arXiv subjects

Geoffrey C. Bower

Publications and source records attributed to Geoffrey C. Bower.

At least 55 records · Page 3Linked to original sources

arXiv2020

Gravitational Test Beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole

The 2017 Event Horizon Telescope (EHT) observations of the central source in M87 have led to the first measurement of the size of a black-hole shadow. This observation offers a new and clean gravitational test of the black-hole metric in the strong-field regime. We show analytically that spacetimes that deviate from the Kerr metric but satisfy weak-field tests can lead to large deviations in the predicted black-hole shadows that are inconsistent with even the current EHT measurements. We use numerical calculations of regular, parametric, non-Kerr metrics to identify the common characteristic among these different parametrizations that control the predicted shadow size. We show that the shadow-size measurements place significant constraints on deviation parameters that control the second post-Newtonian and higher orders of each metric and are, therefore, inaccessible to weak-field tests. The new constraints are complementary to those imposed by observations of gravitational waves from stellar-mass sources.

gr-qc↗

arXiv2020

Monitoring the Morphology of M87* in 2009-2017 with the Event Horizon Telescope

The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature - a ring with azimuthal brightness asymmetry - and to quantify the image variability on event horizon scales. To address this need, we analyze M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain enough information to produce images, they are sufficient to constrain simple geometric models. We develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and validate our procedures using synthetic data. Applying the same approach to the observational data sets, we find the M87* morphology in 2009-2017 to be consistent with a persistent asymmetric ring of ~40 uas diameter. The position angle of the peak intensity varies in time. In particular, we find a significant difference between the position angle measured in 2013 and 2017. These variations are in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simulations. We show that quantifying the variability across multiple observational epochs has the potential to constrain the physical properties of the source, such as the accretion state or the black hole spin.

astro-ph.HE↗

arXiv2020

An Intensity Mapping Detection of Aggregate CO Line Emission at 3 mm

We present a detection of molecular gas emission at $z\sim1-5$ using the technique of line intensity mapping. We make use of a pair of 3 mm interferometric data sets, the first from the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS), and the second from a series of Atacama Compact Array (ACA) observations conducted between 2016 and 2018, targeting the COSMOS field. At 100 GHz, we measure non-zero power at 97.8% and 99.9% confidence in the ACA and ALMA data sets, respectively. In the joint result, we reject the zero-power hypothesis at 99.99% confidence, finding $\tilde{I}^{2}_{s}(ν)=770\pm210\ μ\textrm{K}^2\ \textrm{Hz}\ \textrm{sr}$. After accounting for sample variance effects, the estimated spectral shot power is $\tilde{I}^{2}_{s}(ν)=1010_{-390}^{+550}\ μ\textrm{K}^2\ \textrm{Hz}\ \textrm{sr}$. We derive a model for the various line species our measurement is expected to be sensitive to, and estimate the shot power to be $120_{-40}^{+80}\ μ\textrm{K}^2\ h^{-3}\,\textrm{Mpc}^{3}$, $200^{+120}_{-70}\ μ\textrm{K}^2\ h^{-3}\,\textrm{Mpc}^{3}$, and $90^{+70}_{-40}\ μ\textrm{K}^2\ h^{-3}\,\textrm{Mpc}^{3}$ for CO(2-1) at $z=1.3$, CO(3-2) at $z=2.5$, and CO(4-3) at $z=3.6$, respectively. Using line ratios appropriate for high-redshift galaxies, we find these results to be in good agreement with those from the CO Power Spectrum Survey (COPSS). Adopting $α_{\rm CO}=3.6\ M_{\odot}\ (\textrm{K}\ \textrm{km}\ \textrm{s}^{-1}\ \textrm{pc}^{2})^{-1}$, we estimate a cosmic molecular gas density of $ρ_{\textrm{H}_2}(z)\sim 10^{8}\ M_{\odot}\ \textrm{Mpc}^{-3}$ between $z=1-3$.

