arXiv subjects

Guy G. Pooley

Publications and source records attributed to Guy G. Pooley.

At least 37 records · Page 2Linked to original sources

arXiv2011

The 2010 May Flaring Episode of Cygnus X-3 in Radio, X-Rays, and γ-Rays

In 2009, Cygnus X-3 (Cyg X-3) became the first microquasar to be detected in the GeV γ-ray regime, via the satellites Fermi and AGILE. The addition of this new band to the observational toolbox holds promise for building a more detailed understanding of the relativistic jets of this and other systems. We present a rich dataset of radio, hard and soft X-ray, and γ-ray observations of Cyg X-3 made during a flaring episode in 2010 May. We detect a ~3-d softening and recovery of the X-ray emission, followed almost immediately by a ~1-Jy radio flare at 15 GHz, followed by a 4.3σ γ-ray flare (E > 100 MeV) ~1.5 d later. The radio sampling is sparse, but we use archival data to argue that it is unlikely the γ-ray flare was followed by any significant unobserved radio flares. In this case, the sequencing of the observed events is difficult to explain in a model in which the γ-ray emission is due to inverse Compton scattering of the companion star's radiation field. Our observations suggest that other mechanisms may also be responsible for γ-ray emission from Cyg X-3.

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arXiv2011

AMI-LA radio continuum observations of Spitzer c2d small clouds and cores: Perseus region

We present deep radio continuum observations of the cores identified as deeply embedded young stellar objects in the Perseus molecular cloud by the Spitzer c2d programme at a wavelength of 1.8 cm with the Arcminute Microkelvin Imager Large Array (AMI-LA). We detect 72% of Class 0 objects from this sample and 31% of Class I objects. No starless cores are detected. We use the flux densities measured from these data to improve constraints on the correlations between radio luminosity and bolometric luminosity, infrared luminosity and, where measured, outflow force. We discuss the differing behaviour of these objects as a function of protostellar class and investigate the differences in radio emission as a function of core mass. Two of four possible very low luminosity objects (VeLLOs) are detected at 1.8 cm.

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arXiv2011

10C Survey of Radio Sources at 15.7 GHz: I - Observing, mapping and source extraction

We have observed an area of approximatley 27 deg^2 to an rms noise level of less than 0.2 mJy at 15.7 GHz, using the Arcminute Microkelvin Imager Large Array. These observations constitute the most sensitive radio-source survey of any extent (greater than approximately 0.2 deg^2) above 1.4 GHz. This paper presents the techniques employed for observing, mapping and source extraction. We have used a systematic procedure for extracting information and producing source catalogues, from maps with varying noise and uv-coverage. We have performed simulations to test our mapping and source-extraction procedures, and developed methods for identifying extended, overlapping and spurious sources in noisy images. In an accompanying paper, AMI Consortium: Davies et al. 2010, the first results from the 10C survey, including the deep 15.7-GHz source count, are presented.

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arXiv2011

10C Survey of Radio Sources at 15.7 GHz: II - First Results

The first results from the Tenth Cambridge (10C) Survey of Radio Sources, carried out using the AMI Large Array (LA) at an observing frequency of 15.7 GHz, are presented. The survey fields cover an area of approximately 27 sq. degrees to a flux-density completeness of 1 mJy. Results for some deeper areas, covering approximately 12 sq. degrees, wholly contained within the total areas and complete to 0.5 mJy, are also presented. The completeness for both areas is estimated to be at least 93 per cent. The source catalogue contains 1897 entries and is available at www.mrao.cam.ac.uk/surveys/10C. It has been combined with that of the 9C Survey to calculate the 15.7-GHz source counts. A broken power law is found to provide a good parameterisation of the differential count between 0.5 mJy and 1 Jy. The measured count has been compared to that predicted by de Zotti et al. (2005). The model displays good agreement with the data at the highest flux densities but under-predicts the integrated count between 0.5 mJy and 1 Jy by about 30 per cent. Entries from the source catalogue have been matched to those contained in the catalogues of NVSS and FIRST (both of which have observing frequencies of 1.4 GHz). This matching provides evidence for a shift in the typical 1.4-to-15.7-GHz spectral index of the 15.7-GHz-selected source population with decreasing flux density towards sub-mJy levels - the spectra tend to become less steep. Automated methods for detecting extended sources have been applied to the data; approximately 5 per cent of the sources are found to be extended relative to the LA synthesised beam of approximately 30 arcsec. Investigations using higher-resolution data showed that most of the genuinely extended sources at 16 GHz are classical doubles, although some nearby galaxies and twin-jet sources were also identified.

