156 research outputs found
Conformal gravity rotation curves with a conformal Higgs halo
We discuss the effect of a conformally coupled Higgs field on Conformal Gravity (CG) predictions for the rotation curves of galaxies. The Mannheim-Kazanas (MK) metric is a valid vacuum solution of CG's 4-th order Poisson equation if and only if the Higgs field has a particular radial profile, S(r) = S0 a/(r + a), decreasing from S0 at r = 0 with radial scale length a. Since particle rest masses scale with S(r)/S0, their world lines do not follow time-like geodesics of the MK metric gμν, as previously assumed, but rather those of the Higgs-frame MK metric ḡμν = Ω2 gμν, with the conformal factor Ω(r) = S(r)/S0. We show that the required stretching of the MK metric exactly cancels the linear potential that has been invoked to fit galaxy rotation curves without dark matter. We also formulate, for spherical structures with a Higgs halo S(r), the CG equations that must be solved for viable astrophysical tests of CG using galaxy and cluster dynamics and lensing.Publisher PDFPeer reviewe
Dust and the intrinsic spectral index of quasar variations : hints of finite stress at the innermost stable circular orbit
Funding: The Cosmic Dawn Center (DAWN) is funded by the Danish National Research Foundation under grant No. 140. J.W. acknowledges support from the European Research Council (ERC) Consolidator Grant funding scheme (project ConTExt, grant No. 648179), and from the University of St Andrews Undergraduate Research Assistant Scheme. K.H. acknowledges support from STFC grant ST/R000824/1.We present a study of 9242 spectroscopically-confirmed quasars with multi-epoch ugriz photometry from the SDSS Southern Survey. By fitting a separable linear model to each quasar's spectral variations, we decompose their five-band spectral energy distributions into variable (disc) and non-variable (host galaxy) components. In modelling the disc spectra, we include attenuation by dust on the line of sight through the host galaxy to its nucleus. We consider five commonly used attenuation laws, and find that the best description is by dust similar to that of the Small Magellanic Cloud, inferring a lack of carbonaceous grains from the relatively weak 2175 Å absorption feature. We go on to construct a composite spectrum for the quasar variations spanning 700 to 8000 Å. By varying the assumed power-law Lv ∝ va spectral slope, we find a best-fit value = 0:71 ± 0:02, excluding at high confidence the canonical Lv ∝ v1/3 prediction for a steady-state accretion disc with a T ∝ r-3/4 temperature profile. The bluer spectral index of the observed quasar variations instead supports the model of Mummery & Balbus in which a steeper temperature profile, T ∝ r-7/8, develops as a result of finite magnetically-induced stress at the innermost stable circular orbit extracting energy and angular momentum from the black hole spin.Publisher PDFPeer reviewe
Difference image analysis : automatic kernel design using information criteria
This publication was made possible by NPRP grant # X-019-1-006 from the Qatar National Research Fund (a member of Qatar Foundation).We present a selection of methods for automatically constructing an optimal kernel model for difference image analysis which require very few external parameters to control the kernel design. Each method consists of two components; namely, a kernel design algorithm to generate a set of candidate kernel models, and a model selection criterion to select the simplest kernel model from the candidate models that provides a sufficiently good fit to the target image. We restricted our attention to the case of solving for a spatially invariant convolution kernel composed of delta basis functions, and we considered 19 different kernel solution methods including six employing kernel regularization. We tested these kernel solution methods by performing a comprehensive set of image simulations and investigating how their performance in terms of model error, fit quality, and photometric accuracy depends on the properties of the reference and target images. We find that the irregular kernel design algorithm employing unregularized delta basis functions, combined with either the Akaike or Takeuchi information criterion, is the best kernel solution method in terms of photometric accuracy. Our results are validated by tests performed on two independent sets of real data. Finally, we provide some important recommendations for software implementations of difference image analysis.Publisher PDFPeer reviewe
ROME/REA : a gravitational microlensing search for exoplanets beyond the snow line on a global network of robotic telescopes