astro-ph.GA↗

arXiv2019

Chandra Spectral and Timing Analysis of Sgr A*'s Brightest X-ray Flares

We analyze the two brightest Chandra X-ray flares detected from Sagittarius A*, with peak luminosities more than 600 x and 245 x greater than the quiescent X-ray emission. The brightest flare has a distinctive double-peaked morphology --- it lasts 5.7 ksec ($\sim 2$ hours), with a rapid rise time of 1500 sec and a decay time of 2500 sec. The second flare lasts 3.4 ksec, with rise and decay times of 1700 sec and 1400 sec. These luminous flares are significantly harder than quiescence: the first has a power law spectral index $Γ= 2.06\pm 0.14$ and the second has $Γ= 2.03\pm 0.27$, compared to $Γ= 3.0\pm0.2$ for the quiescent accretion flow. These spectral indices (as well as the flare hardness ratios) are consistent with previously-detected Sgr A* flares, suggesting that bright and faint flares arise from similar physical processes. Leveraging the brightest flare's long duration and high signal-to-noise, we search for intraflare variability and detect excess X-ray power at a frequency of $ν\approx 3$ mHz, but show that it is an instrumental artifact and not of astrophysical origin. We find no other evidence (at the 95% confidence level) for periodic or quasi-periodic variability in either flares' time series. We also search for non-periodic excess power but do not find compelling evidence in the power spectrum. Bright flares like these remain our most promising avenue for identifying Sgr A*'s short timescale variability in the X-ray, which may probe the characteristic size scale for the X-ray emission region.

astro-ph.HE↗

arXiv2019

ALMA Observations of the Terahertz Spectrum of Sagittarius A*

We present ALMA observations at 233, 678, and 870 GHz of the Galactic Center black hole, Sagittarius A*. These observations reveal a flat spectrum over this frequency range with spectral index $α\approx -0.3$, where the flux density $S \propto ν^α$. We model the submm and far infrared spectrum with a one zone synchrotron model of thermal electrons. We infer electron densities $n = (2-5) \times 10^6$ cm$^{-3}$, electron temperatures $T_e = (1-3) \times 10^{11}$ K, and magnetic field strength $B = 10-50$ G. The parameter range can be further constrained using the observed quiescent X-ray luminosity. The flat submm spectrum results in a high electron temperature and implies that the emitting electrons are efficiently heated. We also find that the emission is most likely optically thin at 233 GHz. These results indicate that millimeter and submillimeter wavelength very long baseline interferometry of Sgr A* including those of the Event Horizon Telescope should see a transparent emission region down to event horizon scales.

astro-ph.HE↗

arXiv2018

The JCMT Transient Survey: An Extraordinary Submillimetre Flare in the T Tauri Binary System JW 566

The binary T Tauri system JW 566 in the Orion Molecular Cloud underwent an energetic, short-lived flare observed at submillimetre wavelengths by the SCUBA-2 instrument on 26 November 2016 (UT). The emission faded by nearly 50% during the 31 minute integration. The simultaneous source fluxes averaged over the observation are 500 +/- 107 mJy/beam at 450 microns and 466 +/- 47 mJy/beam at 850 microns. The 850 micron flux corresponds to a radio luminosity of $L_ν=8\times10^{19}$ erg/s/Hz, approximately one order of magnitude brighter (in terms of $νL_ν$) than that of a flare of the young star GMR-A, detected in Orion in 2003 at 3mm. The event may be the most luminous known flare associated with a young stellar object and is also the first coronal flare discovered at sub-mm wavelengths. The spectral index between 450 microns and 850 microns of $α= 0.11$ is broadly consistent with non-thermal emission. The brightness temperature was in excess of $6\times10^{4}$ K. We interpret this event to be a magnetic reconnection that energised charged particles to emit gyrosynchrotron/synchrotron radiation.

astro-ph.SR↗

arXiv2018

Micro-arcsecond structure of Sagittarius A* revealed by high-sensitivity 86 GHz VLBI observations