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arXiv2011

Sunyaev-Zel'dovich observation of the Bullet-like cluster Abell 2146 with the Arcminute Microkelvin Imager

We present 13.9-18.2 GHz observations of the Sunyaev-Zel'dovich (SZ) effect towards Abell 2146 using the Arcminute Microkelvin Imager (AMI). The cluster is detected with a peak SNR ratio of 13 sigma in the radio source subtracted map. Comparison of the SZ and X-ray images suggests that they both have extended regions which lie approximately perpendicular to one another, with their emission peaks significantly displaced. These features indicate non-uniformities in the distributions of the gas temperature and pressure, indicative of a cluster merger. We use a Bayesian cluster analysis to explore the high-dimensional parameter space of the cluster-plus-sources model to obtain cluster parameter estimates in the presence of radio point sources, receiver noise and primordial CMB anisotropy; the probability of SZ + CMB primordial structure + radio sources + receiver noise to CMB + radio sources + receiver noise is 3 x 10^{6}:1. We compare the results from three different cluster models. Our preferred model exploits the observation that the gas fractions do not appear to vary greatly between clusters. Given the relative masses of the two merging systems in Abell 2146, the mean gas temperature can be deduced from the virial theorem (assuming all of the kinetic energy is in the form of internal gas energy) without being affected significantly by the merger event, provided the primary cluster was virialized before the merger. In this model we fit a simple spherical isothermal beta-model, despite the inadequacy of this model for a merging system like Abell 2146, and assume the cluster follows the mass-temperature relation of a virialized, singular, isothermal sphere. We note that this model avoids inferring large-scale cluster parameters internal to r_200 under the widely used assumption of hydrostatic equilibrium. We find that at r_200 M_T= 4.1 \pm 0.5 x 10^{14} h^{-1}M_sun and T=4.5 \pm 0.5 keV.

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arXiv2010

The doubling of the superorbital period of Cyg X-1

We study properties of the superorbital modulation of the X-ray emission of Cyg X-1. We find that it has had a stable period of about 300 d in soft and hard X-rays and in radio since 2005 until at least 2010, which is about double the previously seen period. This new period, seen in the hard spectral state only, is detected not only in the light curves but also in soft X-ray hardness ratios and in the amplitude of the orbital modulation. On the other hand, the spectral slope in hard X-rays, >20 keV, averaged over superorbital bins is constant, and the soft and hard X-rays and the radio emission change in phase. This shows that the superorbital variability consists of changing the normalization of an intrinsic spectrum of a constant shape and of changes of the absorbing column density with the phase. The maximum column density is achieved at the superorbital minimum. The amplitude changes are likely to be caused by a changing viewing angle of an anisotropic emitter, most likely a precessing accretion disc. The constant shape of the intrinsic spectrum shows that this modulation is not caused by a changing accretion rate. The modulated absorbing column density shows the presence of a bulge around the disc centre, as proposed previously. We also find the change of the superorbital period from about 150 d to about 300 d to be associated with almost unchanged average X-ray fluxes, making the period change difficult to explain in the framework of disc-irradiation models. Finally, we find no correlation of the X-ray and radio properties with the reported detections in the GeV and TeV gamma-ray range.

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arXiv2010

AMI Large Array radio continuum observations of Spitzer c2d small clouds and cores

We perform deep 1.8 cm radio continuum imaging towards thirteen protostellar regions selected from the Spitzer c2d small clouds and cores programme at high resolution (25") in order to detect and quantify the cm-wave emission from deeply embedded young protostars. Within these regions we detect fifteen compact radio sources which we identify as radio protostars including two probable new detections. The sample is in general of low bolometric luminosity and contains several of the newly detected VeLLO sources. We determine the 1.8 cm radio luminosity to bolometric luminosity correlation, L_rad -L_bol, for the sample and discuss the nature of the radio emission in terms of the available sources of ionized gas. We also investigate the L_rad-L_IR correlation and suggest that radio flux density may be used as a proxy for the internal luminosity of low luminosity protostars.

