Motivation, Funding and Current Status

The base of our current scientific understanding of the universe at large scales is general relativity (henceforth GR), formulated by Albert Einstein in 1915.

Almost 100 years later, GR has passed all experimental tests. In the first phase of this process, the Solar System was used as a natural laboratory to test GR, a process that continues to this day. A second phase started in 1974 with the discovery of the first binary pulsar, PSR B1913+16. Binary pulsars allowed the detection of phenomena that simply cannot be observed the Solar System, like the emission of gravitational waves, which were originally predicted by Einstein himself in 1916. Confirming the existence of gravitational radiation opened up an entirely new field of research in physics and astronomy and showed that GR, formulated at a time where only weak fields were observable, continues being valid in a completely unforeseen regime.

Since then, the discovery of the double pulsar system J0737−3039 (Lyne et al. 2004, Science, 303, 1153) has allowed five tests of GR in that system alone - and, despite the unprecedented precision of some of these tests, the theory passes all of them with flying colours! (Kramer et al. 2006, Science, 314, 97).

Despite these successes, we know that GR cannot be the ultimate picture of gravity. The theory predicts the appearance of non-physical singularities at the center of black holes. It appears to be incompatible with the quantum-mechanical standard model of particle physics. Furthermore, the discovery of mysterious phenomena like Dark Energy and Dark Matter gave a new impetus to the study of alternative theories of gravity which might be able to naturally explain these phenomena without mysterious new dark components of the Universe.

What is important to us is that many of these alternative theories of gravity generally predict a violation of the strong equivalence principle (SEP) and/or violation of Local Lorentz Invariance of gravity (LLI). These will cause very small and apparently unimportant perturbations in the orbits of distant millisecond pulsar - white dwarf (MSP-WD) binaries: None of these effects have been detected by the most sensitive experiments yet performed: The combination of these results is extremely powerful for testing alternative theories of gravity. Two examples are Scalar-Tensor theories and Tensor-Vector-Scalar theories, for which we havederived the most stringent limits to date (see Freire et al. 2012), in fact TeVeS already needs to be very fine tuned in order to explain the MOND phenomenon.

In an important recent paper, Kent Yagi, Diego Blas, Enrico Barausse and Nicolas Yunes (2013, arXiv:1311.7144) used the LLI and SEP violation results listed above, plus the limits on two previous binary systems (the double pulsar and PSR J1141−6545), to constrain the Einstein-Æther and Hořava gravity theories to the point that the surviving theories predict NS-NS mergers undistinguishable from those of GR. These four classes of theories were among the few well-studied and well-motivated alternatives to GR that were candidates for testing by advanced LIGO and Virgo.

This is just the start. As theoretical work proceeds and the pulsar experiments become more precise, several other gravity theories will no doubt be excluded. This implies that the pulsar experiments are already contributing to a clarification of the current picture of Cosmology, where gravitational theories attempting to explain dark matter, dark energy and inflation have increased exponentially in recent times, but where hard data is still lacking.

None of this means that none of these hypothetical effects occurs at lower levels. We should not forget that, if they exist, their detection would falsify GR. To do that, we need to improve the precision of the pulsar timing technique.

Fig. 1 (left): The UBB receiver, as seen at the end of June 2012, shortly before being sent to Effelsberg

Going further

The precision of pulsar timing is presently limited by two factors. The solution to both problems is the use of an ultra-broadband receiver (UBB), with corresponding broadband back-ends. Since pulsars are broadband sources, increasing the bandwidth should increase the signal-to-noise ratio of the detection and therefore contribute to improve the precision of the timing. More importantly, the large band allows a great improvement in the measurement of the daily variations of the electron column density between the Earth and the pulsar, therefore mitigating the main systematic effect in our measurements.

With such a system, we will be able to do even more precise tests of GR. What is new is that our precision is becoming so high that if we do not falsify GR, we will start falsifying other alternative theories of gravity, like TeVeS. Excluding these theories would have important implications not only for physics, but also for our understanding of the constituents of the the Universe.

This observing system could potentially have vast implications in other areas of research, like the detection of gravitational waves using pulsar timing arrays. Furthermore, to do these precise tests of GR, we need to measure precisely the masses of the millisecond pulsars we are using to do these tests. If we measure a high NS mass, that will be of great importance to the study of nuclear physics. Currently, the most constraining neutron star mass measurement is that of PSR J0348+0432 (see Antoniadis et al. 2013, Science, 340, n. 6131 *), which has partly resulted from our study of the potential of this system as a gravitational laboratory.

