Joe Callingham | The University of Sydney (original) (raw)
Papers by Joe Callingham
The Astrophysical Journal, 2015
We present broadband observations and spectral modeling of PKS B0008-421, and identify it as an e... more We present broadband observations and spectral modeling of PKS B0008-421, and identify it as an extreme gigahertz-peaked spectrum (GPS) source. PKS B0008-421 is characterized by the steepest known spectral slope below the turnover, close to the theoretical limit of synchrotron self-absorption, and the smallest known spectral width of any GPS source. Spectral coverage of the source spans from 0.118 to 22 GHz, which includes data from the Murchison Widefield Array and the wide bandpass receivers on the Australia Telescope Compact Array. We have implemented a Bayesian inference model fitting routine to fit the data with internal free-free absorption, single and double component free-free absorption in an external homogeneous medium, free-free absorption in an external inhomogeneous medium, or single and double component synchrotron self-absorption models, all with and without a high-frequency exponential break. We find that without the inclusion of a highfrequency break these models can not accurately fit the data, with significant deviations above and below the peak in the radio spectrum. The addition of a high-frequency break provides acceptable spectral fits for the inhomogeneous free-free absorption and double-component synchrotron self-absorption models, with the inhomogeneous free-free absorption model statistically favored. The requirement of a high-frequency spectral break implies that the source has ceased injecting fresh particles. Additional support for the inhomogeneous free-free absorption model as being responsible for the turnover in the spectrum is given by the consistency between the physical parameters derived from the model fit and the implications of the exponential spectral break, such as the necessity of the source being surrounded by a dense ambient medium to maintain the peak frequency near the gigahertz region. This implies that PKS B0008-421 should display an internal H I column density greater than 10 20 cm −2 . The discovery of PKS B0008-421 suggests that the next generation of low radio frequency surveys could reveal a large population of GPS sources that have ceased activity, and that a portion of the ultra-steep spectrum source population could be composed of these GPS sources in a relic phase.
We present Murchison Widefield Array observations of the supernova remnant (SNR) 1987A between 72... more We present Murchison Widefield Array observations of the supernova remnant (SNR) 1987A between 72 and 230 MHz, representing the lowest frequency observations of the source to date. This large lever arm in frequency space constrains the properties of the circumstellar medium created by the progenitor of SNR 1987A when it was in its red supergiant phase. As of late-2013, the radio spectrum of SNR 1987A between 72 MHz and 8.64 GHz does not show any deviation from a non-thermal power-law with a spectral index of −0.74 ± 0.02. This spectral index is consistent with that derived at higher frequencies, beneath 100 GHz, and with a shock in its adiabatic phase. A spectral turnover due to free-free absorption by the circumstellar medium has to occur below 72 MHz, which places upper limits on the optical depth of ≤ 0.1 at a reference frequency of 72 MHz, emission measure of 13,000 cm −6 pc, and an electron density of 110 cm −3. This upper limit on the electron density is consistent with the detection of prompt radio emission and models of the X-ray emission from the supernova. The electron density upper limit implies that some hydrodynamic simulations derived a red supergiant mass loss rate that is too high, or a wind velocity that is too low. The mass loss rate of ∼ 5 × 10 −6 solar mass yr −1 and wind velocity of 10 km s −1 obtained from optical observations are consistent with our upper limits, predicting a current turnover frequency due to free-free absorption between 5 and 60 MHz.
We present broadband observations and spectral modeling of PKS B0008-421, and identify it as an e... more We present broadband observations and spectral modeling of PKS B0008-421, and identify it as an extreme gigahertz-peaked spectrum (GPS) source. PKS B0008-421 is characterized by the steepest known spectral slope below the turnover, close to the theoretical limit of synchrotron self-absorption, and the smallest known spectral width of any GPS source. Spectral coverage of the source spans from 0.118 to 22 GHz, which includes data from the Murchison Widefield Array and the wide bandpass receivers on the Australia Telescope Compact Array. We have implemented a Bayesian inference model fitting routine to fit the data with internal free-free absorption, single and double component free-free absorption in an external homogeneous medium, free-free absorption in an external inhomogeneous medium, or single and double component synchrotron self-absorption models, all with and without a high-frequency exponential break. We find that without the inclusion of a high-frequency break these models can not accurately fit the data, with significant deviations above and below the peak in the radio spectrum. The addition of a high-frequency break provides acceptable spectral fits for the inhomogeneous free-free absorption and double-component synchrotron self-absorption models, with the inhomogeneous free-free absorption model statistically favored. The requirement of a high-frequency spectral break implies that the source has ceased injecting fresh particles. Additional support for the inhomogeneous free-free absorption model as being responsible for the turnover in the spectrum is given by the consistency between the physical parameters derived from the model fit and the implications of the exponential spectral break, such as the necessity of the source being surrounded by a dense ambient medium to maintain the peak frequency near the gigahertz region. This implies that PKS B0008-421 should display an internal H I column density greater than 10 20 cm −2. The discovery of PKS B0008-421 suggests that the next generation of low radio frequency surveys could reveal a large population of GPS sources that have ceased activity, and that a portion of the ultra-steep spectrum source population could be composed of these GPS sources in a relic phase.