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arXiv2010

Sunyaev-Zel'dovich observations of galaxy clusters out to the virial radius with the Arcminute Microkelvin Imager

We present observations using the Small Array of the Arcminute Microkelvin Imager (AMI; 14-18 GHz) of four Abell and three MACS clusters spanning 0.171-0.686 in redshift. We detect Sunyaev-Zel'dovich (SZ) signals in five of these without any attempt at source subtraction, although strong source contamination is present. With radio-source measurements from high-resolution observations, and under the assumptions of spherical $β$-model, isothermality and hydrostatic equilibrium, a Bayesian analysis of the data in the visibility plane detects extended SZ decrements in all seven clusters over and above receiver noise, radio sources and primary CMB imprints. Bayesian evidence ratios range from 10^{11}:1 to 10^{43}:1 for six of the clusters and 3000:1 for one with substantially less data than the others. We present posterior probability distributions for, e.g., total mass and gas fraction averaged over radii internal to which the mean overdensity is 1000, 500 and 200, r_200 being the virial radius. Reaching r_200 involves some extrapolation for the nearer clusters but not for the more-distant ones. We find that our estimates of gas fraction are low (compared with most in the literature) and decrease with increasing radius. These results appear to be consistent with the notion that gas temperature in fact falls with distance (away from near the cluster centre) out to the virial radius.

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arXiv2010

Microwave observations of spinning dust emission in NGC6946

We report new cm-wave measurements at five frequencies between 15 and 18GHz of the continuum emission from the reportedly anomalous "region 4" of the nearby galaxy NGC6946. We find that the emission in this frequency range is significantly in excess of that measured at 8.5GHz, but has a spectrum from 15-18GHz consistent with optically thin free-free emission from a compact HII region. In combination with previously published data we fit four emission models containing different continuum components using the Bayesian spectrum analysis package radiospec. These fits show that, in combination with data at other frequencies, a model with a spinning dust component is slightly preferred to those that possess better-established emission mechanisms.

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arXiv2010

High resolution AMI Large Array imaging of spinning dust sources: spatially correlated 8 micron emission and evidence of a stellar wind in L675

We present 25 arcsecond resolution radio images of five Lynds Dark Nebulae (L675, L944, L1103, L1111 & L1246) at 16 GHz made with the Arcminute Microkelvin Imager (AMI) Large Array. These objects were previously observed with the AMI Small Array to have an excess of emission at microwave frequencies relative to lower frequency radio data. In L675 we find a flat spectrum compact radio counterpart to the 850 micron emission seen with SCUBA and suggest that it is cm-wave emission from a previously unknown deeply embedded young protostar. In the case of L1246 the cm-wave emission is spatially correlated with 8 micron emission seen with Spitzer. Since the MIR emission is present only in Spitzer band 4 we suggest that it arises from a population of PAH molecules, which also give rise to the cm-wave emission through spinning dust emission.

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arXiv2009

AMI observations of Lynds Dark Nebulae: further evidence for anomalous cm-wave emission

Observations at 14.2 to 17.9 GHz made with the AMI Small Array towards fourteen Lynds Dark Nebulae with a resolution of 2' are reported. These sources are selected from the SCUBA observations of Visser et al. (2001) as small angular diameter clouds well matched to the synthesized beam of the AMI Small Array. Comparison of the AMI observations with radio observations at lower frequencies with matched uv-plane coverage is made, in order to search for any anomalous excess emission which can be attributed to spinning dust. Possible emission from spinning dust is identified as a source within a 2' radius of the Scuba position of the Lynds dark nebula, exhibiting an excess with respect to lower frequency radio emission. We find five sources which show a possible spinning dust component in their spectra. These sources have rising spectral indices in the frequency range 14.2--17.9 GHz. Of these five one has already been reported, L1111, we report one new definite detection, L675, and three new probable detections (L944, L1103 and L1246). The relative certainty of these detections is assessed on the basis of three criteria: the extent of the emission, the coincidence of the emission with the Scuba position and the likelihood of alternative explanations for the excess. Extended microwave emission makes the likelihood of the anomalous emission arising as a consequence of a radio counterpart to a protostar or a proto-planetary disk unlikely. We use a 2' radius in order to be consistent with the IRAS identifications of dark nebulae (Parker 1988), and our third criterion is used in the case of L1103 where a high flux density at 850 microns relative to the FIR data suggests a more complicated emission spectrum.

