Observations over the past two years with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope have led to a tenfold increase in the number of known γ-ray pulsars. Roughly one third of this population consists of young radio-loud pulsars, one third are radio-quiet pulsars discovered in blind searches of LAT data, and one third are γ-ray millisecond pulsars (MSPs). In this paper I discuss what we have learned about these three populations, as well as prospects for further discoveries

Saz Parkinson, P. M.

English

Observations over the past two years with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope have led to a tenfold increase in the number of known γ-ray pulsars. Roughly one third of this population consists of young radio-loud pulsars, one third are radio-quiet pulsars discovered in blind searches of LAT data, and one third are γ-ray millisecond pulsars (MSPs). In this paper I discuss what we have learned about these three populations, as well as prospects for further discoveries. © Società Italiana di Fisica.link_to_subscribed_fulltex

Saz Parkinson, PM

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Gamma-ray pulsars in the Fermi LAT era

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DOI 10.1393/ncc/i2011-10888-7Colloquia: Scineghe2010IL NUOVO CIMENTO Vol. 34 C, N. 3 Maggio-Giugno 2011Gamma-ray pulsars in the Fermi LAT eraP. M. Saz Parkinson for the Fermi LAT CollaborationSanta Cruz Institute for Particle Physics, University of California Santa Cruz1156 High Street, Santa Cruz, CA 95064(ricevuto il 25 Febbraio 2011; pubblicato online l’1 Giugno 2011)Summary. — Observations over the past two years with the Large Area Telescope(LAT) onboard the Fermi Gamma-ray Space Telescope have led to a tenfold increasein the number of known γ-ray pulsars. Roughly one third of this population consistsof young radio-loud pulsars, one third are radio-quiet pulsars discovered in blindsearches of LAT data, and one third are γ-ray millisecond pulsars (MSPs). In thispaper I discuss what we have learned about these three populations, as well asprospects for further discoveries.PACS 95.85.Pw – γ-ray.PACS 97.60.Gb – Pulsars.1. – IntroductionThe phenomenal success of the Fermi Large Area Telescope (LAT) over the pasttwo years is widely demonstrated by the tremendous progress made in the field of γ-raypulsars; without any doubt we have entered a golden age in γ-ray pulsar astrophysics.This paper gives a brief summary of some of the highlights of this new Fermi LAT era,though given the limited space, it will hardly do justice to all the results presented. Aslightly more extensive review can be found elsewhere [1], and throughout this paper Irefer the reader to the refereed publications where the results are described in full detail.This review is organized as follows: I start with a brief history of γ-ray astronomy beforethe LAT, and summarize the status of our knowledge of γ-ray pulsars prior to the launchof Fermi. I then briefly describe the LAT instrument, pointing out the key advantagesover its predecessors. The main part of the article focuses on the three populations ofγ-ray pulsars being detected by the LAT. I begin with the young γ-ray pulsars foundby folding LAT events with a known radio ephemeris. Next, I discuss the new pulsarsdiscovered by the LAT in blind searches of the γ-ray data. The third type of pulsarsare millisecond pulsars, some of which were known prior to the launch of Fermi, andothers which have recently been discovered by radio telescopes targeting unassociatedLAT sources. I conclude with a brief summary and an outlook for the future.c© Societa` Italiana di Fisica 173174 P. M. SAZ PARKINSON for the FERMI LAT COLLABORATION1.1. Prior γ-ray missions. – Three γ-ray satellites paved the way for Fermi. The firstof these, NASA’s Second Small Astronomy Satellite (SAS-2), was launched in November1972 and collected ∼ 10000 (20MeV–1GeV) photons in approximately six months ofoperations. Among its successes was the discovery of the first unidentified γ-ray source,later known as Geminga [2]. The European satellite, COS-B, launched in 1975, col-lected ∼ 200000 (2 keV–5GeV) photons in its more than six years of operations. COS-Bproduced a catalog of 25 γ-ray sources, of which 21 were unidentified [3]. Finally, theEGRET instrument, onboard NASA’s Compton Gamma-Ray Observatory (CGRO), col-lected ∼ 1.5 million (30MeV–10GeV) events between April 1991 and June 2000, pro-ducing a wealth of results, including a catalog of 271 sources [4], of which more than halfremained unidentified at the time of the launch of Fermi (ne´e GLAST), in June 2008.1.2. γ-ray pulsars in the year 2000 . – CGRO was deorbited in June 2000, and untilthe launch of AGILE and GLAST, provided the best data on γ-ray pulsars. Six pulsarswere firmly detected by EGRET (Crab, Vela, Geminga, PSR B1055–52, PSR