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Jean-Pierre Macquart

Publications and source records attributed to Jean-Pierre Macquart.

45 records · Page 3Linked to original sources
arXiv2009

Annual Cycles in the Interstellar Scintillation Timescales of PKS B1519-273 and PKS B1622-253

We have used the University of Tasmania's 30 m radio telescope at Ceduna in South Australia to regularly monitor the flux density of a number of southern blazars. We report the detection of an annual cycle in the variability timescale of the centimetre radio emission of PKS B1622-253. Observations of PKS B1519-273 over a period of nearly two years confirm the presence of an annual cycle in the variability timescale in that source. These observations prove that interstellar scintillation is the principal cause of inter-day variability at radio wavelengths in these sources. The best-fit annual cycle model for both sources implies a high degree of anisotropy in the scattering screen and that it has a large velocity offset with respect to the Local Standard of Rest. This is consistent with a greater screen distance for these "slow'" IDV sources than for rapid scintillators such as PKS B0405-385 or J1819+3845.

astro-ph.CO↗
arXiv2008

A comprehensive study of GRB 070125, a most energetic gamma ray burst

We present a comprehensive multiwavelength analysis of the bright, long duration gamma-ray burst GRB 070125, comprised of observations in $γ$-ray, X-ray, optical, millimeter and centimeter wavebands. Simultaneous fits to the optical and X-ray light curves favor a break on day 3.78, which we interpret as the jet break from a collimated outflow. Independent fits to optical and X-ray bands give similar results in the optical bands but shift the jet break to around day 10 in the X-ray light curve. We show that for the physical parameters derived for GRB 070125, inverse Compton scattering effects are important throughout the afterglow evolution. While inverse Compton scattering does not affect radio and optical bands, it may be a promising candidate to delay the jet break in the X-ray band. Radio light curves show rapid flux variations, which are interpreted as due to interstellar scintillation, and are used to derive an upper limit of $2.4 \times 10^{17}$ cm on the radius of the fireball in the lateral expansion phase of the jet. Radio light curves and spectra suggest a high synchrotron self absorption frequency indicative of the afterglow shock wave moving in a dense medium. Our broadband modeling favors a constant density profile for the circumburst medium over a wind-like profile ($R^{-2}$). However, keeping in mind the uncertainty of the parameters, it is difficult to unambiguously distinguish between the two density profiles. Our broadband fits suggest that \event is a burst with high radiative efficiency ($> 60 %$).

astro-ph↗
arXiv2007

Emergence and disappearance of micro-arcsecond structure in the scintillating quasar J1819+3845

The 4.8 GHz lightcurves of the scintillating intra-day variable quasar J1819+3845 during 2004-5 exhibit sharp structure, down to a time scale of 15 minutes, that was absent from lightcurves taken prior to this period and from the 2006 lightcurves. Analysis of the lightcurve power spectra show that the variations must be due to the emergence of new structure in the source. The power spectra yield a scattering screen distance of 3.8 +/- 0.3 pc for a best-fit v_ISS=59 +/- 0.5 km/s or 2.0 +/- 0.3 pc for the scintillation velocity reported by Dennett-Thorpe & de Bruyn (2003). The turbulence is required to be exceptionally turbulent, with C_N^2 > 0.7 Delta L_pc^{-1} m^{-20/3} for scattering material of thickness Delta L_{pc} pc along the ray path. The 2004 power spectrum can be explained in terms of a double source with a component separation 240 +/- 15 microas in 2004.

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arXiv2006

Scattering of Gravitational Radiation: Intensity Fluctuations

The effect of gravitational microlensing on the intensity of gravitational radiation as it propagates through an inhomogeneous medium containing small-scale structure is considered. Lensing by both stars and a power law spectrum of density perturbations is examined. The long wavelengths characteristic of gravitational radiation mandate a statistical, physical-optics approach to treat the effect of stellar microlensing. A model for the mass power spectrum of a starfield, including the effects of clustering and allowing for a distribution of stellar masses, is constructed and used to determine both the amplitude of fluctuations in the gravitational wave strain and its associated temporal fluctuation spectrum. For a uniformly distributed starfield the intensity variance scales linearly with stellar density, sigma, but is enhanced by a factor >sigma r_F^2 when clustering is important, where r_F is the Fresnel scale. The effect of lensing by a power law mass spectrum is also considered. For spectra with indices steeper than -2 the wave amplitude exhibits rms fluctuations 1.3 < Delta Sigma^2 >^0.25 (D_eff/1 Gpc)^0.5 %, where is the variance in the mass surface density measured in M_sun^2 pc^-4 and D_eff is the effective distance to the lensing medium. For shallower spectra the amplitude of the fluctuations depends additionally on the inner length scale and power law index of the density fluctuations. The intensity fluctuations are dominated by temporal fluctuations on too long a timescale to be observable.