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arXiv2009

Follow-up observations at 16 and 33 GHz of extragalactic sources from WMAP 3-year data: I - Spectral properties

We present follow-up observations of 97 point sources from the Wilkinson Microwave Anisotropy Probe (WMAP) 3-year data, contained within the New Extragalactic WMAP Point Source (NEWPS) catalogue between declinations of -4 and +60 degrees; the sources form a flux-density-limited sample complete to 1.1 Jy (approximately 5 sigma) at 33 GHz. Our observations were made at 16 GHz using the Arcminute Microkelvin Imager (AMI) and at 33 GHz with the Very Small Array (VSA). 94 of the sources have reliable, simultaneous -- typically a few minutes apart -- observations with both telescopes. The spectra between 13.9 and 33.75 GHz are very different from those of bright sources at low frequency: 44 per cent have rising spectra (alpha < 0.0), where flux density is proportional to frequency^-alpha, and 93 per cent have spectra with alpha < 0.5; the median spectral index is 0.04. For the brighter sources, the agreement between VSA and WMAP 33-GHz flux densities averaged over sources is very good. However, for the fainter sources, the VSA tends to measure lower values for the flux densities than WMAP. We suggest that the main cause of this effect is Eddington bias arising from variability.

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arXiv2009

Follow-up observations at 16 and 33 GHz of extragalactic sources from WMAP 3-year data: II - Flux-density variability

Using the Arcminute Microkelvin Imager (AMI) at 16 GHz and the Very Small Array (VSA) at 33 GHz to make follow-up observations of sources in the New Extragalactic WMAP Point Source Catalogue, we have investigated the flux-density variability in a complete sample of 97 sources over timescales of a few months to approximately 1.5 years. We find that 53 per cent of the 93 sources, for which we have multiple observations, are variable, at the 99 per cent confidence level, above the flux density calibration uncertainties of approximately 4 per cent at 16 GHz; the fraction of sources having varied by more than 20 per cent is 15 per cent at 16 GHz and 20 per cent at 33 GHz. Not only is this common occurrence of variability at high frequency of interest for source physics, but strategies for coping with source contamination in CMB work must take this variability into account. There is no strong evidence of a correlation between variability and flux density for the sample as a whole. Using a maximum-likelihood method, we calculate the correlation in the variability at the two frequencies in a subset of sources classified as variable from both the AMI and VSA data and find the Pearson product-moment correlation coefficient to be very high (0.955 +/- 0.034). We also find the degree of variability at 16 GHz (0.202 +/- 0.028) to be very similar to that at 33 GHz (0.224 +/- 0.039). Finally, we have investigated the relationship between variability and spectral index, alpha_13.9^33.75 (where S proportional to nu^-alpha), and find a significant difference in the spectral indices of the variable sources (-0.06 +/- 0.05) and non-variable sources (0.13 +/- 0.04).

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arXiv2008

Opacity effects and shock-in-jet modelling of low-level activity in Cygnus X-3

We present simultaneous dual-frequency radio observations of Cygnus X-3 during a phase of low-level activity. We constrain the minimum variability timescale to be 20 minutes at 43 GHz and 30 minutes at 15 GHz, implying source sizes of 2 to 4 AU. We detect polarized emission at a level of a few per cent at 43 GHz which varies with the total intensity. The delay of approximately 10 minutes between the peaks of the flares at the two frequencies is seen to decrease with time, and we find that synchrotron self-absorption and free-free absorption by entrained thermal material play a larger role in determining the opacity than absorption in the stellar wind of the companion. A shock-in-jet model gives a good fit to the lightcurves at all frequencies, demonstrating that this mechanism, which has previously been used to explain the brighter, longer-lived giant outbursts in this source, is also applicable to these low-level flaring events. Assembling the data from outbursts spanning over two orders of magnitude in flux density shows evidence for a strong correlation between the peak brightness of an event, and the timescale and frequency at which this is attained. Brighter flares evolve on longer timescales and peak at lower frequencies. Analysis of the fitted model parameters suggests that brighter outbursts are due to shocks forming further downstream in the jet, with an increased electron normalisation and magnetic field strength both playing a role in setting the strength of the outburst.