B1706–44,and PSR B1951+32), and one more, PSR B1509–58, was detected at lower energies byCOMPTEL. A few others were detected with marginal significance, including a millisec-ond pulsar. Only one pulsar, Geminga, was radio quiet, and predictions of the number(and type) of pulsars the LAT would detect varied widely. For a review of the observa-tions of γ-ray pulsars at the end of the EGRET era, see [5]. While it was thought that alarge number of unidentified EGRET sources (especially along the Galactic plane) couldbe pulsars [6], and a few pulsars were indeed discovered in radio searches of the EGRETerror circles (e.g. [7]), the insufficient sensitivity of EGRET, and its poor angular reso-lution, resulted in the majority of the EGRET sources remaining unidentified, makingtheir identification one of the primary goals of the GLAST mission.1.3. The Fermi Large Area Telescope. – The LAT is the primary instrument onboardthe Fermi Gamma-ray Space Telescope. It is a pair-conversion telescope that consists of asilicon tracker, sitting on a cesium iodide calorimeter, each made of 16 modules arrangedin a 4 × 4 array, covered by a segmented anticoincidence detector. The key advantagesof the LAT, compared to EGRET, are a larger field of view (∼ 2 sr), larger effectivearea (9500 cm2, at 1GeV, normal incidence), a better point spread function (∼ 0.6◦ at1GeV), much broader energy range (from 20MeV to > 300GeV), and a significantlyshorter deadtime. In addition, the LAT operates in continuous sky survey mode, makinga more efficient use of its time in orbit. For a detailed description of the LAT, see [8].2. – The Pulsar Timing and Search ConsortiaPrior to Fermi, all γ-ray pulsars were first discovered in other wavelengths (mostlyradio, except for Geminga, which was discovered in X-rays(1)). The γ-ray pulsationswere subsequently obtained by folding the photons with the known timing solution. Rec-ognizing the importance of contemporaneous radio and X-ray observations of pulsars inthe Fermi era, the Pulsar Timing Consortium was set up, between the LAT Collabora-tion and observers using the largest radio telescopes in the world, and RXTE, with thegoal of timing nearby energetic pulsars, to enable the detection of γ-ray pulsations withthe LAT [9]. Similarly, the Pulsar Search Consortium was set up after the Fermi launch,(1) Although it could have been discovered in blind searches of EGRET γ-ray data [19].GAMMA-RAY PULSARS IN THE FERMI LAT ERA 175between essentially the same group of radio astronomers(2) and members of the LATCollaboration, with the somewhat different goal of searching for radio pulsations fromLAT-discovered pulsars, as well as for new radio pulsars in LAT unassociated sources.3. – Young radio-loud gamma-ray pulsars3.1. The EGRET pulsars as seen by the LAT . – Before discussing any new pulsarsdetected by the LAT, it is important to highlight some results from LAT observations ofthe so-called EGRET pulsars. Because these pulsars are so bright, and the LAT sensitiv-ity so much greater than that of EGRET, these observations offer a great opportunity tostudy features of the pulsar light curves and spectra that were until now unimaginable.The huge number of photons collected in LAT observations of Vela [10, 11], allow us tosee some exquisite details, as well as to extend the detection of pulsed emission up to atleast 20GeV, while at the same time detecting a third peak in the pulse profile whichwas hitherto unknown, and which has an evolution with energy that is markedly differentto the other two peaks, hinting at an origin in a different part of the magnetosphere.Observations of the Crab pulsar [12], with the improved timing resolution of the LAT,make it possible to determine that the γ-ray peak leads the radio peak by 281±12±21μs,whereas it was previously thought that the two peaks were aligned [12]. LAT observa-tions of Geminga [13], and of the three remaining EGRET pulsars [14] similarly illustratethe enormous leap in capabilities of the LAT, allowing, thanks to the increased statistics,phase-resolved spectral analyses to be carried out.3.2. Young gamma-ray pulsars found using ephemerides. – As of the time of thisconference(3) there were 22 new young radio selected γ-ray pulsars detected by the LAT(not counting the EGRET pulsars). Early detections included some bright γ-ray pulsars,such as PSRs J1048–5832 and J2229+6114 [15]. In the case of J1048–5832, the LATdetection confirmed a previous claim of a marginal detection with EGRET [16]. Morerecently, the LAT has detected the highest B-field γ-ray pulsar so far, J1119–6127, withan implied surface magnetic field of ∼ 4× 1013 G [17].4. – Blind-search pulsarsThe sparseness of gamma-ray data makes traditional FFT searches computationallyexpensive. A new method, based on taking time differences of events was devised [18]and demonstrated to work very effectively