The compact radio source Sagittarius~A$^*$ (Sgr~A$^*$)in the Galactic Center is the primary supermassive black hole candidate. General relativistic magnetohydrodynamical (GRMHD) simulations of the accretion flow around Sgr\,A$^*$ predict the presence of sub-structure at observing wavelengths of $\sim 3$\,mm and below (frequencies of 86\,GHz and above). For very long baseline interferometry (VLBI) observations of Sgr\,A$^*$ at this frequency the blurring effect of interstellar scattering becomes subdominant, and arrays such as the High Sensitivity Array (HSA) and the global mm-VLBI Array (GMVA) are now capable of resolving potential sub-structure in the source. Such investigations improve our understanding of the emission geometry of the mm-wave emission of Sgr\,A$^*$, which is crucial for constraining theoretical models and for providing a background to interpret 1\,mm VLBI data from the Event Horizon Telescope (EHT). We performed high-sensitivity very long baseline interferometry (VLBI) observations of Sgr\,A$^*$ at 3\,mm using the Very Long Baseline Array (VLBA) and the Large Millimeter Telescope (LMT) in Mexico on two consecutive days in May 2015, with the second epoch including the Green Bank Telescope (GBT). We find an overall source geometry that matches previous findings very closely, showing a deviation in fitted model parameters less than 3\% over a time scale of weeks and suggesting a highly stable global source geometry over time. The reported sub-structure in the 3\,mm emission of Sgr\,A$^*$ is consistent with theoretical expectations of refractive noise on long baselines. However, comparing our findings with recent results from 1\,mm and 7\,mm VLBI observations, which also show evidence for east-west asymmetry, an intrinsic origin cannot be excluded. Confirmation of persistent intrinsic substructure will require further VLBI observations spread out over multiple epochs.

astro-ph.GA↗

arXiv2018

ALMA Polarimetry of Sgr A*: Probing the Accretion Flow from the Event Horizon to the Bondi Radius

Millimeter polarimetry of Sgr A* probes the linearly polarized emission region on a scale of $\sim 10$ Schwarzschild radii ($R_S$) as well as the dense, magnetized accretion flow on scales out to the Bondi radius ($\sim 10^5 R_S$) through Faraday rotation. We present here multi-epoch ALMA Band 6 (230 GHz) polarimetry of Sgr A*. The results confirm a mean rotation measure, ${\rm RM} \approx -5 \times 10^5 {\rm\ rad\ m^{-2}}$, consistent with measurements over the past 20 years and support an interpretation of the RM as originating from a radiatively inefficient accretion flow (RIAF) with $\dot{M} \approx 10^{-8} { \rm\ M_{\odot}\ y^{-1} }$. Variability is observed for the first time in the RM on time scales that range from hours to months. The long-term variations may be the result of changes in the line of sight properties in a turbulent accretion flow. Short-term variations in the apparent RM are not necessarily the result of Faraday rotation and may be the result of complex emission and propagatation effects close to the black hole, some of which have been predicted in numerical modeling. We also confirm the detection of circular polarization at a mean value of $-1.1 \pm 0.2 \%$. It is variable in amplitude on time scales from hours to months but the handedness remains unchanged from that observed in past centimeter- and millimeter-wavelength detections. These results provide critical constraints for the analysis and interpretation of Event Horizon Telescope data of Sgr A*, M87, and similar sources.

astro-ph.HE↗

arXiv2018

Galactic Center Pulsars with the ngVLA

Pulsars in the Galactic Center (GC) are important probes of General Relativity, star formation, stellar dynamics, stellar evolution, and the interstellar medium. Despite years of searching, only a handful of pulsars in the central 0.5 deg are known. The high-frequency sensitivity of ngVLA will open a new window for discovery and characterization of pulsars in the GC. A pulsar in orbit around the GC black hole, Sgr A*, will provide an unprecedented probe of black hole physics and General Relativity.

astro-ph.HE↗

arXiv2018

Science with an ngVLA: Tidal disruption events

The tidal disruption and subsequent accretion of a star by a supermassive black hole can be used as a laboratory to study the physics of relativistic jets. The ngVLA is the only planned instrument that can both discover and characterize a large number of these short-lived radio sources. In particular the high-frequency capabilities of the ngVLA enable this important leap forward. Multi-frequency radio follow-up observations (3 - 100 GHz) of tidal disruption events found in optical or X-ray surveys will provide a measurement of the jet efficiency as a function of black hole spin, thus enabling a direct test of the prediction that relativistic jets require high spin. Hundreds of tidal disruption jets will be discovered in a blind ngVLA survey for radio transients. By including VLBI observations with the ngVLA Long Baseline Array, we can resolve some of these sources, obtaining a robust measurement of the jet launch date and the magnetic field strength. From the thermal emission of the tidal disruption flare we can measure the accretion rate at this launch date, thus providing another unique opportunity to identify the conditions that lead to jet production.