Funding: KH acknowledges support from STFC grant ST/R000824/1.Planet population synthesis models predict an abundance of planets with semimajor axes between 1 and 10 au, yet they lie at the edge of the detection limits of most planet finding techniques. Discovering these planets and studying their distribution is critical to understanding the physical processes that drive planet formation. ROME/REA is a gravitational microlensing project whose main science driver is to discover exoplanets in the cold outer regions of planetary systems. To achieve this, it uses a novel approach combining a multiband survey with reactive follow-up observations, exploiting the unique capabilities of the Las Cumbres Observatory global network of robotic telescopes combined with a Target and Observation Manager system. We present the main science objectives and a technical overview of the project, including initial results.PostprintPeer reviewe
Continuum reverberation mapping of Mrk 876 over three years with remote robotic observatories
Funding: Research at UC Irvine is supported by NSF grant AST-1907290. HL acknowledges a Daphne Jackson Fellowship sponsored by the Science and Technology Facilities Council (STFC), UK. ERC acknowledges support by the NRF of South Africa. TT acknowledges support from NSF through grant NSF-AST-1907208.Continuum reverberation mapping probes the sizescale of the optical continuum-emitting region in active galactic nuclei (AGN). Through 3 years of multiwavelength photometric monitoring in the optical with robotic observatories, we perform continuum reverberation mapping on Mrk~876. All wavebands show large amplitude variability and are well correlated. Slow variations in the light curves broaden the cross-correlation function (CCF) significantly, requiring detrending in order to robustly recover interband lags. We measure consistent interband lags using three techniques (CCF, JAVELIN, PyROA), with a lag of around 13~days from u to z. These lags are longer than the expected radius of 12~days for the self-gravitating radius of the disk. The lags increase with wavelength roughly following λ4/3, as would be expected from thin disk theory, but the lag normalization is approximately a factor of 3 longer than expected, as has also been observed in other AGN. The lag in the i band shows an excess which we attribute to variable Hα broad-line emission. A flux-flux analysis shows a variable spectrum that follows fν ∝ λ-1/3 as expected for a disk, and an excess in the i band that also points to strong variable Hα emission in that band.Publisher PDFPeer reviewe
NGC 5548 in a Low-Luminosity State: Implications for the Broad-Line Region
We describe results from a new ground-based monitoring campaign on NGC 5548,
the best studied reverberation-mapped AGN. We find that it was in the lowest
luminosity state yet recorded during a monitoring program, namely L(5100) = 4.7
x 10^42 ergs s^-1. We determine a rest-frame time lag between flux variations
in the continuum and the Hbeta line of 6.3 (+2.6/-2.3) days. Combining our
measurements with those of previous campaigns, we determine a weighted black
hole mass of M_BH = 6.54 (+0.26/-0.25) x 10^7 M_sun based on all broad emission
lines with suitable variability data. We confirm the previously-discovered
virial relationship between the time lag of emission lines relative to the
continuum and the width of the emission lines in NGC 5548, which is the
expected signature of a gravity-dominated broad-line region. Using this lowest
luminosity state, we extend the range of the relationship between the
luminosity and the time lag in NGC 5548 and measure a slope that is consistent
with alpha = 0.5, the naive expectation for the broad line region for an
assumed form of r ~ L^alpha. This value is also consistent with the slope
recently determined by Bentz et al. for the population of reverberation-mapped
AGNs as a whole.Comment: 24 pages, 3 tables, 7 figures, accepted for publication in Ap
The pre-main sequence binary HK Ori : Spectro-astrometry and EXPORT data
In this paper we present multi-epoch observations of the pre-main sequence
binary HK Ori. These data have been drawn from the EXPORT database and are
complemented by high quality spectro-astrometric data of the system. The
spectroscopic data appear to be very well represented by a combination of an A
dwarf star spectrum superposed on a (sub-)giant G-type spectrum. The radial
velocity of the system is consistent with previous determinations, and does not
reveal binary motion, as expected for a wide binary. The spectral, photometric
and polarimetric properties and variability of the system indicate that the
active object in the system is a T Tauri star with UX Ori characteristics. The
spectro-astrometry of HK Ori is sensitive down to milli-arcsecond scales and
