Capítulo 2. HÁBITAT GENERAL DE Juniperus thurifera EN CASTILLA Y LEÓN
2.4. DISCUSIÓN
2.4.3. Del análisis multivariante
This chapter has presented the main methods and techniques used throughout this thesis, in particular the pulsar search algorithm and the data reduction of millimetre wavelength observations. The main challenges arose from the use of newly developed receivers, technologies that in some cases were applied to the observations of pulsars for the rst time (like the EMIR receiver of the IRAM 30-m telescope). In this sense, we had to deal with problems for which, in many cases, we had little or no previous experience. Some examples are the large amount of red noise present in the millimetre pulsar data, the absolute ux density calibration of the IRAM 30-m data, or the fact that the eects of dispersion in the ISM are minimal or negligible at very high radio frequencies.
The pulsar searching observations and data reduction follow a standard method- ology whose principles have not changed in decades. However, during this thesis we make use of the most modern algorithms and hardware, which improved our analysis speed and sensitivity to pulsar binaries considerably. New areas where improvement is possible have at the same time been identied. One is the quantitative assessment of the impact of the red noise on the detection of slow spinning pulsars in a blind search performed on our high frequency surveys. This eect is not taken into account in the sensitivity limits presented here or in any other similar work (with exception ofLazarus et al. (2015) for the P-ALFA L−band survey with the Arecibo telescope). A second
pending task is the development of a robust method to distinguish RFI from celestial signals when the eect of dispersion is not of help. This should reduce signicantly the uncertainty that we face when detecting a pulsar-like signal in our high frequency surveys (see Section 3.2.2.3).
The PAF and the UBB receivers at Eelsberg are currently under commissioning, and will be made available to the scientic community after the tests are completed. The commissioning works follow systematics measurements of the specications and robustness against RFI of the systems. This includes a combination of engineering and scientic work. More details about these commissioning test are presented in Chapter7.
Searching for pulsar binaries in the
Galactic Centre with the Eelsberg
100-m radio telescope
This chapter is based on an article in preparation to be submitted to Monthly Notices of the Royal Astronomical Society when completed. I am the lead author of the article. My main contributions include the development of the pulsar searching pipeline, its installation in the Max-Planck-Gesellschaft's supercomputer Hydra, the processing of the data, the review of the pulsar candidates, the simulations of the impact of extreme accelerations on the signal recovery, and the writing of the article. Most observations were undertaken prior to this work as part of a Galactic Centre pulsar monitoring campaign. I observed on two occasions.The full list of authors is:
P. Torne, R. P. Eatough, R. Karuppusamy, M. Kramer, and B. Klein
Abstract
Pulsars located in the innermost region of the Milky Way can be excellent tools to improve our understanding of gravity and the Galactic Centre environment. The high stellar densities of the Galactic Centre, that includes many massive stars, increases the probabilities of nding there exotic binaries like pulsars in orbit with stellar-mass black holes or the supermassive black hole Sgr A* itself. Those extreme systems would provide unique laboratories for black hole physics and to test General Relativity. However, and despite many eorts surveying the centre of the Galaxy, only six pulsars have been discovered within 15 arcmin from Sgr A*, and none of them in binary systems. This paucity of discoveries highly contrasts with the high number of pulsars predicted to exist in the region. With the objective of nding the elusive Galactic Centre pulsars, we present a new, multiepoch survey of the Galactic Centre at four dierent central frequencies, 4.85, 8.35, 14.60, and 18.95 GHz, using the Eelsberg 100-m radio telescope. The data analysis included acceleration searches to increase our sensitivity to binary pulsars. At the moment of writing, 100 per cent of the observations were processed, 100 per cent of the results from the analysis through single pulses were reviewed, and 23 per cent of the candidates from the periodicity search have been inspected, with no new discoveries until now. Although the lack of more discoveries is certainly puzzling, we conclude that we cannot rule out the possibility that the non-detections are simply due to a lack of sucient sensitivity in our survey.
3.1 Introduction
The Galactic Centre (GC) is a region of particular interest to nd pulsars. It contains a∼4.3·106Msuncompact object at its centre, coincident with the radio source Sgr A*, believed to be a supermassive black hole (SMBH) at a distance of 8.3 kpc (Eckart & Genzel, 1996; Gillessen et al.,2009). Furthermore, the stellar densities in the GC are large and contain many massive stars (Genzel et al., 2010; Pfuhl et al., 2014), the progenitors of neutron stars and black holes. The presence of these massive stars are an indirect indication that pulsars should exist in the GC, and recent works based on previous observations and population analysis estimate in up to ∼1000 the number of
pulsars in the GC (Pfahl & Loeb,2004;Wharton et al.,2012). Assuming that pulsars are formed in or arrive at the GC, the high stellar densities of the region will increase the interactions between objects. Such interactions have the potential to create a very rich menagerie of binary pulsar systems. Therefore, it is reasonable to expect not only a high number of pulsar binaries in the GC, but also an increased probability of exotic systems, like for instance pulsars orbiting black holes (Faucher-Giguère & Loeb,2011). If a detected pulsar in the GC is in a close orbit1 with Sgr A* or even a lower mass
black hole, General Relativity (GR) and other theories of gravity could be tested to an unprecedented precision (Wex & Kopeikin,1999;Liu et al.,2012,2014;Psaltis et al., 2016). Additionally, any pulsar, even isolated, is a potential precision tool to study the GC medium properties (e.g. Eatough et al., 2013c;Schnitzeler et al., 2016), and will help to understand better the GC stellar population and evolutionary history.
