Bijan Berenji | California State University, Los Angeles (original) (raw)

arXiv (Cornell University), Apr 19, 2020

Axions are hypothetical particles proposed to solve the strong CP problem in QCD and may constitu... more Axions are hypothetical particles proposed to solve the strong CP problem in QCD and may constitute a significant fraction of the dark matter in the Universe. Axions are expected to be produced in neutron stars and subsequently decay, producing gamma-rays detectable by the Fermi Large Area Telescope (Fermi-LAT). Considering that light QCD axions, as opposed to axions > 1eV, may travel a long range before they decay into gamma rays, neutron stars may appear as a spatially-extended source of gamma rays. We extend our previous search for gamma rays from axions, based on a point source model, to consider the neutron star as an extended source of gamma rays. The extended consideration of neutron stars' leads to higher sensitivity to searches for axions, as it will be shown. We investigate the spatial emission of gamma rays using phenomenological models of neutron star axion emission. We present models including the fundamental astrophysics and relativistic, extended gamma-ray emission from axions around neutron stars. A Monte Carlo simulation of the LAT gives us an expectation for the extended angular profile and spectrum. For a source of 100 pc, we predict a mean angular spread of 2 • with gamma-ray energies in the range 10-200 MeV, due to the cutoff of the spin-structure function S σ (ω). We demonstrate the feasibility of setting more stringent limits for axions in this mass range, excluding a range not probed by observations before. We consider projected sensitivities for mass limits on axions from J0108-1431, a neutron star at a distance of 130 pc. Based on the extended angular profile of the source, the expected sensitivity of the 95% CL upper limit on the axion mass from J0108-1431 is 10 meV. The limit based on 7.9 years of Fermi-LAT data is 0.76 meV for an inner temperature of the neutron star of 20 MeV. In this work, we consider only QCD axions, but the results may of course be generalized to axion-like particles (ALPs).

arXiv: High Energy Astrophysical Phenomena, 2020

Physical Review D, 2016

We present constraints on the nature of axions and axion-like particles (ALPs) by analyzing gamma... more We present constraints on the nature of axions and axion-like particles (ALPs) by analyzing gamma-ray data from neutron stars using the Fermi Large Area Telescope. In addition to axions solving the strong CP problem of particle physics, axions and ALPs are also possible dark matter candidates. We investigate axions and ALPs produced by nucleon-nucleon bremsstrahlung within neutron stars. We derive a phenomenological model for the gamma-ray spectrum arising from subsequent axion decays. By analyzing 5 years of gamma-ray data (between 60 MeV and 200 MeV) for a sample of 4 nearby neutron stars, we do not find evidence for an axion or ALP signal, thus we obtain a combined 95% confidence level upper limit on the axion mass of 7.9×10 −2 eV, which corresponds to a lower limit for the Peccei-Quinn scale f a of 7.6×10 7 GeV. Our constraints are more stringent than previous results probing the same physical process, and are competitive with results probing axions and ALPs by different mechanisms.