on EGRET data [19]. The application of thetime differencing technique on LAT data has been one of the early successes of Fermi.4.1. Gamma-selected pulsars in blind searches. – Following the discovery of the first 16γ-ray pulsars found in blind searches of LAT data [20], an additional 8 pulsars were foundusing one year of data [21]. A large number of these LAT-discovered pulsars (13 of thefirst 16) had a corresponding EGRET source, thus at least partly solving the problemof EGRET unidentified sources. Some of these sources had been the object of deepsearches (both in radio and X-ray) and were sometimes even expected to harbor radio(2) Telescopes used by the Pulsar Timing and/or Search Consortia: Arecibo (US/Puerto Rico),Effelsberg (Germany), GBT (US), GMRT (India), Jodrell Bank (England), Nanc¸ay (France),and Parkes (Australia). In addition, RXTE is used for timing certain X-ray pulsars.(3) 8–10 September 2010.176 P. M. SAZ PARKINSON for the FERMI LAT COLLABORATIONquiet pulsars; such was the case of 3EG J1835+5918, the brightest unidentified EGRETsource off the Galactic plane, affectionately known by some as the Next Geminga, and nowshown to contain the fairly old (τ ∼ 1.8 million years) PSR J1836+5925 [22]. Many ofthe pulsars found in LAT blind searches are associated with previously known supernovaremnants (SNRs) or pulsar wind nebulae (PWNe). The discovery of many of thesepulsars by the LAT not only ends some long-running searches for individual “missing”pulsars, but also improves our understanding of such systems and ultimately will resultin a more complete census of the neutron star population in our Galaxy.4.2. Multiwavelength connections . – As discussed in sect. 2, in order to search forpossible radio pulsations from LAT-discovered pulsars, the Pulsar Search Consortiumwas set up between all the major radio telescopes and the LAT Collaboration. Deepsearches of all 24 gamma-selected pulsars discovered by the LAT(4) resulted in only 3radio detections [23, 24], meaning that approximately 50% of all young γ-ray pulsarsdetected by the LAT remain radio quiet, or at least radio faint. The very low radioflux (∼ 3.4μJy) from PSR J1907+0602, detected in a long observation using the 305-mArecibo telescope [24], suggests that maybe sometimes only a faint section of the radiobeam sometimes could be crossing the Earth, blurring the distinction between the so-called radio loud and radio quiet pulsars; this means that future more sensitive radiotelescopes may actually be able to detect radio pulsations from some of the pulsarscurrently classified as radio quiet.X-ray observations have proven invaluable in the discovery and further study of LATblind-search pulsars. In some cases, X-ray observations served to identify, a priori, likelyPWNe being powered by a radio-quiet pulsar. This was the case for PSRs J0007+7303(in CTA1), J1418–6058 (aka the rabbit), or J1809–2332 (aka Taz ). In other cases, X-rayobservations served, a posteriori, to refine the LAT timing solution, by providing a moreaccurate position of the likely counterpart. Such was the case for PSRs J0633+0632 andJ1741–2054, where short (∼ 5 ks) Swift observations were sufficient to reveal the likelyX-ray counterparts [20].A strong connection exists between GeV γ-ray pulsars detected by the LAT and TeVsources seen by ground-based Cerenkov telescopes. In a study of the TeV emission coin-cident with sources from the Fermi Bright Source List, the Milagro detector found a sur-prisingly high number of young LAT pulsars coincident with significant sources of emis-sion (presumably from the PWNe) at TeV energies [25]. For example, the blind-searchpulsar PSR J2032+4127 is coincident with TeV J2032+4130 (aka MGRO J2031+41), thefirst unidentified TeV source, originally discovered by the HEGRA experiment, about tenyears ago [26]. In some cases, the discovery of an energetic γ-ray pulsar coincident witha TeV source can change the probable interpretation of the TeV emission. A case inpoint is PSR J1023–5746 (the most energetic γ-ray pulsar found in blind searches sofar), coincident with HESS J1023–575. Originally, the TeV emission from this sourcewas believed to be primarily due to colliding winds from massive stars, while the TeVPWNe interpretation now seems more likely [21].