Many authors have surveyed the GC for pulsars in the past (e.g. Johnston et al., 1995;Kramer et al.,2000;Manchester et al., 2001;Klein,2005;Johnston et al.,2006; Deneva et al.,2009;Macquart et al., 2010; Bates et al.,2011;Eatough et al.,2013a). In spite of the eorts, only six pulsars were found within 15 arcmin from Sgr A*, none of them in binary systems (Johnston et al.,2006; Deneva et al.,2009;Eatough et al., 2013c). Before the detection of the GC radio magnetar SGR J1745−2900 in 2013 (Mori
et al., 2013; Kennea et al., 2013;Eatough et al., 2013c; Shannon & Johnston, 2013), the lack of detections close to the centre of the Galaxy was explained by the strong scattering predicted in the direction of the GC. The temporal pulse broadening time was estimated from electron distribution models to be up to τs ∼2000ν−4s, where ν is the radiation frequency in gigahertz (Cordes & Lazio, 2002). Such hyperstrong scattering toward the GC, if real, would render almost any pulsar undetectable at low radio frequencies, specially those spinning fast. Since the frequency dependence of scattering is strong, τs∝ν−4, the paucity of pulsar discoveries at low frequencies led to more recent surveys to be carried out at higher frequencies, in order to alleviate the negative eects of scattering (e.g. Macquart et al.,2010;Eatough et al.,2013a). The main drawback of observing at high radio frequencies is that pulsars, being typically steep spectrum sources (Maron et al.,2000,hαi=−1.8±0.2, forSν ∝να), are much
fainter and become more dicult to detect. Thus, we require of the largest and most
1Close orbits around Sgr A* are those with orbital periods of the order of a year or less. In the case of orbits around stellar-mass black holes, close orbits refer to those with orbital periods of the order a few days or less.
sensitive radio telescopes to carry out pulsar surveys at high radio frequencies.
The hyperstrong scattering scenario toward the GC was put into question by the pulse broadening measurement of the radio emission from SGR 1745−2900 (Spitler
et al.,2014), which is by far the closest pulsar to Sgr A* with a projected separation of
≈3 arcsec = 0.1 pc (Bower et al.,2015). Spitler et al.(2014) showed that the temporal
scattering for this pulsar is much less than the one predicted byCordes & Lazio(2002), with a measured value of only τs∼1.3ν−3.8s (Spitler et al.,2014), making it dicult to explain the previous lack of pulsar discoveries due to hyperstrong scattering alone. With such low temporal broadening we should already have detected many more pulsars in past surveys, and it is not clear why this has not been the case.
In addition to scattering, other proposed causes for the non-detection of pulsars at the GC include the existence of a singular pulsar population in the region, dominated perhaps by magnetars that rarely emit in radio (Dexter & O'Leary, 2014), or MSPs that are more challenging to detect, since they exist often in binary systems and are much more aected by scattering broadening (Macquart & Kanekar,2015). It has been even suggested that the lack of detections may be due to the rapid collapse of pulsars into black holes by accretion of dark matter (Bramante & Linden,2014).
The absence of many more pulsars discoveries despite previous eorts, together with the high predicted number of pulsars in the GC indicates that we do not fully under- stand yet the scattering medium in this direction, nor the population of pulsars that may exist there. The single recent discovery of the radio magnetar SGR J1745−2900,
an inherently rare object, within one parsec of the central back hole suggests the ex- istence of a much larger GC population of more common pulsars. Despite all these uncertainties, the unique and extraordinary scientic potential of nding new pulsars located in the GC justify further and continuous eorts to survey the region; par- ticularly as new observational hardware that can exploit larger bandwidths becomes available.
Whereas the scattering could still have hindered the detections of pulsars in previ- ous surveys, in particular of millisecond pulsars (MSPs) due to their very short spin periods, the processing of the data did not include a full search in the acceleration pa- rameter space. Consequently, previous works may not have been sensitive to pulsars in tight binary systems or orbiting very massive companions (such as other neutron stars, stellar-mass black holes or Sgr A*), which may be the dominant pulsar population of the GC given its high stellar densities.
With the objective of nding new pulsars in the GC, and with emphasis in highly- accelerated pulsar binary systems, we present a new, multiepoch, multifrequency Galac- tic Centre pulsar survey at frequencies 4.85, 8.35, 14.60, and 18.95 GHz. The observing frequencies used in our observations are high in order to alleviate the scattering eects toward the GC, and the data analysis used a newly developed searching pipeline with capabilities to detect pulsars in binary systems over a large range of accelerations.