astro-ph↗
arXiv2006

The Rotation Measure and 3.5mm Polarization of Sgr A*

We report the detection of variable linear polarization from Sgr A* at a wavelength of 3.5mm, the longest wavelength yet at which a detection has been made. The mean polarization is 2.1 +/- 0.1% at a position angle of 16 +/- 2 deg with rms scatters of 0.4% and 9 deg over the five epochs. We also detect polarization variability on a timescale of days. Combined with previous detections over the range 150-400GHz (750-2000 microns), the average polarization position angles are all found to be consistent with a rotation measure of -4.4 +/- 0.3 x 10^5 rad/m^2. This implies that the Faraday rotation occurs external to the polarized source at all wavelengths. This implies an accretion rate ~0.2 - 4 x 10^-8 Msun/yr for the accretion density profiles expected of ADAF, jet and CDAF models and assuming that the region at which electrons in the accretion flow become relativistic is within 10 R_S. The inferred accretion rate is inconsistent with ADAF/Bondi accretion. The stability of the mean polarization position angle between disparate polarization observations over the frequency range limits fluctuations in the accretion rate to less than 5%. The flat frequency dependence of the inter-day polarization position angle variations also makes them difficult to attribute to rotation measure fluctuations, and suggests that both the magnitude and position angle variations are intrinsic to the emission.

astro-ph↗
arXiv2005

Understanding the Radio Variability of Sgr A*

We determine the characteristics of the 7mm to 20cm wavelength radio variability in Sgr A* on time scales from days to three decades. The amplitude of the intensity modulation is between 30 and 39% at all wavelengths. Analysis of uniformly sampled data with proper accounting of the sampling errors associated with the lightcurves shows that Sgr A* exhibits no 57- or 106-day quasi-periodic oscillations, contrary to previous claims. The cause of the variability is investigated by examining a number of plausible scintillation models, enabling those variations which could be attributed to interstellar scintillation to be isolated from those that must be intrinsic to the source. Thin-screen scattering models do not account for the variability amplitude on most time scales. However, models in which the scattering region is extended out to a radius of 50-500pc from the Galactic Center account well for the broad characteristics of the variability on >4-day time scales. The ~ 10% variability on <4-day time scales at 0.7-3cm appears to be intrinsic to the source. The degree of scintillation variability expected at millimeter wavelengths depends sensitively on the intrinsic source size; the variations, if due to scintillation, would require an intrinsic source size smaller than that expected.

astro-ph↗
arXiv2005

Diffractive Interstellar Scintillation of the Quasar J1819+3845 at 21cm

We report the discovery of fast, frequency-dependent intensity variations from the scintillating intra-day variable quasar J1819+3845 at a wavelength of 21cm which resemble diffractive interstellar scintillations observed in pulsars. The timescale (down to 20 min) and the bandwidth (frequency decorrelation bandwidth of 160 MHz) of the observed variations jointly imply that the component of the source exhibiting this scintillation must possess a brightness temperature well in excess of the inverse Compton limit. A specific model in which both the source and scintillation pattern are isotropic implies a brightness temperature 0.5 x 10^13 z_{pc} K, where previous estimates place the distance to the scattering medium in the range z_{pc}=4-12pc, yielding a minimum brightness temperature >20 times the inverse Compton limit. An independent estimate of the screen distance using the 21cm scintillation properties alone indicates a minimum screen distance of z approx 40pc and a brightness temperature above 2 x 10^14 K. There is no evidence for anisotropy in the scattering medium or source from the scintillation characteristics, but these estimates may be reduced by a factor comparable to the axial ratio if the source is indeed elongated.

astro-ph↗
arXiv2004

Scintillation-induced Variability in Radio Absorption Spectra against Extragalactic Sources

Spectral features absorbed against some radio quasars exhibit ~50mJy variations, with the lines varying both relative to the continuum and, when several lines are present, even relative to one another. We point out that such variability can be expected as a consequence of refractive scintillation caused by the interstellar medium of our Galaxy. Scintillation can cause independent variations between closely-spaced spectral lines, and can even alter the line profile. The background source need not be compact to exhibit spectral variability. The variability can be used to infer the parsec to sub-parsec scale structure of the intervening absorbing material. We discuss the importance of scintillation relative to other possible origins of spectral variability. The present theory is applied to account for the variations observed in the HI-absorbed quasar PKS 1127-145.

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