astro-ph.HE↗

arXiv2018

The Scattering and Intrinsic Structure of Sagittarius A* at Radio Wavelengths

Radio images of the Galactic Center supermassive black hole, Sagittarius A* (Sgr A*), are dominated by interstellar scattering. Previous studies of Sgr A* have adopted an anisotropic Gaussian model for both the intrinsic source and the scattering, and they have extrapolated the scattering using a purely $λ^2$ scaling to estimate intrinsic properties. However, physically motivated source and scattering models break all three of these assumptions. They also predict that refractive scattering effects will be significant, which have been ignored in standard model fitting procedures. We analyze radio observations of Sgr A* using a physically motivated scattering model, and we develop a prescription to incorporate refractive scattering uncertainties when model fitting. We show that an anisotropic Gaussian scattering kernel is an excellent approximation for Sgr A* at wavelengths longer than 1cm, with an angular size of $(1.380 \pm 0.013) λ_{\rm cm}^2\,{\rm mas}$ along the major axis, $(0.703 \pm 0.013) λ_{\rm cm}^2\,{\rm mas}$ along the minor axis, and a position angle of $81.9^\circ \pm 0.2^\circ$. We estimate that the turbulent dissipation scale is at least $600\,{\rm km}$, with tentative support for $r_{\rm in} = 800 \pm 200\,{\rm km}$, suggesting that the ion Larmor radius defines the dissipation scale. We find that the power-law index for density fluctuations in the scattering material is $β< 3.47$, shallower than expected for a Kolmogorov spectrum ($β=11/3$), and we estimate $β= 3.38^{+0.08}_{-0.04}$ in the case of $r_{\rm in} = 800\,{\rm km}$. We find that the intrinsic structure of Sgr A* is nearly isotropic over wavelengths from 1.3mm to 1.3cm, with a size that is roughly proportional to wavelength. We discuss implications for models of Sgr A*, for theories of interstellar turbulence, and for imaging Sgr A* with the Event Horizon Telescope.

astro-ph.GA↗

arXiv2018

Detection of intrinsic source structure at ~3 Schwarzschild radii with Millimeter-VLBI observations of SAGITTARIUS A*

We report results from very long baseline interferometric (VLBI) observations of the supermassive black hole in the Galactic center, Sgr A*, at 1.3 mm (230 GHz). The observations were performed in 2013 March using six VLBI stations in Hawaii, California, Arizona, and Chile. Compared to earlier observations, the addition of the APEX telescope in Chile almost doubles the longest baseline length in the array, provides additional {\it uv} coverage in the N-S direction, and leads to a spatial resolution of $\sim$30 $μ$as ($\sim$3 Schwarzschild radii) for Sgr A*. The source is detected even at the longest baselines with visibility amplitudes of $\sim$4-13% of the total flux density. We argue that such flux densities cannot result from interstellar refractive scattering alone, but indicate the presence of compact intrinsic source structure on scales of $\sim$3 Schwarzschild radii. The measured nonzero closure phases rule out point-symmetric emission. We discuss our results in the context of simple geometric models that capture the basic characteristics and brightness distributions of disk- and jet-dominated models and show that both can reproduce the observed data. Common to these models are the brightness asymmetry, the orientation, and characteristic sizes, which are comparable to the expected size of the black hole shadow. Future 1.3 mm VLBI observations with an expanded array and better sensitivity will allow a more detailed imaging of the horizon-scale structure and bear the potential for a deep insight into the physical processes at the black hole boundary.