Publications of the Astronomical Society of Australia, 2015
The Murchison Widefield Array (MWA) is a new low-frequency interferometric radio telescope built ... more The Murchison Widefield Array (MWA) is a new low-frequency interferometric radio telescope built in Western Australia at one of the locations of the future Square Kilometre Array (SKA). We describe the automated radio-frequency interference (RFI) detection strategy implemented for the MWA, which is based on the AOFLAGGER platform, and present 72-231-MHz RFI statistics from 10 observing nights. RFI detection removes 1.1% of the data. RFI from digital TV (DTV) is observed 3% of the time due to occasional ionospheric or atmospheric propagation. After RFI detection and excision, almost all data can be calibrated and imaged without further RFI mitigation efforts, including observations within the FM and DTV bands. The results are compared to a previously published Low-Frequency Array (LOFAR) RFI survey. The remote location of the MWA results in a substantially cleaner RFI environment compared to LOFAR's radio environment, but adequate detection of RFI is still required before data can be analysed. We include specific recommendations designed to make the SKA more robust to RFI, including: the availability of sufficient computing power for RFI detection; accounting for RFI in the receiver design; a smooth band-pass response; and the capability of RFI detection at high time and frequency resolution (second and kHz-scale respectively).
We present detailed analysis of the transient X-ray source 2XMMi J003833.3+402133 detected by XMM... more We present detailed analysis of the transient X-ray source 2XMMi J003833.3+402133 detected by XMM-Newton in 2008 January during a survey of M31. The X-ray spectrum is well fitted by either a steep power law plus a blackbody model or a double blackbody model. Prior observations with XMM-Newton, Chandra, Swift, and ROSAT spanning 1991-2007, as well as an additional Swift observation in 2011, all failed to detect this source. No counterpart was detected in deep optical imaging with the Canada-France-Hawaii Telescope down to a 3σ lower limit of g = 26.5 mag. This source has previously been identified as a black hole X-ray binary in M31. While this remains a possibility, the transient behavior, X-ray spectrum, and lack of an optical counterpart are equally consistent with a magnetar interpretation for 2XMMi J003833.3+402133. The derived luminosity and blackbody emitting radius at the distance of M31 argue against an extragalactic location, implying that if it is indeed a magnetar it is located within the Milky Way but 22 • out of the plane. The high Galactic latitude could be explained if 2XMMi J003833.3+402133 were an old magnetar, or if its progenitor was a runaway star that traveled away from the plane prior to going supernova.
The Astrophysical Journal, 2015
We present broadband observations and spectral modeling of PKS B0008-421, and identify it as an e... more We present broadband observations and spectral modeling of PKS B0008-421, and identify it as an extreme gigahertz-peaked spectrum (GPS) source. PKS B0008-421 is characterized by the steepest known spectral slope below the turnover, close to the theoretical limit of synchrotron self-absorption, and the smallest known spectral width of any GPS source. Spectral coverage of the source spans from 0.118 to 22 GHz, which includes data from the Murchison Widefield Array and the wide bandpass receivers on the Australia Telescope Compact Array. We have implemented a Bayesian inference model fitting routine to fit the data with internal free-free absorption, single and double component free-free absorption in an external homogeneous medium, free-free absorption in an external inhomogeneous medium, or single and double component synchrotron self-absorption models, all with and without a high-frequency exponential break. We find that without the inclusion of a highfrequency break these models can not accurately fit the data, with significant deviations above and below the peak in the radio spectrum. The addition of a high-frequency break provides acceptable spectral fits for the inhomogeneous free-free absorption and double-component synchrotron self-absorption models, with the inhomogeneous free-free absorption model statistically favored. The requirement of a high-frequency spectral break implies that the source has ceased injecting fresh particles. Additional support for the inhomogeneous free-free absorption model as being responsible for the turnover in the spectrum is given by the consistency between the physical parameters derived from the model fit and the implications of the exponential spectral break, such as the necessity of the source being surrounded by a dense ambient medium to maintain the peak frequency near the gigahertz region. This implies that PKS B0008-421 should display an internal H I column density greater than 10 20 cm −2 . The discovery of PKS B0008-421 suggests that the next generation of low radio frequency surveys could reveal a large population of GPS sources that have ceased activity, and that a portion of the ultra-steep spectrum source population could be composed of these GPS sources in a relic phase.