According to the Large Extra Dimensions (LED) model of Arkani-Hamed, Dimopoulos, and Dvali (ADD),... more According to the Large Extra Dimensions (LED) model of Arkani-Hamed, Dimopoulos, and Dvali (ADD), in addition to the (3+1) observed space-time dimensions, there exist n gravity-only spatial dimensions. Due to the presence of the additional dimensions, the Planck scale of gravity should be brought down from 10 16 TeV to the TeV scale, near the electroweak scale, and thus solve the hierarchy problem. Based on the ADD theory, Kaluza-Klein (KK) gravitons, having masses of the order 100 MeV and lifetimes of the order of billions of years, are expected to be produced within supernova cores by nucleon-nucleon gravi-bremsstrahlung in the LED model. Once produced, they are predicted to be trapped by the gravitational potential of subsequently formed neutron stars (NS), and their decay is predicted to contribute to a measurable gamma-ray flux from NS. In this dissertation, refinements to past theoretical models are made, including modifications for the expected spectral energy distribution based on orbital motion of the KK gravitons, and magnetic fields and age. n = 2, 3,. .. , 7 extra dimensions are considered. A sample of 6 gamma-ray faint NS sources not reported in the first Fermi gamma-ray source catalog that are good candidates are selected for this analysis, based on age, surface magnetic field, distance, and galactic latitude. Based on 11 months of data from Fermi-LAT, 95% CL upper limits on the size of extra dimensions R from each source are obtained, as well as 95% CL lower limits on the (n+4)dimensional Planck scale M D. In addition, the limits from all of the analyzed NSs have been combined statistically using two likelihood-based methods. The results indicate more stringent limits on LED than quoted previously from individual neutron star sources in gamma-rays. In addition, the results are more stringent than current v collider limits, from the LHC, for n < 4, and comparable for n = 4. If the Planck scale is around a TeV, then for n = 2, 3, the compactification topology must be more complicated than a torus. For n > 3, the toroidal topology is still allowed. vi Preface This dissertation is dedicated to physicists, past and present, theoretical and experimental, who have tried to unify gravity with the the other fundamental forces. Hopefully, this work will help the experimental effort in this respect. vii viii I would like to thank my Mom and Dad for supporting me through the years of graduate school at Stanford. I would like to thank my sister Manijeh for being supportive. My brother Shahin, you always had entertaining stories and anecdotes, thanks for keeping me in positive spirits. I would like to thank my graduate student colleagues Yvonne Edmonds, Ping Wang, and Alex Drlica-Wagner for helpful discussions on Fermi-LAT analysis and camaraderie. I would like to thank the other Fermi-LAT graduate students for their friendship and help. I would like to thank the postdocs who gave me advice along the way. In particular, I would like to thank Dr. Simona Murgia, for being supportive of my research in general, and for promoting my presentation at Aspen Winter Conference in February 2011. I would like to recognize the dedication of all of the members of the Fermi-LAT collaboration, without whom this work couldn't proceed. I am in gratitude to my adviser, Elliott Bloom, who contributed immensely to my progress as a experimental particle astrophysicist, and encouraged me to be a critical thinker in physics. I would like to thank all the physics professors that I have encountered, who have nurtured my skills in physics Regarding institutions, I would like to thank the Department of Energy for providing the source of funding for my work. I would like to thank the people of SLAC National Accelerator Laboratory for making it an inviting place to do research for Stanford University students. I would also like to express my appreciation for the faculty and staff of the

The Astrophysical Journal, 2010

We present multiwavelength studies of the 106.6 ms γ-ray pulsar PSR J1907+06 near the TeV source ... more We present multiwavelength studies of the 106.6 ms γ-ray pulsar PSR J1907+06 near the TeV source MGRO J1908+06. Timing observations with Fermi result in a precise position determination for the pulsar of R.A. = 19 h 07 m 54. s 7(2), decl. = +06 • 02 ′ 16(2) ′′ placing the pulsar firmly within the TeV

The Astrophysical Journal, 2009

We report the discovery of gamma-ray pulsations from the nearby isolated millisecond pulsar PSR J... more We report the discovery of gamma-ray pulsations from the nearby isolated millisecond pulsar PSR J0030+0451 with the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope (formerly GLAST). This discovery makes PSR J0030+0451 the second millisecond pulsar to be detected in gamma-rays after PSR J0218+4232, observed by the EGRET instrument on the Compton Gamma Ray Observatory. The spin-down powerĖ = 3.5 × 10 33 ergs s −1 is an order of magnitude lower than the empirical lower bound of previously known gamma-ray pulsars. The emission profile is characterized by two narrow peaks, respectively 0.07 ± 0.01 and 0.08 ± 0.02 wide, separated by 0.44 ± 0.02 in phase. The first gamma-ray peak falls 0.15 ± 0.01 after the main radio peak. The pulse shape is similar to that of the "normal" gamma-ray pulsars. An exponentially cutoff power-law fit of the emission spectrum leads to an integral photon flux above 100 MeV of (6.76 ± 1.05 ± 1.35) ×10 −8 cm −2 s −1 with cutoff energy (1.7 ± 0.4 ± 0.5) GeV. Based on its parallax distance of (300±90) pc, we obtain a gamma-ray efficiency L γ /Ė ≃ 15% for the conversion of spin-down energy rate into gamma-ray radiation, assuming isotropic emission.

The Astrophysical Journal, 2010

The Astrophysical Journal, 2009

We report the detection of pulsed γ-rays for PSRs J0631+1036, J0659+1414, J0742-2822, J1420-6048,... more We report the detection of pulsed γ-rays for PSRs J0631+1036, J0659+1414, J0742-2822, J1420-6048, J1509-5850, and J1718-3825 using the Large Area Telescope on board the Fermi Gamma-ray Space Telescope (formerly known as GLAST). Although these six pulsars are diverse in terms of their spin parameters, they share an important feature: their γ-ray light curves are (at least given the current count statistics) single peaked. For two pulsars, there are hints for a double-peaked structure in the light curves. The shapes of the observed light curves of this group of pulsars are discussed in the light of models for which the emission originates from high up in the magnetosphere. The observed phases of the γ-ray light curves are, in general, consistent with those predicted by high-altitude models, although we speculate that the γ-ray emission of PSR J0659+1414, possibly featuring the softest spectrum of all Fermi pulsars coupled with a very low efficiency, arises from relatively low down in the magnetosphere. High-quality radio polarization data are available showing that all but one have a high degree of linear polarization. This allows us to place some constraints on the viewing geometry and aids the comparison of the γ-ray light curves with high-energy beam models.