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arXiv2008

Observations of the Corona Borealis supercluster with the superextended Very Small Array: further constraints on the nature of the non-Gaussian CMB cold spot

We present interferometric imaging at 33 GHz, with the new superextended configuration of the Very Small Array (VSA), of a very deep decrement in the cosmic microwave background (CMB) temperature. This decrement is located in the direction of the Corona Borealis supercluster, at a position with no known galaxy clusters, and was discovered by a previous VSA survey (Genova-Santos et al.). A total area of 3 sq.deg. has now been imaged, with an angular resolution of 7 arcmin and a flux sensitivity of 5 mJy/beam. These observations confirm the presence of this strong and resolved negative spot at -41+/-5 mJy/beam (-258+/-29 muK), with a signal to noise level of 8. This structure is also present in the WMAP 5-year data. The temperature of the W-band (94 GHz) data at the position of the decrement agrees within 1.2-sigma with that observed by the VSA at 33 GHz, and within 0.2-sigma with the Sunyaev-Zel'dovich (SZ) spectrum. Our analyses show that it is a non-Gaussian feature in the CMB at a level of 4.8-sigma. The probability of finding such a deviation or larger in CMB Gaussian simulations is only 0.19 per cent. Therefore, an explanation other than primordial CMB is required. We have considered the possibility of an SZ effect generated in a diffuse, extended warm/hot gas distribution. This hypothesis is especially relevant, as the presence of such structures, if confirmed, could provide the location for a significant fraction of the missing baryons in the Local Universe.

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arXiv2007

HST/NICMOS Observations of Fast Infrared Flickering in the Microquasar GRS 1915+105

We report infrared observations of the microquasar GRS 1915+105 using the NICMOS instrument of the Hubble Space Telescope during 9 visits in April-June 2003. During epochs of high X-ray/radio activity near the beginning and end of this period, we find that the $1.87 $\um infrared flux is generally low ($\sim 2$ mJy) and relatively steady. However, during the X-ray/radio ``plateau'' state between these epochs, we find that the infrared flux is significantly higher ($\sim 4-6$ mJy), and strongly variable. In particular, we find events with amplitudes $\sim 20-30$% occurring on timescales of $\sim 10-20$s (e-folding timescales of $\sim 30$s). These flickering timescales are several times faster than any previously-observed infrared variability in GRS 1915+105 and the IR variations exceed corresponding X-ray variations at the same ($\sim 8s$) timescale. These results suggest an entirely new type of infrared variability from this object. Based on the properties of this flickering, we conclude that it arises in the plateau-state jet outflow itself, at a distance $<2.5$ AU from the accretion disk. We discuss the implications of this work and the potential of further flickering observations for understanding jet formation around black holes.

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arXiv2007

Correlated radio--X-ray variability of Galactic Black Holes: A radio--X-ray flare in Cygnus X-1

We report on the first detection of a quasi-simultaneous radio-X-ray flare of Cygnus X-1. The detection was made on 2005 April 16 with pointed observations by the Rossi X-ray Timing Explorer and the Ryle telescope, during a phase where the black hole candidate was close to a transition from the its soft into its hard state. The radio flare lagged the X-rays by approximately 7 minutes, peaking at 3:20 hours barycentric time (TDB 2453476.63864). We discuss this lag in the context of models explaining such flaring events as the ejection of electron bubbles emitting synchrotron radiation.

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arXiv2007

Constraints on spinning dust towards Galactic targets with the VSA: a tentative detection of excess microwave emission towards 3C396

We present results from observations made at 33 GHz with the Very Small Array (VSA) telescope towards potential candidates in the Galactic plane for spinning dust emission. In the cases of the diffuse HII regions LPH96 and NRAO591 we find no evidence for anomalous emission and, in combination with Effelsberg data at 1.4 and 2.7 GHz, confirm that their spectra are consistent with optically thin free--free emission. In the case of the infra-red bright SNR 3C396 we find emission inconsistent with a purely non-thermal spectrum and discuss the possibility of this excess arising from either a spinning dust component or a shallow spectrum PWN, although we conclude that the second case is unlikely given the strong constraints available from lower frequency radio images.

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