2019-A, Extended

Authors: Dr. C. Weniger (University of Amsterdam), Dr. Y. Kahn (Uni-versity of Chicago/University of Illinois), Dr. A. Possenti (INAF-Osservatorio Astronomico di Cagliari), C.D. Tremblay (ICRAR-Curtin Univeristy), Dr. F. Calore (CNRS, LAPTh), K. Kel- ley (ICRAR - University of Western Australia), N. D. R. Bhat (ICRAR - Curtin), V. Kondratiev (ASTRON), Dr. B. R. Safdi (University of Michigan)

Abstract: Astrophysical observations of galaxies and galaxy clusters, and cosmological observations of the temperature fluctuations of the cosmic microwave background provide strong evidence for the existence of dark matter (DM) in the Universe. One of the oldest and theoretically best motivated DM candidates are QCD axions, ultralight particle that solves the strong CP problem of the Standard Model of particle physics. For masses in the μeV range, these particles also naturally explain the observed amount of cold DM. Substantial experimental efforts are underway to search for these particles in laboratory experiments. Here, we propose to leverage on the fact that DM axions can resonantly convert into photons when traversing the strong magnetic field in the magnetosphere of neutron stars (NS). The observable signal is a narrow spectral line, with frequency that depends on the (unknown) axion mass. Detailed calculations show that slowly-spinning NSs lead to the most efficient conversion. An optimal target for axion searches is hence the large population of dead NSs, which can be inferred from population synthesis arguments. We propose here to use the unique capabilities of the Murchison Widefield Array to search for narrow spectral lines from the Galactic center (GC) region in the 80 – 300 MHz frequency range. The proposed searches are complementary to similar proposed work with the Green Bank, Effelsberg, and Sardinia radio telescopes. A discovery of an axion line would be a major breakthrough, but even null results will provide the strongest limits on the axion photon coupling and will significantly deepen our understanding of DM in the Universe.

Authors: Mr. Andrew Zic (The University of Sydney/CSIRO Astronomy and Space Science), Dr. Christene Lynch (International Centre for Radio Astronomy Research/ASTRO-3D), A/ Prof. Tara Murphy (The University of Sydney), A/Prof. David Kaplan (University of Wisconsin-Milwaukee), Dr. Emil Lenc (CSIRO Astronomy and Space Science), Dr. Steve Croft (University of California, Berkeley)

Abstract: Stellar flares and coronal mass ejections (CMEs) play an important role in the habitability of their planetary companions. M-dwarf stars are particularly interesting in this regard, as they are known to flare more frequently and powerfully than the Sun. At the same time, a large number of these stars are likely to host planets within their habitable zones. While stellar flares have been observed ubiquitously across the electromagnetic spectrum, observational identification and characterisation of space weather events such as CMEs around other stars is lacking. These space weather events are expected to cause effects such as magnetospheric compression and atmospheric stripping of planetary companions, degrading their habitability. One promising method to detect and characterise stellar CMEs is to detect low-frequency Type II bursts, associated with radio emission from the CME shock front. Analysis of these bursts from the Sun have enabled characterisation of the CME shock-front propagation speed, the density profile of the corona and interplanetary medium, and the mass and energy of the ejected material. Therefore, the detection and characterisation of Type II bursts from other stars may lead to rich insights into the activity of these stars, and its impact on the habitability of their planetary companions. We propose 54 hours of observations targeting the closest star to the Sun, Proxima Centauri. This star is a well-known active flare star, but its low-frequency emission remains observationally unconstrained. This program will enable us to understand the low-frequency emission of this star, and may allow us to make the first detection of a Type II burst from another star beyond the Sun. This will permit a more detailed characterisation of the impact of Proxima Centauri’s activity on various stellar parameters, as well as the habitability of any planetary companions such as Proxima Centauri b.