The Astrophysical Journal, 2009

We report the detection of pulsed gamma-rays from the young, spin-powered radio pulsar PSR J2021+... more We report the detection of pulsed gamma-rays from the young, spin-powered radio pulsar PSR J2021+3651 using data acquired with the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope (formerly GLAST). The light curve consists of two narrow peaks of similar amplitude separated by 0.468 ± 0.002 in phase. The first peak lags the maximum of the 2 GHz radio pulse by 0.162 ± 0.004 ± 0.01 in phase. The integral gamma-ray photon flux above 100 MeV is (56 ±

The Astrophysical Journal, 2009

We report the detection of γ-ray pulsations (≥ 0.1 GeV) from PSR J2229+6114 and PSR J1048−5832, t... more We report the detection of γ-ray pulsations (≥ 0.1 GeV) from PSR J2229+6114 and PSR J1048−5832, the latter having been detected as a low-significance pulsar by EGRET. Data in the γ-ray band were acquired by the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope, while the radio rotational ephemerides used to fold the γ-ray light curves were obtained using the Green Bank Telescope, the Lovell telescope at Jodrell Bank, and the Parkes telescope. The two young radio pulsars, located within the error circles of the previously unidentified EGRET sources 3EG J1048−5840 and 3EG J2227+6122, present spin-down characteristics similar to the Vela pulsar. PSR J1048−5832 shows two sharp peaks at phases 0.15 ± 0.01 and 0.57 ± 0.01 relative to the radio pulse confirming the EGRET light curve, while PSR J2229+6114 presents a very broad peak at phase 0.49 ± 0.01. The γ-ray spectra above 0.1 GeV of both pulsars are fit with power laws having exponential cutoffs near 3 GeV, leading to integral photon fluxes of (2.19 ± 0.22 ± 0.32) × 10 −7 cm −2 s −1 for PSR J1048−5832 and (3.77 ± 0.22 ± 0.44) × 10 −7 cm −2 s −1 for PSR J2229+6114. The first uncertainty is statistical and the second is systematic. PSR J1048−5832 is one of two LAT sources which were entangled together as 3EG J1048−5840. These detections add to the growing number of young γ-ray pulsars that make up the dominant population of GeV γ-ray sources in the Galactic plane.

The Astrophysical Journal, 2010

Millisecond pulsars (MSPs) have been firmly established as a class of γ-ray emitters via the dete... more Millisecond pulsars (MSPs) have been firmly established as a class of γ-ray emitters via the detection of pulsations above 0.1 GeV from eight MSPs by the Fermi Large Area Telescope (LAT). Using 13 months of LAT data, significant γ-ray pulsations at the radio period have been detected from the MSP PSR J0034−0534, making it the ninth clear MSP detection by the LAT. The γ-ray light curve shows two peaks separated by 0.274 ± 0.015 in phase which are very nearly aligned with the radio peaks, a phenomenon seen only in the Crab pulsar until now. The 0.1 GeV spectrum of this pulsar is well fit by an exponentially cutoff power law with a cutoff energy of 1.8 ± 0.6 ± 0.1 GeV and a photon index of 1.5 ± 0.2 ± 0.1, first errors are statistical and second are systematic. The near-alignment of the radio and γ-ray peaks strongly suggests that the radio and γ-ray emission regions are co-located and both are the result of caustic formation.

The Astrophysical Journal, 2009

This Letter presents the first results from the observations of LS I +61 • 303 using Large Area T... more This Letter presents the first results from the observations of LS I +61 • 303 using Large Area Telescope data from the Fermi Gamma-Ray Space Telescope between 2008 August and 2009 March. Our results indicate variability that is consistent with the binary period, with the emission being modulated at 26.6 ± 0.5 days. This constitutes the first detection of orbital periodicity in high-energy gamma rays (20 MeV-100 GeV, HE). The light curve is characterized by a broad peak after periastron, as well as a smaller peak just before apastron. The spectrum is best represented by a power law with an exponential cutoff, yielding an overall flux above 100 MeV of 0.82 ± 0.03(stat) ± 0.07(syst) 10 −6 ph cm −2 s −1 , with a cutoff at 6.3 ± 1.1(stat) ± 0.4(syst) GeV and photon index Γ = 2.21 ± 0.04(stat) ± 0.06(syst). There is no significant spectral change with orbital phase. The phase of maximum emission, close to periastron, hints at inverse Compton scattering as the main radiation mechanism. However, previous very high-energy gamma ray (>100 GeV, VHE) observations by MAGIC and VERITAS show peak emission close to apastron. This and the energy cutoff seen with Fermi suggest the link between HE and VHE gamma rays is nontrivial.

