Enabled by the Fermi Large Area Telescope, we now know young and recycled
pulsars fill the gamma-ray sky, and we are beginning to understand their
emission mechanism and their distribution throughout the Galaxy. However, key
questions remain: Is there a large population of pulsars near the Galactic
center? Why do the most energetic pulsars shine so brightly in MeV gamma rays
but not always at GeV energies? What is the source and nature of the pair
plasma in pulsar magnetospheres, and what role does the polar cap accelerator
play? Addressing these questions calls for a sensitive, wide-field MeV
telescope, which can detect the population of MeV-peaked pulsars hinted at by
Fermi and hard X-ray telescopes and characterize their spectral shape and
polarization.Comment: 8 pages, 4 figures, Astro2020 Science White Paper submitted to the
  National Academies of Science

Ajello, Marco

Bernard, Denis

Blumer, Harsha

Grenier, Isabelle

Guiriec, Sylvain

Harding, Alice K.

Kerr, Matthew

Longo, Francesco

Manousakis, Antonios

Prescod-Weinstein, Chanda

Saz-Parkinson, Pablo

Wadiasingh, Zorawar

Younes, George

Zane, Silvia

Zhang, Bing

English

arXiv

Enabled by the Fermi Large Area Telescope, we now know that pulsars fill the gamma-ray sky, and we are beginning to understand their emission mechanism and their distribution throughout the Galaxy. To address key questions calls for a sensitive, wide-field MeV telescope, which can detect the population of MeV-peaked pulsars hinted at by Fermi