astro-ph.GA↗

arXiv2018

A Search for Molecular Gas in the Host Galaxy of FRB 121102

We present SMA and NOEMA observations of the host galaxy of FRB 121102 in the CO 3-2 and 1-0 transitions, respectively. We do not detect emission from either transition. We set $3σ$ upper limits to the CO luminosity $L_{CO} < 2.5 \times 10^7\,{\rm K\,km\,s}^{-1} {\, \rm pc^{-2}}$ for CO 3-2 and $L_{CO} < 2.3 \times 10^9\, {\rm K\,km\,s}^{-1} {\, \rm pc^{-2}}$ for CO 1-0. For Milky-Way-like star formation properties, we set a $3σ$ upper limit on the $H_2$ mass of $2.5 \times 10^8 \rm\ M_{\odot}$, slightly less than the predictions for the $H_2$ mass based on the star formation rate. The true constraint on the $H_2$ mass may be significantly higher, however, because of the reduction in CO luminosity that is common forlow-metallicity dwarf galaxies like the FRB host galaxy. These results demonstrate the challenge of identifying the nature of FRB progenitors through study of the host galaxy molecular gas. We also place a limit of 42 $μ$Jy ($3σ$) on the continuum flux density of the persistent radio source at 97 GHz, consistent with a power-law extrapolation of the low frequency spectrum, which may arise from an AGN or other nonthermal source.

astro-ph.HE↗

arXiv2018

The JCMT Transient Survey: Stochastic and Secular Variability of Protostars and Disks In the Sub-Millimeter Observed Over Eighteen Months

We analyze results from the first eighteen months of monthly sub-mm monitoring of eight star-forming regions in the JCMT Transient Survey. In our search for stochastic variability in 1643 bright peaks, only the previously identified source, EC53, shows behavior well above the expected measurement uncertainty. Another four sources, two disks and two protostars, show moderately-enhanced standard deviations in brightness, as expected for stochastic variables. For the two protostars, this apparent variability is the result of single epochs that are much brighter than the mean. In our search for secular brightness variations that are linear in time, we measure the fractional brightness change per year for 150 bright peaks, fifty of which are protostellar. The ensemble distribution of slopes is well fit by a normal distribution with sigma ~ 0.023. Most sources are not rapidly brightening or fading in the sub-mm. Comparison against time-randomized realizations shows that the width of the distribution is dominated by the uncertainty in the individual brightness measurements of the sources. A toy model for secular variability reveals that an underlying Gaussian distribution of linear fractional brightness change sigma = 0.005 would be unobservable in the present sample, whereas an underlying distribution with sigma = 0.02 is ruled out. Five protostellar sources, 10% of the protostellar sample, are found to have robust secular measures deviating from a constant flux. The sensitivity to secular brightness variations will improve significantly with a larger time sample, with a factor of two improvement expected by the conclusion of our 36-month survey.

astro-ph.SR↗

arXiv2017

What is the Hidden Depolarization Mechanism in Low Luminosity AGN?

Millimeter wavelength polarimetry of accreting black hole systems can provide a tomographic probe of the accretion flow on a wide range of linear scales. We searched for linear polarization in two low luminosity active galactic nuclei (LLAGN), M81 and M84, using the Combined Array for Millimeter Astronomy (CARMA) and the Submillimeter Array (SMA). We find upper limits of $\sim 1 - 2\%$ averaging over the full bandwidth and with a rotation measure (RM) synthesis technique. These low polarization fractions, along with similar low values for LLAGN M87 and 3C84, suggest that LLAGN have qualitatively different polarization properties than radio-loud sources and Sgr A*. If the sources are intrinsically polarized and then depolarized by Faraday rotation then we place lower limits on the RM of a few times $10^7\, {\rm rad\, m^{-2}}$ for the full bandwidth case and $\sim 10^9\, {\rm rad\, m^{-2}}$ for the RM synthesis analysis. These limits are inconsistent with or marginally consistent with expected accretion flow properties. Alternatively, the sources may be depolarized by cold electrons within a few Schwarzschild radii from the black hole, as suggested by numerical models.

astro-ph.HE↗

arXiv2017

The JCMT Transient Survey: Identifying Submillimetre Continuum Variability over Several Year Timescales Using Archival JCMT Gould Belt Survey Observations