The Astrophysical Journal, 2009

We report on γ-ray observations of the Crab Pulsar and Nebula using 8 months of survey data with ... more We report on γ-ray observations of the Crab Pulsar and Nebula using 8 months of survey data with the Fermi Large Area Telescope (LAT). The high quality light curve obtained using the ephemeris provided by the Nançay and Jodrell Bank radio telescopes shows two main peaks stable in phase with energy. The first γ-ray peak leads the radio main pulse by (281 ± 12 ± 21) µs, giving new constraints on the production site of non-thermal emission in pulsar magnetospheres. The first uncertainty is due to γ-ray statistics, and the second arises from the rotation parameters. The improved sensitivity and the unprecedented statistics afforded by the LAT enable precise measurement of the Crab Pulsar spectral parameters: cutoff energy at E c = (5.8 ± 0.5 ± 1.2) GeV, spectral index of Γ = (1.97 ± 0.02 ± 0.06) and integral photon flux above 100 MeV of (2.09 ± 0.03 ± 0.18) × 10 −6 cm −2 s −1. The first errors represent the statistical error on the fit parameters, while the second ones are the systematic uncertainties. Pulsed γ-ray photons are observed up to ∼ 20 GeV which precludes emission near the stellar surface, below altitudes of around 4 to 5 stellar radii in phase intervals encompassing the two main peaks. A detailed phase-resolved spectral analysis is also performed: the hardest emission from the Crab Pulsar comes from the bridge region between the two γ-ray peaks while the softest comes from the falling edge of the second peak. The spectrum of the nebula in the energy range 100 MeV-300 GeV is well described by the sum of two power-laws of indices Γ sync = (3.99 ± 0.12 ± 0.08) and Γ IC = (1.64 ± 0.05 ± 0.07), corresponding to the falling edge of the synchrotron and the rising edge of the inverse Compton components, respectively. This latter, which links up naturally with the spectral data points of Cherenkov experiments, is well reproduced via inverse Compton scattering from standard Magnetohydrodynamics (MHD) nebula models, and does not require any additional radiation mechanism.

The Astrophysical Journal, 2009

We report the discovery of γ-ray pulsations (≥ 0.1 GeV) from the young radio and X-ray pulsar PSR... more We report the discovery of γ-ray pulsations (≥ 0.1 GeV) from the young radio and X-ray pulsar PSR J0205+6449 located in the Galactic supernova remnant 3C 58. Data in the γ-ray band were acquired by the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope (formerly GLAST), while the radio rotational ephemeris used to fold γ-rays was obtained using both the Green Bank Telescope and the Lovell telescope at Jodrell Bank. The light curve consists of two peaks separated by 0.49 ± 0.01 ± 0.01 cycles which are aligned with the X-ray peaks. The first γ-ray peak trails the radio pulse by 0.08 ± 0.01 ± 0.01, while its amplitude decreases with increasing energy as for the other γ-ray pulsars. Spectral analysis of the pulsed γ-ray emission suggests a simple power law of index −2.1 ± 0.1 ± 0.2 with an exponential cutoff at 3.0 +1.1 −0.7 ± 0.4 GeV. The first uncertainty is statistical and the second is systematic. The integral γ-ray photon flux above 0.1 GeV is (13.7±1.4±3.0)×10 −8 cm −2 s −1 , which implies for a distance of 3.2 kpc and assuming a broad fan-like beam a luminosity of 8.3 × 10 34 ergs s −1 and an efficiency η of 0.3%. Finally, we report a 95% upper limit on the flux of 1.7 × 10 −8 cm −2 s −1 for off-pulse emission from the object.

