52 research outputs found
Direct Observation of Cosmic Strings via their Strong Gravitational Lensing Effect: II. Results from the HST/ACS Image Archive
We have searched 4.5 square degrees of archival HST/ACS images for cosmic
strings, identifying close pairs of similar, faint galaxies and selecting
groups whose alignment is consistent with gravitational lensing by a long,
straight string. We find no evidence for cosmic strings in five large-area HST
treasury surveys (covering a total of 2.22 square degrees), or in any of 346
multi-filter guest observer images (1.18 square degrees). Assuming that
simulations ccurately predict the number of cosmic strings in the universe,
this non-detection allows us to place upper limits on the unitless Universal
cosmic string tension of G mu/c^2 < 2.3 x 10^-6, and cosmic string density of
Omega_s < 2.1 x 10^-5 at the 95% confidence level (marginalising over the other
parameter in each case). We find four dubious cosmic string candidates in 318
single filter guest observer images (1.08 square degrees), which we are unable
to conclusively eliminate with existing data. The confirmation of any one of
these candidates as cosmic strings would imply G mu/c^2 ~ 10^-6 and Omega_s ~
10^-5. However, we estimate that there is at least a 92% chance that these
string candidates are random alignments of galaxies. If we assume that these
candidates are indeed false detections, our final limits on G mu/c^2 and
Omega_s fall to 6.5 x 10^-7 and 7.3 x 10^-6. Due to the extensive sky coverage
of the HST/ACS image archive, the above limits are universal. They are quite
sensitive to the number of fields being searched, and could be further reduced
by more than a factor of two using forthcoming HST data.Comment: 21 pages, 18 figure
Now You See It, Now You Don't: The Disappearing Central Engine of the Quasar J1011+5442
We report the discovery of a new "changing-look" quasar, SDSS
J101152.98+544206.4, through repeat spectroscopy from the Time Domain
Spectroscopic Survey. This is an addition to a small but growing set of quasars
whose blue continua and broad optical emission lines have been observed to
decline by a large factor on a time scale of approximately a decade. The 5100
Angstrom monochromatic continuum luminosity of this quasar drops by a factor of
> 9.8 in a rest-frame time interval of < 9.7 years, while the broad H-alpha
luminosity drops by a factor of 55 in the same amount of time. The width of the
broad H-alpha line increases in the dim state such that the black hole mass
derived from the appropriate single-epoch scaling relation agrees between the
two epochs within a factor of 3. The fluxes of the narrow emission lines do not
appear to change between epochs. The light curve obtained by the Catalina Sky
Survey suggests that the transition occurs within a rest-frame time interval of
approximately 500 days. We examine three possible mechanisms for this
transition suggested in the recent literature. An abrupt change in the
reddening towards the central engine is disfavored by the substantial
difference between the timescale to obscure the central engine and the observed
timescale of the transition. A decaying tidal disruption flare is consistent
with the decay rate of the light curve but not with the prolonged bright state
preceding the decay, nor can this scenario provide the power required by the
luminosities of the emission lines. An abrupt drop in the accretion rate onto
the supermassive black hole appears to be the most plausible explanation for
the rapid dimming.Comment: Submitted to MNRA
Towards an Understanding of Changing-Look Quasars: An Archival Spectroscopic Search in SDSS
The uncertain origin of the recently-discovered `changing-looking' quasar
phenomenon -- in which a luminous quasar dims significantly to a quiescent
state in repeat spectroscopy over ~10 year timescales -- may present unexpected
challenges to our understanding of quasar accretion. To better understand this
phenomenon, we take a first step to building a sample of changing-look quasars
with a systematic but simple archival search for these objects in the Sloan
Digital Sky Survey Data Release 12. By leveraging the >10 year baselines for
objects with repeat spectroscopy, we uncover two new changing-look quasars, and
a third discovered previously. Decomposition of the multi-epoch spectra and
analysis of the broad emission lines suggest that the quasar accretion disk
emission dims due to rapidly decreasing accretion rates (by factors of >2.5),
while disfavoring changes in intrinsic dust extinction for the two objects
where these analyses are possible. Broad emission line energetics also support
intrinsic dimming of quasar emission as the origin for this phenomenon rather
than transient tidal disruption events or supernovae. Although our search
criteria included quasars at all redshifts and transitions from either
quasar-like to galaxy-like states or the reverse, all of the clear cases of
changing-look quasars discovered were at relatively low-redshift (z ~ 0.2 -
0.3) and only exhibit quasar-like to galaxy-like transitions.Comment: 15 pages, 8 figures. Updated to accepted versio
The Time Domain Spectroscopic Survey: Variable Object Selection and Anticipated Results
We present the selection algorithm and anticipated results for the Time Domain Spectroscopic Survey (TDSS). TDSS is an Sloan Digital Sky Survey (SDSS)-IV Extended Baryon Oscillation Spectroscopic Survey (eBOSS) subproject that will provide initial identification spectra of approximately 220,000 luminosity-variable objects (variable stars and active galactic nuclei across 7500 deg2 selected from a combination of SDSS and multi-epoch Pan-STARRS1 photometry. TDSS will be the largest spectroscopic survey to explicitly target variable objects, avoiding pre-selection on the basis of colors or detailed modeling of specific variability characteristics. Kernel Density Estimate analysis of our target population performed on SDSS Stripe 82 data suggests our target sample will be 95% pure (meaning 95% of objects we select have genuine luminosity variability of a few magnitudes or more). Our final spectroscopic sample will contain roughly 135,000 quasars and 85,000 stellar variables, approximately 4000 of which will be RR Lyrae stars which may be used as outer Milky Way probes. The variability-selected quasar population has a smoother redshift distribution than a color-selected sample, and variability measurements similar to those we develop here may be used to make more uniform quasar samples in large surveys. The stellar variable targets are distributed fairly uniformly across color space, indicating that TDSS will obtain spectra for a wide variety of stellar variables including pulsating variables, stars with significant chromospheric activity, cataclysmic variables, and eclipsing binaries. TDSS will serve as a pathfinder mission to identify and characterize the multitude of variable objects that will be detected photometrically in even larger variability surveys such as Large Synoptic Survey Telescope
The First High Redshift Quasar from Pan-STARRS
We present the discovery of the first high redshift (z > 5.7) quasar from the
Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1 or PS1).