Beacons in the dark

The ``Beacons in the Dark'' project (short form BEACON), submitted by P. Freire to the European Research Council (ERC) in October 2010, requested a total of 1.9 million euro. This included, among other items, funding for the construction of an ultra broadband receiver (UBB) with a frequency between 0.6 and 3 GHz - the optimal frequencies for pulsar timing. The grant also covered the construction of the associated back-ends and their installation, maintenance, and operation at the Effelsberg 100-m telescope for a period of 5 years. Recognizing the scientific importance of the project and its potential scientific impacts (marked with top grade of all proposals in Europe!), the ERC decided to fund the project in June 2011.

Implementation started in September 2011 by the MPIfR workshops and the electronics labs. The main back-end system consists of a high-speed (12.5 GHz) Tektronix ADC, capable of fully Nyquist sampling two 3.125 GHz polarization channels, supported by a complex and challenging interface module. This forwards the 8-bit voltage data to a digital polyphase spectrometer, to be i mplemented in a FPGA-based Uniboard 1 computer. The firmware for this is being implemented at INAF/Arcetri, our Italian partner institution in the BEACON project. This produces discrete 25-MHz bands which are then coherently dedispersed and folded in real time by a GPU cluster.

In parallel, we are also increasing the bandwidth of the existing pulsar timing system (ASTERIX - based on ROACH boards and conventional CPU coherent dedispersion). This allows for early science to be done with the UBB. Furthermore, when the main back-end comes online, it will provide important redundancy, allowing a very important cross-verification of its timing accuracy.

In June 2012, the receiver (Fig. 1) was being tested in the lab. In Fig. 2, we look down into the quad-ridge horn, designed by Sander Weinreb at the Jet Propulsion Labs within the US Technology Development Project (TDP). The feed is one of the innovative features of this receiver, machining it was a major task, requiring inputs from all parts of the electronic division and many man-months from the technicians at the MPIfR workshop. In Fig. 3, we look down on the back of the horn and we can see how it is connected to the dewar. These connectors are of a novel design and they lower the receiver noise significantly - a very important quantity for pulsar experiments. The preliminary tests indicate at extremely low system temperature of < 25 K.

Fig. 2: The unique quad ridge design of the horn, which makes the receiver sensitive to frequencies between 0.6 and 3 GHz.

Fig. 3: Top view, illustrating the connection between the horn and the dewar, where the cryogenic low-noise amplifiers are installed.

At the beginning of July 2012 the receiver was hoisted into the focus cabin of the Effelsberg telescope and started undergoing pointing and focusing tests. On July 18, the first pulsar observations were made using the current ROACH board/Asterix pulsar timing system. Amazingly, everything worked very well: we pointed the receiver at a pulsar and immediately a bright detect ion of PSR B1937+21 appeared on the screen.

The observations gave us a preliminary indication of the sensitivity of the system, which was constrained by very strong radio frequency to a higher degree than expected. This was later found to be due to a TETRA transmitter near the Effelsberg telescope, with an unexpectedly strong line at 396 MHz. Although this is out of our nominal detection band, it is so strong that the LNA was still going in saturation for a large fraction of the time.

Because of this, the receiver was fitted with a high-pass filter in front of the first Low-Noise amplifier. This has only slightly degraded the system temperature (by about 1 K), making it a reasonable compromise. The receiver was taken back to the focus cabin of Effelsberg in May 2013 and the results showed that it has substantially decreased the number of times the receiver saturates. The situation is still far from ideal, and it will remain so. Because of RFI, substantial parts of the band have to be deleted, either with filters or later when reducing data (see Fig. 4). This reduces sensitivity. We are studying several ways of combating radio interference and optimizing the system performance in this difficult environment. But eventually, we will have the most accurate pulsar timing system in the world.

Fig. 4: Integrated pulse profile for PSR B1933+16 as a function of radio frequency, as measured by the UBB using a broadband version of the Asterix back-end. The red dot-dashed line represents the delay as a function of frequency expected from interstellar dispersion for this pulsar, the yellow line against dark background represents the measurements. The UBB allows here, for the first time, the simultaneous detection of three frequency bands, centered at 650, 870 and 1300 MHz, where RFI levels were found to be acceptable. This allows a much more precise measurement of the interstellar dispersion relation for this pulsar, which allows for more precise timing.

* The BEACON project includes a large scientific component, which aims to maximize the scientific output from the new receiver. This includes a) a careful study of the systems we will be targeting with the UBB observing system, b) a detailed description of the theoretical aspects of the project, and c) the use of the UBB observing system for precise timing of these systems. The papers indicated with an asterisk fulfill items a) and b) and were partly supported by BEACON.

P. Freire, 2012 October 23.
Last updated: 2014 April 15.