The Astrophysical Journal, 2010

The Astrophysical Journal, 2009

The Large Area Telescope (Fermi/LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ra... more The Large Area Telescope (Fermi/LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy γ-ray telescope, covering the energy range from below 20 MeV to more than 300 GeV. The LAT was built by an international collaboration with contributions from space agencies, high-energy particle physics institutes, and universities in France, Italy, Japan, Sweden, and the United States. This paper describes the LAT, its pre-flight expected performance, and summarizes the key science objectives that will be addressed. On-orbit performance will be presented in detail in a subsequent paper. The LAT is a pair-conversion telescope with a precision tracker and calorimeter, each consisting of a 4 × 4 array of 16 modules, a segmented anticoincidence detector that covers the tracker array, and a programmable trigger and data acquisition system. Each tracker module has a vertical stack of 18 x, y tracking planes, including two layers (x and y) of single-sided silicon strip detectors and high-Z converter material (tungsten) per tray. Every calorimeter module has 96 CsI(Tl) crystals, arranged in an 8 layer hodoscopic configuration with a total depth of 8.6 radiation lengths, giving both longitudinal and transverse information about the energy deposition pattern. The calorimeter's depth and segmentation enable the highenergy reach of the LAT and contribute significantly to background rejection. The aspect ratio of the tracker (height/width) is 0.4, allowing a large field-of-view (2.4 sr) and ensuring that most pair-conversion showers initiated in the tracker will pass into the calorimeter for energy measurement. Data obtained with the LAT are intended to (i) permit rapid notification of high-energy γ-ray bursts (GRBs) and transients and facilitate monitoring of variable sources, (ii) yield an extensive catalog of several thousand high-energy sources obtained from an all-sky survey, (iii) measure spectra from 20 MeV to more than 50 GeV for several hundred sources, (iv) localize point sources to 0.3-2 arc minutes, (v) map and obtain spectra of extended sources such as SNRs, molecular clouds, and nearby galaxies, (vi) measure the diffuse isotropic γ-ray background up to TeV energies, and (vii) explore the discovery space for dark matter.

The Astrophysical Journal, 2011

We report on the first year of Fermi γ-ray observations of pulsed high-energy emission from the o... more We report on the first year of Fermi γ-ray observations of pulsed high-energy emission from the old PSR J2043+2740. The study of the γ-ray efficiency of such old pulsars gives us an insight into the evolution of pulsars' ability to emit in γ rays as they age. The γ-ray lightcurve of this pulsar above 0.1 GeV is clearly defined by two sharp peaks, 0.353 ± 0.035 periods apart. We have combined the γ-ray profile characteristics of PSR J2043+2740 with the geometrical properties of the pulsar's radio emission, derived from radio polarization data, and constrained the pulsar-beam geometry in the framework of a Two Pole Caustic and an Outer Gap model. The ranges of magnetic inclination and viewing angle were determined to be {α, ζ} ∼ {52 •-57 • , 61 •-68 • } for the Two Pole Caustic model, and {α, ζ} ∼ {62 •-73 • , 74 •-81 • } and {α, ζ} ∼ {72 •-83 • , •-75 • } for the Outer Gap model. Based on this geometry, we assess possible birth locations for this pulsar and derive a likely proper motion, sufficiently high to be measurable with VLBI. At a characteristic age of 1.2 Myr, PSR J2043+2740 is the third oldest of all discovered, non-recycled, γ-ray pulsars: it is twice as old as the next oldest, PSR J0357+32, and younger only than the recently discovered PSR J1836+5925 and PSR J2055+25, both of which are at least 5 and 10 times less energetic, respectively.

The Astrophysical Journal, 2010

The Large Area Telescope (LAT) onboard the F ermi Gamma-ray Space Telescope detected a γ-ray sour... more The Large Area Telescope (LAT) onboard the F ermi Gamma-ray Space Telescope detected a γ-ray source that is spatially consistent with the location of Eta Carinae This source has been persistently bright since the beginning of the LAT survey observations (from 2008 August to 2009 July, the time interval considered here). The γ-ray signal is detected significantly throughout the LAT energy band (i.e., up to ∼100 GeV). The 0.1-100 GeV energy spectrum is well represented by a combination of a cutoff power-law model (< 10 GeV) and a hard power-law component (> 10 GeV). The total flux (> 100 MeV) is 3.7 +0.3 −0.1 × 10 −7 photons s −1 cm −2 , with additional systematic uncertainties of 10%, and consistent with the average flux measured by AGILE (Tavani et al. 2009). The light curve obtained by F ermi is consistent with steady emission. Our observations do not confirm the presence of a γ-ray flare in 2008 October as reported by Tavani et al. (2009), although we cannot exclude that a flare lasting only a few hours escaped detection by the F ermi LAT. We also do not find any evidence for γ-ray variability that correlates with the large X-ray variability of Eta Carinae observed during 2008 December and 2009 January. We are thus not able to establish an unambiguous identification of the LAT source with Eta Carinae.