This quasar was initially detected as an i dropoutout in PS1, confirmed
photometrically with the SAO Widefield InfraRed Camera (SWIRC) at Arizona's
Multiple Mirror Telescope (MMT) and the Gamma-Ray Burst Optical/Near-Infrared
Detector (GROND) at the MPG 2.2 m telescope in La Silla. The quasar was
verified spectroscopically with the the MMT Spectrograph, Red Channel and the
Cassegrain Twin Spectrograph (TWIN) at the Calar Alto 3.5 m telescope. It has a
redshift of 5.73, an AB z magnitude of 19.4, a luminosity of 3.8 x 10^47 erg/s
and a black hole mass of 6.9 x 10^9 solar masses. It is a Broad Absorption Line
quasar with a prominent Ly-beta peak and a very blue continuum spectrum. This
quasar is the first result from the PS1 high redshift quasar search that is
projected to discover more than a hundred i dropout quasars, and could
potentially find more than 10 z dropout (z > 6.8) quasars.Comment: 8 pages, 7 figure
Recommended from our members
Direct Observation of Cosmic Strings Via Their Strong Gravitational Lensing Effect. 1. Predictions for High Resolution Imaging Surveys
We use current theoretical estimates for the density of long cosmic strings to predict the number of strong gravitational lensing events in astronomical imaging surveys as a function of angular resolution and survey area. We show that angular resolution is the single most important factor, and that interesting limits on the dimensionless string tension G{mu}/c{sup 2} can be obtained by existing and planned surveys. At the resolution of the Hubble Space Telescope (0'.14), it is sufficient to survey of order a square degree -- well within reach of the current HST archive -- to probe the regime G{mu}/c{sup 2} {approx} 10{sup -8}. If lensing by cosmic strings is not detected, such a survey would improve the limit on the string tension by an order of magnitude on that available from the cosmic microwave background. At the resolution (0'.028) attainable with the next generation of large ground based instruments, both in the radio and the infra-red with adaptive optics, surveying a sky area of order ten square degrees will allow us to probe the G{mu}/c{sup 2} {approx} 10{sup -9} regime. These limits will not be improved significantly by increasing the solid angle of the survey
The Time-Domain Spectroscopic Survey: Understanding the Optically Variable Sky with SEQUELS in SDSS-III
The Time-Domain Spectroscopic Survey (TDSS) is an SDSS-IV eBOSS subproject
primarily aimed at obtaining identification spectra of ~220,000
optically-variable objects systematically selected from SDSS/Pan-STARRS1
multi-epoch imaging. We present a preview of the science enabled by TDSS, based
on TDSS spectra taken over ~320 deg^2 of sky as part of the SEQUELS survey in
SDSS-III, which is in part a pilot survey for eBOSS in SDSS-IV. Using the
15,746 TDSS-selected single-epoch spectra of photometrically variable objects
in SEQUELS, we determine the demographics of our variability-selected sample,
and investigate the unique spectral characteristics inherent in samples
selected by variability. We show that variability-based selection of quasars
complements color-based selection by selecting additional redder quasars, and
mitigates redshift biases to produce a smooth quasar redshift distribution over
a wide range of redshifts. The resulting quasar sample contains systematically
higher fractions of blazars and broad absorption line quasars than from
color-selected samples. Similarly, we show that M-dwarfs in the TDSS-selected
stellar sample have systematically higher chromospheric active fractions than
the underlying M-dwarf population, based on their H-alpha emission. TDSS also
contains a large number of RR Lyrae and eclipsing binary stars with
main-sequence colors, including a few composite-spectrum binaries. Finally, our
visual inspection of TDSS spectra uncovers a significant number of peculiar
spectra, and we highlight a few cases of these interesting objects. With a
factor of ~15 more spectra, the main TDSS survey in SDSS-IV will leverage the
lessons learned from these early results for a variety of time-domain science
applications.Comment: 17 pages, 14 figures, submitted to Ap
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