Investigating variability at the earliest stages of low-mass star formation is fundamental in understanding how a protostar assembles mass. While many simulations of protostellar disks predict non-steady accretion onto protostars, deeper investigation requires robust observational constraints on the frequency and amplitude of variability events characterised across the observable SED. In this study, we develop methods to robustly analyse repeated observations of an area of the sky for submillimetre variability in order to determine constraints on the magnitude and frequency of deeply embedded protostars. We compare \mbox{850 $μ$m} JCMT Transient Survey data with archival JCMT Gould Belt Survey data to investigate variability over 2-4 year timescales. Out of 175 bright, independent emission sources identified in the overlapping fields, we find 7 variable candidates, 5 of which we classify as \textit{Strong} and the remaining 2 as \textit{Extended} to indicate the latter are associated with larger-scale structure. For the \textit{Strong} variable candidates, we find an average fractional peak brightness change per year of |4.0|\% yr$^{-1}$ with a standard deviation of $2.7\%\mathrm{\:yr}^{-1}$. In total, 7\% of the protostars associated with \mbox{850 $μ$m} emission in our sample show signs of variability. Four of the five \textit{Strong} sources are associated with a known protostar. The remaining source is a good follow-up target for an object that is anticipated to contain an enshrouded, deeply embedded protostar. In addition, we estimate the \mbox{850 $μ$m} periodicity of the submillimetre variable source, EC 53, to be \mbox{567 $\pm$ 32 days} based on the archival Gould Belt Survey data.

astro-ph.SR↗

arXiv2017

How do stars gain their mass? A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star Forming Regions

Most protostars have luminosities that are fainter than expected from steady accretion over the protostellar lifetime. The solution to this problem may lie in episodic mass accretion -- prolonged periods of very low accretion punctuated by short bursts of rapid accretion. However, the timescale and amplitude for variability at the protostellar phase is almost entirely unconstrained. In "A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star Forming Regions", we are monitoring monthly with SCUBA-2 the sub-mm emission in eight fields within nearby (<500 pc) star forming regions to measure the accretion variability of protostars. The total survey area of ~1.6 sq.deg. includes ~105 peaks with peaks brighter than 0.5 Jy/beam (43 associated with embedded protostars or disks) and 237 peaks of 0.125-0.5 Jy/beam (50 with embedded protostars or disks). Each field has enough bright peaks for flux calibration relative to other peaks in the same field, which improves upon the nominal flux calibration uncertainties of sub-mm observations to reach a precision of ~2-3% rms, and also provides quantified confidence in any measured variability. The timescales and amplitudes of any sub-mm variation will then be converted into variations in accretion rate and subsequently used to infer the physical causes of the variability. This survey is the first dedicated survey for sub-mm variability and complements other transient surveys at optical and near-IR wavelengths, which are not sensitive to accretion variability of deeply embedded protostars.

astro-ph.SR↗

arXiv2017

The JCMT Transient Survey: Data Reduction and Calibration Methods

Though there has been a significant amount of work investigating the early stages of low-mass star formation in recent years, the evolution of the mass assembly rate onto the central protostar remains largely unconstrained. Examining in depth the variation in this rate is critical to understanding the physics of star formation. Instabilities in the outer and inner circumstellar disk can lead to episodic outbursts. Observing these brightness variations at infrared or submillimetre wavelengths sets constraints on the current accretion models. The JCMT Transient Survey is a three-year project dedicated to studying the continuum variability of deeply embedded protostars in eight nearby star-forming regions at a one month cadence. We use the SCUBA-2 instrument to simultaneously observe these regions at wavelengths of 450 $μ$m and 850 $μ$m. In this paper, we present the data reduction techniques, image alignment procedures, and relative flux calibration methods for 850 $μ$m data. We compare the properties and locations of bright, compact emission sources fitted with Gaussians over time. Doing so, we achieve a spatial alignment of better than 1" between the repeated observations and an uncertainty of 2-3\% in the relative peak brightness of significant, localised emission. This combination of imaging performance is unprecedented in ground-based, single dish submillimetre observations. Finally, we identify a few sources that show possible and confirmed brightness variations. These sources will be closely monitored and presented in further detail in additional studies throughout the duration of the survey.

astro-ph.IM↗