(4) Not all LAT-discovered pulsars required new radio follow-up observations. For example,3EG J1835+5925 had been searched deeply enough prior to the launch of Fermi, so that foldingthe radio data with the LAT timing solution already produced a very low upper limit on theradio pulsations of PSR J1836+5925 [22]. In the case of PSR J1741–2054, on the other hand,radio pulsations were detected in archival Parkes multibeam survey data from the year 2000 [23].GAMMA-RAY PULSARS IN THE FERMI LAT ERA 1775. – Millisecond pulsars5.1. Previously known millisecond pulsars. – Aside from the marginal detection ofPSR J0218+4232 with EGRET [27], millisecond pulsars were not known to be a classof γ-ray emitters prior to Fermi, so it was one of the early surprises when the LATdetected, within a few months, strong γ-ray emission from a population of MSPs [28].Perhaps more surprising was the fact that both the light curves and spectra of thesefirst 8 γ-ray MSPs resembled so much those of young γ-ray pulsars, indicating that theemission mechanism for both types of pulsars is likely to be the same. Following theinitial detection of the first 8 MSPs with the LAT, γ-ray millisecond pulsars continue tobe detected, including PSR J0034-0534, of particular interest due to the co-alignmentof the radio and γ-ray peaks, suggesting that at least in some cases the two regions ofemission might be co-located [29].5.2. New millisecond pulsars in LAT sources. – Perhaps a bigger surprise than thedetection of γ-ray emission from MSPs was the discovery, in a matter of months, of∼ 20 new radio MSPs in searches of LAT unassociated sources. Unlike in the case ofEGRET, where the large positional uncertainty required numerous pointings with radiotelescopes, LAT sources typically have error regions that are comparable in size to theradio beam. Initial searches with the GBT of promising bright unassociated sources fromthe Bright Source List uncovered the first three MSPs [30], followed in rapid successionby others found with the Nanc¸ay [31], and Parkes [32] radio telescopes. The discovery, ina relatively short time (under a year), of such a large number of MSPs outside of globularclusters illustrates the difficulty of finding these pulsars in standard radio surveys. Unlikeyoung pulsars, MSPs are distributed isotropically in the sky and therefore are harder tofind. This, perhaps, explains why some of the extremely bright MSPs being discoveredthanks to Fermi, would have been missed. It is also encouraging to note the apparentlack of correlation between the γ-ray and radio fluxes of these pulsars, suggesting thatadditional MSP discoveries can be expected as fainter LAT unassociated sources aresearched [30]. Because these radio MSPs were discovered by pointing at LAT sources, itis also reasonable to expect that most of these will eventually turn into γ-ray pulsars too,and in fact that has already turned out to be the case for at least seven of them [30-32].6. – Conclusions and future prospectsThe enormous progress made by Fermi in the field of γ-ray pulsars is attested to bythe fact that the discovery of new pulsars with the LAT was considered the runner-upscientific breakthrough of 2009 by the journal Science [33]. While the results so far havebeen impressive, the prospects for future discoveries with the LAT remain just as excit-ing. In addition to finding increasing numbers of γ-ray pulsars, there are different typesof pulsars yet to be fully explored by the LAT: high B-field pulsars, young binary pul-sars, and maybe even radio quiet millisecond pulsars. In addition to this, the increase instatistics will no doubt result in more precise measurements of the relevant parameters ofeach and every pulsar detected by the LAT. Work has already begun towards the gener-ation of a second Fermi LAT pulsar catalog, which will contain approximately twice thenumber of pulsars contained in the first pulsar catalog and will provide an unprecedentedlevel of detail for theorists and modellers to measure their predictions against.178 P. M. SAZ PARKINSON for the FERMI LAT COLLABORATION∗ ∗ ∗I thank the organizers for inviting me to this conference and for their hospitalityduring my stay in the wonderful city of Trieste, and I congratulate them on the smoothorganization of such a stimulating and productive meeting. The Fermi LAT Collaborationacknowledges support from a number of agencies and institutes for both development andthe operation of the LAT as well as scientific data analysis. These include NASA andDOE in the United States, CEA/Irfu and IN2P3/CNRS in France, ASI and INFN inItaly, MEXT, KEK, and JAXA in Japan, and the K. A. Wallenberg Foundation, theSwedish Research Council and the National Space Board in Sweden. Additional supportfrom INAF in Italy and CNES in France for science analysis during the operations phaseis also gratefully acknowledged.REFERENCES[1] Ray P. S. and Saz Parkinson P. M., Proceedings of ICREA Workshop on The High-Energy Emission from Pulsars and their Systems (2010) [arXiv:1007.2183].[2] Thompson D. J. et al., Astrophys. J., 213 (1977) 252.[3] Swanenburg B. N. et al., Astrophys. J. Lett., 243L (1981) 69.[4] Hartman R. C. et al., Astrophys. J. Suppl., 123 (1999) 79.[5] Thompson D. J. et al., High Energy Gamma-Ray Astronomy: International Symposium,AIP Conf. 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Gamma-ray pulsars in the Fermi LAT era

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