Authors: Gianni Bernardi (INAF-IRA), Franco Vazza, (Dipartimento di Fisica e Astronomia, Bologna University), Nicola Locatelli (Dipartimento di Fisica e Astronomia, Bologna University), Federica Govoni (INAF-Osservatorio Astronomico di Cagliari), Benjamin McKinley (ICRAR - Curtin)

Abstract: Detecting radio emission from gas bridges in between galaxy clusters before major mergers has the potential of unveiling so far underlooked pre-processing of gas matter and magnetic fields in large scale structures. We propose here to use the MWA observe the gas bridge in the A3391—A3395 interacting pair of galaxy clusters, whose emission has already been detected in the X-ray band and through the Sunyaev-Zeldovich effect. Through simulations, we predict that the radio emission from the bridge will be at the 10-20 mJy level, on scales of 10-20’ in ~150-200 MHz frequency range - mostly powered by weak shocks in the interaction regions of the two structures - in presence of ~0.1-0.2 μG magnetic fields. Such expectations are within the capabilities of a 4 hour long MWA observation. The proposed observations will shed light into acceleration mechanism in the outer regions of galaxy clusters which are important in the pre-processing of gas matter and magnetic fields well before more classic major mergers takes place.

Authors: Zhenghao Zhu (Shanghai Jiao Tong University, SJTU), Haiguang Xu (SJTU), Dan Hu (SJTU) Weitian Li (SJTU), Chenxi Shan (SJTU)

Abstract: Galaxy clusters, located at the nodes of cosmic web, are thought to be grown by the infalling mass from surrounding environment. This process are expected to produce virial shocks and deposit a certain fraction of infalling energy into magnetic fields and electrons, latter of which would be accelerated into high relativistic speed. During this process, both γ-ray from inverse Compton emission and synchrotron emission in radio frequency are expected. Recently, Keshet et al. (2017) has found a preliminary evidence of the virial shock outside Coma cluster in γ-ray, but the very existence is still not confirmed, which makes the cluster virial shock remain a theory. With high sensitivity of MWA phase II, it is likely that a clear detection of radio emission from virial shock for the first time can be made by observing Coma cluster. Thus, we propose a deep pointed observation to Coma cluster across full MWA Phase II band 72-231 MHz, for a total observation time of 10 hours to investigate the virial shock. The high-quality radio images and spectral information would help us to confirm the existence of virial shock. Using this signal, we can also give constrains to the magnetic fields of the intergalactic medium (IGM) and trace the possible warm-hot IGM around. Besides, a detailed study of such diffuse structure could also help implement the task of removing the complicated foreground of the 21 cm signal from the era of reionization.

Authors: Mr. Vivek Venkatraman Krishnan (Max Planck Institute for Radio Astronomy), Dr. Willem van Straten (Auckland University of Technology), Dr. Aris Noutsos (Max Planck Institute for Radio Astronomy), Dr. Ramesh Bhat (Curtin University), Prof. Michael Kramer (Max Planck Institute for Radio Astronomy), Mr. Bradley Meyers (Curtin University), Dr. Chris Flynn (Swinburne University of Technology), Dr. Steven Tremblay (Curtin University)

Abstract: We propose timing observations of the relativistic pulsar - white-dwarf binary, PSR J1141−6545. This unique system offers access to a wide range of physics, from relativistic phenomena through pulsar emission mechanism studies, to the structure and distribution of interstellar plasma along our line-of-sight. Our recent analysis of data obtained over almost 2 decades suggests that the pulsar’s 1.4 GHz beam will precess away from our line-of-sight in the next 3-5 years. This provides a unique opportunity to perform regular observations of the pulsar across a wide range of frequencies, which might provide constraints on the latitudinal radius to frequency mapping of the pulsar beam. Our analysis of the 1.4 GHz data also revealed significant phase-resolved, secular Rotation Measure variations. The proposed observations of this pulsar at low frequencies will help resolve if these variations are magnetospheric, due to the intra-binary medium or to interstellar scattering, and result in reliable estimates of the pulsar’s geometry from the Position Angle sweep of its linear polarisation.