We present Murchison Widefield Array observations of the supernova remnant (SNR) 1987A between 72... more We present Murchison Widefield Array observations of the supernova remnant (SNR) 1987A between 72 and 230 MHz, representing the lowest frequency observations of the source to date. This large lever arm in frequency space constrains the properties of the circumstellar medium created by the progenitor of SNR 1987A when it was in its red supergiant phase. As of late-2013, the radio spectrum of SNR 1987A between 72 MHz and 8.64 GHz does not show any deviation from a non-thermal power-law with a spectral index of −0.74 ± 0.02. This spectral index is consistent with that derived at higher frequencies, beneath 100 GHz, and with a shock in its adiabatic phase. A spectral turnover due to free-free absorption by the circumstellar medium has to occur below 72 MHz, which places upper limits on the optical depth of ≤ 0.1 at a reference frequency of 72 MHz, emission measure of 13,000 cm −6 pc, and an electron density of 110 cm −3. This upper limit on the electron density is consistent with the detection of prompt radio emission and models of the X-ray emission from the supernova. The electron density upper limit implies that some hydrodynamic simulations derived a red supergiant mass loss rate that is too high, or a wind velocity that is too low. The mass loss rate of ∼ 5 × 10 −6 solar mass yr −1 and wind velocity of 10 km s −1 obtained from optical observations are consistent with our upper limits, predicting a current turnover frequency due to free-free absorption between 5 and 60 MHz.
We present broadband observations and spectral modeling of PKS B0008-421, and identify it as an e... more We present broadband observations and spectral modeling of PKS B0008-421, and identify it as an extreme gigahertz-peaked spectrum (GPS) source. PKS B0008-421 is characterized by the steepest known spectral slope below the turnover, close to the theoretical limit of synchrotron self-absorption, and the smallest known spectral width of any GPS source. Spectral coverage of the source spans from 0.118 to 22 GHz, which includes data from the Murchison Widefield Array and the wide bandpass receivers on the Australia Telescope Compact Array. We have implemented a Bayesian inference model fitting routine to fit the data with internal free-free absorption, single and double component free-free absorption in an external homogeneous medium, free-free absorption in an external inhomogeneous medium, or single and double component synchrotron self-absorption models, all with and without a high-frequency exponential break. We find that without the inclusion of a high-frequency break these models can not accurately fit the data, with significant deviations above and below the peak in the radio spectrum. The addition of a high-frequency break provides acceptable spectral fits for the inhomogeneous free-free absorption and double-component synchrotron self-absorption models, with the inhomogeneous free-free absorption model statistically favored. The requirement of a high-frequency spectral break implies that the source has ceased injecting fresh particles. Additional support for the inhomogeneous free-free absorption model as being responsible for the turnover in the spectrum is given by the consistency between the physical parameters derived from the model fit and the implications of the exponential spectral break, such as the necessity of the source being surrounded by a dense ambient medium to maintain the peak frequency near the gigahertz region. This implies that PKS B0008-421 should display an internal H I column density greater than 10 20 cm −2. The discovery of PKS B0008-421 suggests that the next generation of low radio frequency surveys could reveal a large population of GPS sources that have ceased activity, and that a portion of the ultra-steep spectrum source population could be composed of these GPS sources in a relic phase.
Publications of the Astronomical Society of Australia, 2015
The Murchison Widefield Array (MWA) is a new low-frequency interferometric radio telescope built ... more The Murchison Widefield Array (MWA) is a new low-frequency interferometric radio telescope built in Western Australia at one of the locations of the future Square Kilometre Array (SKA). We describe the automated radio-frequency interference (RFI) detection strategy implemented for the MWA, which is based on the AOFLAGGER platform, and present 72-231-MHz RFI statistics from 10 observing nights. RFI detection removes 1.1% of the data. RFI from digital TV (DTV) is observed 3% of the time due to occasional ionospheric or atmospheric propagation. After RFI detection and excision, almost all data can be calibrated and imaged without further RFI mitigation efforts, including observations within the FM and DTV bands. The results are compared to a previously published Low-Frequency Array (LOFAR) RFI survey. The remote location of the MWA results in a substantially cleaner RFI environment compared to LOFAR's radio environment, but adequate detection of RFI is still required before data can be analysed. We include specific recommendations designed to make the SKA more robust to RFI, including: the availability of sufficient computing power for RFI detection; accounting for RFI in the receiver design; a smooth band-pass response; and the capability of RFI detection at high time and frequency resolution (second and kHz-scale respectively).
We present detailed analysis of the transient X-ray source 2XMMi J003833.3+402133 detected by XMM... more We present detailed analysis of the transient X-ray source 2XMMi J003833.3+402133 detected by XMM-Newton in 2008 January during a survey of M31. The X-ray spectrum is well fitted by either a steep power law plus a blackbody model or a double blackbody model. Prior observations with XMM-Newton, Chandra, Swift, and ROSAT spanning 1991-2007, as well as an additional Swift observation in 2011, all failed to detect this source. No counterpart was detected in deep optical imaging with the Canada-France-Hawaii Telescope down to a 3σ lower limit of g = 26.5 mag. This source has previously been identified as a black hole X-ray binary in M31. While this remains a possibility, the transient behavior, X-ray spectrum, and lack of an optical counterpart are equally consistent with a magnetar interpretation for 2XMMi J003833.3+402133. The derived luminosity and blackbody emitting radius at the distance of M31 argue against an extragalactic location, implying that if it is indeed a magnetar it is located within the Milky Way but 22 • out of the plane. The high Galactic latitude could be explained if 2XMMi J003833.3+402133 were an old magnetar, or if its progenitor was a runaway star that traveled away from the plane prior to going supernova.