Harding, A

Kerr, M

Ajello, M

Bernard, D

Blumer, H

Grenier, I

Guiriec, S

Longo, F

Manousakis, A

Saz-Parkinson, P

Prescod-Weinstein, C

Wadiasingh, Z

Younes, G

Zane, S

Zhang, B

UCL Discovery

Astro2020 Science White PaperProspects for Pulsar Studies at MeV EnergiesThematic Areas:  Planetary Systems  Star and Planet FormationX Formation and Evolution of Compact Objects  Cosmology and Fundamental Physics Stars and Stellar Evolution  Resolved Stellar Populations and their Environments Galaxy Evolution Multi-Messenger Astronomy and AstrophysicsPrincipal Author:Name: Alice K. HardingInstitution: NASA Goddard Space Flight CenterEmail: Alice.K.Harding@nasa.govPhone: (301) 286-7824Co-authors: Matthew Kerr, Naval Research Laboratory, Washington, DC;matthew.kerr@nrl.navy.milMarco Ajello, Clemson UniversityDenis Bernard, LLR, Ecole Polytechnique, CNRS/IN2P3, FranceHarsha Blumer, West Virginia UniversityIsabelle Grenier, Universite´ Paris Diderot/CEA Saclay, FranceSylvain Guiriec, George Washington University/NASA Goddard Space Flight CenterFrancesco Longo, University and INFN, TriesteAntonios Manousakis, University of Sharjah, Sharjah, UAEPablo Saz-Parkinson, University of Hong Kong, Santa Cruz Institute for Particle PhysicsChanda Prescod-Weinstein, University of New HampshireZorawar Wadiasingh, NASA Goddard Space Flight Center, USRA/NPP FellowGeorge Younes, George Washington UniversitySilvia Zane, University College LondonBing Zhang, University of Nevada-Las VegasAbstract Enabled by the Fermi Large Area Telescope, we now know young and recycled pulsarsfill the gamma-ray sky, and we are beginning to understand their emission mechanism and theirdistribution throughout the Galaxy. However, key questions remain: Is there a large population ofpulsars near the Galactic center? Why do the most energetic pulsars shine so brightly in MeVgamma rays but not always at GeV energies? What is the source and nature of the pair plasma inpulsar magnetospheres, and what role does the polar cap accelerator play? Addressing thesequestions calls for a sensitive, wide-field MeV telescope, which can detect the population ofMeV-peaked pulsars hinted at by Fermi and hard X-ray telescopes and characterize their spectralshape and polarization.11 IntroductionOur understanding of rotation-powered pulsars (RPP) has undergone a revolution over the pastten years with the Fermi Gamma-Ray Space Telescope enabling discoveries of more than 230new γ-ray pulsars [1, 2] above 100 MeV. Although ten times as many RPPs have been discoveredin the radio, the high-energy emission from γ-ray pulsars more directly reveals the particleproduction and acceleration in their magnetospheres and dominates the energy budget.Recent rapid advances in global models of the pulsar magnetosphere [6, 21] suggest that theemission in the Fermi band comes from the most energetic particles that are accelerated in thecurrent sheet outside the light cylinder. But, to account for the narrow peaks of the observed lightcurves, these models require a source of abundant electron-positron pairs to screen the largeelectric fields and establish a near force-free magnetosphere. Many emission models suggest thatthese pairs produce a synchrotron radiation component whose spectral power peaks in the 0.1–10MeV range. Detection of this radiation can give us crucial information on the origin and spectrumof the pairs. Unfortunately, the emission in this band has been detected for only a few of theyoungest RPPs, notably the Crab and PSR B1509-58. Hard X-ray observations have detected apopulation of young RPPs exhibiting hard, non-thermal spectra extending from 20 to 200 keV,strongly implying that their spectral power may peak at MeV energies. Most of these, a group of10 “MeV pulsars”, have pulsed detection in neither the radio nor GeV band. From the data wehave, it appears that the broad-band spectra of very young pulsars have dominant hard X-raycomponents that become much less dominant relative to the GeV emission as pulsars age. Thetransition between X-ray and GeV components lies in the MeV band.Thus, even after the amazing advances from Fermi, we are left with a series of questions: How doRPP spectra evolve with age? Where are the pairs produced in pulsar magnetospheres and howdoes their production evolve? If pairs are produced near the polar caps, why don’t we see theiremission? How large is the MeV pulsar population and why are they radio and GeV quiet? Whatis the pulsar population near the Galactic center? As we will detail below, telescopes sensitive inthe 0.2–100 MeV band can answer many of these questions, and the planned capabilities ofproposed telescopes such as AMEGO (the All-sky Medium Energy Gamma-ray Observatory)[25], e-ASTROGAM [8], AdEPT [20] and HARPO [13] are well suited to the task. If consideredfor the next decade, such telescopes will be primed to take advantage of the huge expansion in thepopulation of radio RPPs expected from the Square Kilometre Array [22].2 Studying the Pulsar Life Cycle from MeV to GeVYoung RPPs (1-10 kyr) with large spin-down power produce electron-positron pair cascades thatare thought to take place at the polar caps (PCs) [16], the outer gaps [35], or current sheets [27].The pairs produce synchrotron radiation (SR) in the outer magnetosphere that peaks in the hardX-ray to soft γ-ray band. A smaller number of particles are accelerated to high energy (∼ 10 TeV)in the current sheet and emit curvature radiation (CR) or SR at a few GeV, but the copious pairseffectively screen the accelerating electric fields in most of the rest of the magnetosphere andthereby limit the GeV emission. As these pulsars age, the pair multiplicity, SR, and electric fieldscreening drop, dramatically increasing the ratio of GeV-to-X-ray emission. This spectral life2Figure 1: Left panel: Theoretical spectra for young pulsars [16, 15] showing strong evolution ofthe spectra as the pair multiplicity decreases with characteristic age. Additionally, direct γ-rayemission from the polar cap is expected to peak at 10 MeV [31]. Right panel: X-ray and γ-raydata from a selection of pulsars with broadband detections, along with the AMEGO sensitivity(data from [24, 23]).cycle, based on a number of models [16, 30], is clearly illustrated in Figure 1. This picture canexplain why the GeV luminosity varies so weakly with age and spin-down rate (∼ E˙1/2), but wedo not understand why the pair emission is so sensitive to age. Indeed, the understanding we haveis largely theoretical, since there are only a few pulsars for which we have this full portrait, fromMeV to GeV (Figure 1, right). With the pair SR component expected to decrease with age, bettersensitivity in the MeV band is needed to explore how this emission evolves. The sensitivity of anAMEGO-like telescope will allow exquisite measurements of the MeV spectrum and polarizationof these known broadband emitters as well as discovery of new MeV pulsars (see below).We compared two pulsar population synthesis models [11, 28] that satisfy constraints from pulsarsurveys and found them largely consistent. Adopting the population of [11], and scaling the γ-rayefficiency as η = 0.1(1034erg s−1/E˙)0.5 above E˙ = 1034 erg s−1 and η = 0.1 for smaller E˙, weestimate that an MeV telescope with flux sensitivity of ∼ 10−6 MeV cm−2 s−1 could detect a fewhundred pulsars, primarily through their GeV components (which are broad and detectable belowa GeV; see Figure 2). On the other hand, the SR component from pairs scales as ∼ 10−3 E˙ [18]and we expect to detect a fraction, ∼ 10% with pair components peaking near 0.1–1 MeV. Fermihas detected a strong trend for young RPPs, namely a decrease in the spectral energy distribution(SED) peak of the GeV component with E˙. This trend may explain why there are no γ-raypulsars detected with E˙ <∼ 1033 erg s−1. Theoretical studies of expected γ-ray spectra using globalsimulations [21] have found that the spectra cutoffs, or SED peaks, move below 100 MeV forE˙ ∼ 1033−34 erg s−1 as the maximum particle acceleration energy drops. Therefore, detecting the3γ-ray emission from these low-E˙ pulsars requires a telescope with good sensitivity below 100MeV. We can roughly estimate that a telescope with flux sensitivity of ∼ 10−6 MeV cm−2 s−1between 10 and 100 MeV could discover about 10 pulsars with E˙ <∼ 1033 erg s−1.The open question of whether the GeV emission is CR or SR can be definitively addressed bypolarimetry measurements in the MeV-to-GeV component transition region of 1 - 1000 MeV.Models have predicted [29, 17] that SR should have a low phase-averaged polarization degreearound 10% - 20% while CR should have a much higher polarization degree around 40% - 60%.Time projection chamber telescopes like AdEPT [20] and HARPO [13], that are capable ofmeasuring polarization with pair production, would be able to detect such a polarization-degreetransition as well as an expected change in position angle if CR is the GeV emission mechanism.If, as expected, many of the electron-positron pairs are produced in cascades by QED magneticpair creation near the PCs, the accelerated particles and secondary pairs should emit CR and SRγ-rays [9]. The PC γ-ray emission was originally proposed to be the main observed GeVemission, but the spectra and light curves observed by Fermi have ruled this out in favor of outermagnetosphere emission. However, the PC emission is still expected and can be unambiguouslyidentified by its pulse phase near the radio peak and by phase-resolved polarimetry. A recentestimate of its spectrum [31] for a Vela-like pulsar, shown in Figure 1, cuts off below the Fermiband around 10 MeV. It is therefore expected that an MeV telescope will detect the PC emissionfor the first time, a milestone that will establish the PC as a source of pairs and enable detailedstudy of the high-field microphysics near neutron stars.- 8 - 7 - 6 - 5 - 4 - 311 01 0 01 0 0 0  Numberl o g ( F l u x )  ( M e V / c m 2 / s )Figure 2: Estimated flux distribution of pulsarsfrom a population synthesis model [11]. Thered dashed line indicates the expected sensitivitylimit of an AMEGO-like MeV telescope.Understandingthe production of pairs by RPPs has broadimplications. The mechanism and evolutionof pulsed radio emission from over 2500RPPs is still not understood and constrainingthe multiplicity and spectrum of the PCpairs would provide a direct input to the theradio emission models. The pair cascades alsoprovide the particles that radiate broadbandemission from radio to very-high-energyγ-rays in pulsar wind nebulaeand the pair multiplicity is crucial to modelsof acceleration and evolution in these sources.Pairs from pulsars have also been invoked toexplain the observed local cosmic ray positronexcess [19] and ultra-high-energy cosmic rays.3 The Mystery of MeV PulsarsA significant sub-population of young and energetic RPPs show high fluxes of pulsednon-thermal X-rays but no detected radio or GeV emission. These so-called “MeV pulsars”4present a mystery since most pulsars with high spin down power have pulsed γ-ray and radioemission. Is their anomaly due to geometry, that our line-of-sight misses both radio and GeVbeams [17, 32], or to intrinsic weakness of acceleration and radio emission? Intriguingly, of the18 known RPPs with X-ray emission above 20 keV [24], 8 have narrow, double-peaked hardX-ray profiles and GeV emission while 10 (MeV pulsars) have broad, single-peaked hard X-rayprofiles and no GeV emission. Only one MeV pulsar has a detected spectral power peak, PSRB1509-58 at around 3 MeV, and one other, PSR J1846-0258 has a Fermi-LAT detection [23] thatimplies a peak around 5 MeV (see Figure 3). Many of the others have possible spectral peaks atleast one order of magnitude above an AMEGO-like sensitivity.Figure 3: A selection of MeV pulsars with broad-band detections, along with the AMEGO sensi-tivity (data from [24, 23]).Measuring the propertiesof the MeV pulsar spectra can determinethe location and constrain the spectrum anddensity of the pairs that are the prime sourceof plasma for the pulsar magnetosphere. Thepeak of the SED of SR from pairs in the outermagnetosphere would be Esyn ∼ γ2+BLC,where γ+ is the maximum energy of the pairs,and BLC = B0(R0/RLC)3 is the magneticfield at the light cylinder, RLC = c/Ω, with Ωthe pulsar rotation rate. In models where polarcap pairs produce the SR, there is a strongdependence of γ+, and therefore the SEDpeak, on surface magnetic field strength, B0,Esyn ∝ B0BLC [17]. On the other hand, theSED peak of SR from outer gap and currentsheet pairs has a stronger dependence on BLC,Esyn ∝ B7/2LC [35]. For example, measuringthe spectral power peaks of PSRs J1846-0258and J1838-0655 will provide a strongdiscrimination between these models since both their values of B0 = 9× 1013 G and 3.8× 1012G, and of BLC = 2.5× 104 G and 1.0× 105 G are very different. Discovering more MeV pulsarswith a telescope like AMEGO, whose sensitivity matches their power peak, is very likely andcould double the existing population [8]. The shape of the light curves of a larger populationwould show whether source geometry is the cause of their GeV and radio silence.4 Galactic Center PulsarsAn important legacy from Fermi is the “GeV excess” [19, 7, 3], a GeV-peaked, degree-scalesignal at the Galactic center in excess of expectations from known diffuse and point sources.Various explanations advanced include annihilating dark matter [5] and cumulative emission froman unresolved population of millisecond [4, 34] or young pulsars [26]. Finding these pulsars ischallenging with radio searches due to interstellar scattering and dispersion.The young pulsar hypothesis is particularly appealing: there is a deficit of millisecond pulsar5progenitors [14], and the detection of a few radio pulsars suggests a population of up to ∼ 1000RPPs with E˙ >∼ 1035 erg s−1 near the Galactic center [10, 33]. In addition, the high star formationrate there (e.g. Arches cluster) suggests that a few very energetic newborn neutron stars lie within100 pc of Sgr A*. As we show below, an MeV telescope could definitively detect both thesenewborn pulsars and a substantial portion of this population.1035 1036 1037 1038 1039Spindown Luminosity (E)100101102103Galactic Center Pulsars (NE)10 810 710 610 510 4Gamma-ray Flux (MeV/cm2 /s)PulsarsCR ComponentSR ComponentFigure 4: Left: The Galactic center populationusing an estimated shape of the population fromsynthesis models [28] and normalization from[10, 33]. Right: The γ-ray flux for two radia-tion components as described in §4. The notionalAMEGO sensitivity is shown by the dashed blackline.Using the pulsar population synthesismodels introduced in §2, we determined thatthe cumulative distribution of high-E˙ pulsars(E˙ > 1035 erg s−1) is well described by apower law N(≥ E˙) ∼ E˙− 45 , and we show thispopulation and the following flux estimatesin Figure 4. Because the SR luminosityfrom pairs scales as 10−3E˙ (see §2), the mostenergetic pulsars are preferentially detectedthrough this channel. With a sensitivity of10−6 MeV cm−2 s−1, the E˙ threshold for SR at8 kpc is 1037 erg s−1. For a population of 1000pulsars, about 3% satisfy this threshold, and anMeV telescope like AMEGO could discover30 energetic pulsars near the Galactic center,firmly establishing the presence of youngneutron stars. Moreover, because newbornpulsars generally satisfy E˙ > 1037 erg s−1,AMEGO would strongly detect any young(few kyr) pulsars produced by the intense starformation near Sgr A*. Although the primary(CR) component is in principle detectablefor pulsars with E˙ > 1036 erg s−1, Fermi couldnot necessarily have detected members of thispopulation, as its angular resolution is only∼1◦ at 1 GeV. An MeV telescope relying onan all-silicon tracker will have a substantially better angular resolution [25] and thus be betterable to detect sources through either channel. Indeed, this may be critical, because “blindly”detecting pulsations requires accumulating many source photons as quickly as possible in order toovercome timing noise.Finally, an MeV telescope could also detect a population of millisecond pulsars (MSPs).Although less energetic than unrecycled pulsars, MSPs may be particularly efficient emitters ofMeV γ-rays [18, 16]. Indeed, recent observations of hard X-ray emission from three energeticMSPs [12] suggest the bulk of their energy is emitted in the MeV band. If their efficienciessurpass 1%, then MSPs like B1821−24A and B1937+21, with E˙ > 1036 erg s−1, could bedetected at the Galactic center and signal the presence of a larger population. Detection of eitherpopulation of pulsars would be a major step forward in understanding both the “GeV excess” andthe Galactic center more generally.6References[1] Abdo, A. A. et al. 2013, ApJS, 208, 17.[2] https://confluence.slac.stanford.edu/display/GLAMCOG/Public+List+of+LAT-Detected+Gamma-Ray+Pulsars[3] Ackermann, M. et al. 2017, ApJ, 840, 43.[4] Brandt, T. D., & Kocsis, B. 2015, ApJ, 812, 15.[5] Calore, F., Cholis, I., McCabe, C., & Weniger, C. 2015, Phs. Rev. D, 91, 063003[6] Cerutti, B. and Philippov, A. A. and Spitkovsky, A. 2016, MNRAS, 457, 2401.[7] Daylan, T., Finkbeiner, D. 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Prospects for Pulsar Studies at MeV Energies

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Prospects for Pulsar Studies at MeV Energies

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