor of ~8 more precise than its first estimate and twice as precise as those without the SNe Ia discovered with HST. Our constraints are consistent with the static nature of and value of w expected for a cosmological constant (i.e., w_0 = -1.0, dw/dz = 0), and are inconsistent with very rapid evolution of dark energy. We address consequences of evolving dark energy for the fate of the Universe.

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