confirms the speckle interferometric results from Leinert et al. The
spectro-astrometry allows with fair certainty the identification of the active
star within the binary, which we suggest to be a G-type T Tauri star based on
its spectral characteristics.Comment: MNRAS in press 8 pages 7 figure
Steps toward Determination of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei. XII. Ground-Based Monitoring of 3C 390.3
Results of a ground-based optical monitoring campaign on 3C 390.3 in 1994-95
are presented. The broad-band fluxes (B, V, R, and I), the spectrophotometric
optical continuum flux F(5177) and the integrated emission-line fluxes of Ha,
Hb, Hg, HeI, and HeII all show a nearly monotonic increase with episodes of
milder short-term variations superposed. The amplitude of the continuum
variations increases with decreasing wavelength (4400-9000 A). The optical
continuum variations follow the variations in the ultraviolet and X-ray with
time delays, measured from the centroids of the cross- correlation functions,
typically around 5 days, but with uncertainties also typically around 5 days;
zero time delay between the high-energy and low-energy continuum variations
cannot be ruled out. The strong optical emission lines Ha, Hb, Hg, and HeI
respond to the high-energy continuum variations with time delays typically
about 20 days, with uncertainties of about 8 days. There is some evidence that
HeII responds somewhat more rapidly, with a time delay of around 10 days, but
again, the uncertainties are quite large (~8 days). The mean and rms spectra of
the Ha and Hb line profiles provide indications for the existence of at least
three distinct components located at +-4000 and 0 km/s relative to the line
peak. The emission-line profile variations are largest near line center.Comment: 42 pages (Latex), 13 figures, 14 table
Supermassive black holes with high accretion rates in active galactic nuclei. XIII. Ultraviolet time lag of Hβ emission in Mrk 142
Funding: We acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), Discovery Grant RGPIN/04157. V.C.K. acknowledges the support of the Ontario Graduate Scholarships. C.H. acknowledges support from the National Science Foundation of China (12122305). The research of V.C.K. was partially supported by the New Technologies for Canadian Observatories, an NSERC CREATE program.We performed a rigorous reverberation-mapping analysis of the broad-line region (BLR) in a highly accreting (L/LEdd = 0.74–3.4) active galactic nucleus, Markarian 142 (Mrk 142), for the first time using concurrent observations of the inner accretion disk and the BLR to determine a time lag for the Hβ λ4861 emission relative to the ultraviolet (UV) continuum variations. We used continuum data taken with the Niel Gehrels Swift Observatory in the UVW2 band, and the Las Cumbres Observatory, Dan Zowada Memorial Observatory, and Liverpool Telescope in the g band, as part of the broader Mrk 142 multi-wavelength monitoring campaign in 2019. We obtained new spectroscopic observations covering the Hβ broad emission line in the optical from the Gemini North Telescope and the Lijiang 2.4-meter Telescope for a total of 102 epochs (over a period of eight months) contemporaneous to the continuum data. Our primary result states a UV-to-Hβ time lag of 8.68+0.75−0.72 days in Mrk 142 obtained from light-curve analysis with a Python-based Running Optimal Average algorithm. We placed our new measurements for Mrk 142 on the optical and UV radius-luminosity relations for NGC 5548 to understand the nature of the continuum driver. The positions of Mrk 142 on the scaling relations suggest that UV is closer to the “true” driving continuum than the optical. Furthermore, we obtain log(M•/M⊙) = 6.32 ± 0.29 assuming UV as the primary driving continuum.Publisher PDFPeer reviewe
A Reverberation-Based Mass for the Central Black Hole in NGC 4151
We have undertaken a new ground-based monitoring campaign to improve the
estimates of the mass of the central black hole in NGC 4151. We measure the lag
time of the broad H beta line response compared to the optical continuum at
5100 A and find a lag of 6.6 (+1.1/-0.8) days. We combine our data with the
recent reanalysis of UV emission lines by Metzroth et al. to calculate a
weighted mean of the black hole mass, M_BH = 4.57 (+0.57/-0.47) x 10^7 M_sun.
The absolute calibration of the black hole mass is based on normalization of
the AGN black hole mass - stellar velocity dispersion (M_BH - sigma_*)
relationship to that of quiescent galaxies by Onken et al. The scatter in the
M_BH - sigma_* relationship suggests that reverberation-mapping based mass
measurements are typically uncertain by a factor of 3-4.Comment: 17 pages, 3 tables, 3 figures, accepted for publication in Ap
- …