Authors: Mr. Hongyu Gong (PI) (Shanghai Astronomical Observatory, CAS), Dr. Zhongli Zhang (Shanghai Astronomical Observatory, CAS), Mr. Xuzhi Hu (Shanghai Astronomical Observatory, CAS), Dr. Ramesh Bhat (Curtin University), Ms. Mengyao Xue (Curtin University), Dr. Franz Kirsten (Chalmers University of Technology, Sweden)

Abstract: Pulsars experience a profile broadening when their emissions traverse through turbulent interstellar medium (ISM). This multi-path propagation effect can be quantified by the scattering time scale τ which is strongly dependent on frequency (τ ∝ ν −α ), while the index α represents intrinsic properties of the ISM in depth. In literature, the scattering time scale indices are predicted by models to be α = 4 or 4.4, while showing large discrepancy in observational results. This is mainly caused by the local ISM complications through the pulsar signal transmission, however, could also be biased by observations taken in different frequencies at different time, and with dissimilar telescopes. Recently, using three pulsars, Kirsten et al. (2018) has proved that we are able to estimate τ with single epoch, multi-band MWA observations. Hence we are proposing a single 1.5- hours MWA VCS observation on a line-of-sight toward PSR J1820-0427, distributing the 24×128MHz channels into two different centre frequencies. Using this data set, we will be able to estimate τ at two MWA frequency bands and calculate α for at least six pulsars covered in this field. Results from this project could be used to accurately characterize the turbulence of the ISM and develop the Galactic electron density model. This project will also demonstrate that MWA is capable to measure scattering spectra indices for a large sample of pulsars with high efficiency.

Authors: John Morgan (Curtin University), Rajan Chhetri (Curtin University), Jean-Pierre Macquart (Curtin University), Ron Ekers (CSIRO), Steven Tingay (Curtin University), Elaine Sadler (University of Sydney/CSIRO)

Abstract: During the first half of 2016 we conducted IPS observations for approximately 1 hour daily using Director’s time. Just two of these observations have been used to write a series of papers [1-5], and work is ongoing to reduce a larger fraction these data. Here we propose to conduct a similar number of observations during 2019-A. With one hour of observing per day we can cover all solar elongations where we can make optimum IPS measurements. By using the Phase II extended MWA, we expect to approximately double our sensitivity.

In our Guaranteed Time proposal we requested 1 hour per day. This is sufficient to cover all pointings relative to the Sun where we can make ‘optimum IPS measurements’. We have used this time to schedule 6✕10-minute observations per day at approximately 30° elongation from the Sun at Position Angles 60, 90, 120, 240, 270 and 300 (degrees relative to ecliptic North). Position angles further North than this are not accessible at this time of year. However a further 30 minutes per day would allow us to supplement these with Position Angles 150, 180 and 210, giving coverage of the areas of the sky affected by the polar solar wind. This wind is less dense and therefore produces weaker scintillation, however measurements made here will increase our astrophysical survey area and will be just as interesting from a Space Weather point of view. Additionally, the original call for proposals envisaged the semester finishing on 1 Jul. We would like to extend our observation campaign to the end of the semester which will give us considerably more sky coverage. Therefore we request:

• 30 minutes per day 8 Apr - 30 Jun inclusive (41.5 hours);
• 1h30m per day 1 Jul - 18 Aug inclusive (72 hours);
for a total of 113.5 hours (in addition to our 150 hours of Guaranteed time)

Authors: N. D. R. Bhat (Curtin; PI), M. Xue (Curtin), D. Li (NAOC), W. Zhu (NAOC), P. Wang (NAOC), Z. Pan (NAOC)

Abstract: The Five-hundred metre Aperture Spherical radio Telescope (FAST) in China is fast gearing up for science. With a sensitivity that surpasses all currently operational facilities, this largest single-dish telescope ever built will push the boundaries of parameter space in many areas of science including pulsar searches. Ongoing early science efforts have already led to a number of new pulsar discover- ies, underscoring the exciting scientific promise of this new facility. Here we propose to undertake low-frequency follow-up of a curious pulsar candidate that is potentially detectable with the MWA. The candidate is seemingly bright, with an estimated flux density of ∼4-5 mJy, albeit prominent only within the lower 100 MHz of the 300-800 MHz wide-band system that was used to make the de- tection. The indicated dispersion measure (DM) is highly anomalous for the line of sight and the observed degree of scattering is significantly lower than expected for such a DM. The candidate pul- sar is potentially detectable in a dedicated 1.5 hr observation with the MWA over a 200-230 MHz band. If confirmed, this pulsar will be an extremely interesting object to probe one of the strangely anomalous lines of sight in terms of both the degree of scattering and the dispersion measure. This will also serve as an excellent demonstration of the MWAs potential to undertake fruitful follow-ups of new pulsar discoveries and candidates to come from the FAST, which may open up promising avenues for synergetic science in the areas of pulsars using the two facilities.

Authors: Prof. A. A. Deshpande (RRI and IIT-Indore; PI), Ms. Shilpi Chakraborty (IIT-Indore), Dr. N. D. R. Bhat (Curtin), Dr. S. E. Tremblay (Curtin), Ms. Mengyao Xue (Curtin)

Abstract: Despite numerous deep searches over the past 20+ years, the radio pulses from the famous Geminga pulsar (the nearest and brightest gamma-ray pulsar) have remained so far illusive, in the midst of some claims of their detections and many reporting non-detections. The apparent radio-quiet nature of this famous gamma-ray pulsar presents a yet-unsolved puzzle, if not a mystery. The claimed detections, all at the low radio frequencies (~35-110 MHz), are essentially either of low significance (e.g., Malov et al. 2015) or of extremely rare ultra- bright pulses (at 34 MHz; see Maan 2015) with significant spread in the associated dispersion measures. A sensitive search for both periodic and occasional bright pulses done in coordinated manner from at least two distinct locations and simultaneously across a wide range of frequencies appears essential to improve chances of possible conclusive radio detection. With this view, we propose to conduct a coordinated search for radio pulses from the Geminga pulsar using the MWA in its wide-band mode and the Gauribidanur Decametre- wave Radio Telescope (GEETEE) observing in a narrow band around 35 MHz, both recording the data in voltage-capture mode. We request for four 45-min sessions (on 3 separate days) with the MWA to conduct the proposed search using 3 sessions, and one session to observe a control pulsar in the same observing mode. Success of the proposed observations will also pave way for an attractive coordinated mode between the MWA and the GEETEE, offering advantages in both, namely, wider spectral range and finer localization of direction.

Authors: Dilpreet Kaur (Curtin), Dr Ramesh Bhat (Curtin), Dr Steven Tremblay (Curtin), Dr Ryan Shannon (Swinburne)

Abstract: Removal of dispersive smearing at lower observing frequencies requires phase coherent de-dispersed data. The newly augmented pulsar beamforming pipeline which re-constructs the data at much higher time resolution (∼ 1 μs) enables us to perform high time resolution studies of millisecond pulsars (MSPs). Using this we have made successful detections of a MSP J2241 − 5236 down to 80 MHz. This pulsar is rapidly emerging as a highly promising target for pulsar timing array experiments (PTA). The pulsar is in a 3.5 hour circular orbit with a low mass (0.012 M ) white dwarf companion. The timing studies from MeerKAT observations showed some systematic offset in pulse arrival times (TOAs), related to the binary parameters of this pulsar. These drifts in the TOAs could be due to minute variations in DM introduced by the companion’s stellar winds. The high-quality detections of this MSP with the MWA has enhanced the sensitivity of the telescope, to measure minute DM variations of the order of ∼ 10−4 pc cm−3 in our line of sight. Such subtle DM change is hard to recognize at any other timing frequencies with single observation. Taking advantage of the MWA’s large frequency leverage and the achievable timing precision, we propose the observations of full orbital period of this interesting target to investigate any contribution by the companion in the measured DM.