FERMI. YOUNG PULSAR SPECTRA WITH THE LAT FERMI TELESCOPE Ateliers pulsars. 25 novembre 2008 Damien Parent. Gamma-ray Large Area Space Telescope

Transcription

1 FERMI Gamma-ray Large Area Space Telescope YOUNG PULSAR SPECTRA WITH THE LAT FERMI TELESCOPE Ateliers pulsars 25 novembre

2 Topics 1. Young pulsars, our best candidates 2 examples : J and J What does the flux teach us? 3. Instrument Response Functions 4. How to access the source flux with Fermi? 5. Vela & J spectrum 2

3 Young pulsars, our best candidates Above some value Edot, gamma-ray emitting electron-positron cascades occur (Arrons 1996). We have listed 224 best gamma-ray pulsar candidates with Edot > 1e34 erg/s (Smith et al. 2008) astro-ph/ : Young, energetic and noisy pulsars The most energetic of the known galactic pulsar are : Crab : 4.6e+38 erg/s J : 3.4e+37 erg/s J : 2.7e+37 erg/s J : 2.2e+37 erg/s 3 First Light Image of the LAT Fermi telescope

4 Young pulsars, J (1) This very young pulsar in the SNR 3C58 was detected in radio after the end of CGRO. F. Camilo et al, Ap J Lett 571, (2002). The radio discovery was made possible by the Chandra X-ray discovery of the pulsar shortly before that (Murray et al 2002). With a real weak signal in radio: ~45 ujy at 1400 MHz. Associated to the SNR and the PWN 3C58, the system is the possible remnant of the SN It has a whopping spin down energy (Crab/15) of 2.7e37 erg/s Optical image of 3C 58 Fesen et al VLA images Slane et al

5 Young pulsars, J (2) An archival observation of RXTE has confirmed the periodic signal of PSR J (image). Estimated characteristic age ~ 5.37e+03 years Estimated distance ~ 3.2 kpc P ~65.7 ms, Pdot ~ 1.93e-13 Estimated surface B ~ 3.61e+12 Image of 3C 58 at 1.4 GHz. The arrow marks the position of the pusar BietenHolz

6 J The high Edot (2.2e37 erg/s) pulsar was discovered at the end of the CGRO mission (2001). Halpern, J. P., Camilo, F., Gotthelf, E. V., Helfand, D. J., Kramer, M., Lyne, A. G., Leighly, K. M. & Eracleous, M., PSR J : Discovery of an Energetic Young Pulsar in the Error Box of the EGRET Source 3EG J Associated to the unidentified 3EG J and to the SNR G , surrounded by an X-ray PWN. Halpern, J. P.; Gotthelf, E. V.; Camilo, F.; Collins, B.; Helfand, D. J.The New Gamma-ray Pulsar PSR J , its Pulsar Wind Nebula, and Comparison with the Vela Pulsar X-ray pulsations Estimated age ~ 1e+04 years Estimated distance ~ 7.25 kpc P ~ 51.6 ms, Pdot ~ 7.83e-14 Estimated surface B ~ 2e+12 G Chandra ACIS-I image showing the pulsar and its associated PWN (greyscale). 6

7 A search in EGRET data Best EGRET light curve for PSR J After taking N trials into account, the statistical significance of a positive detection is low, and no detection is claimed. Thompson, Reimer, Digel and Nolan, ASP Conference Series, arxiv:astro-ph/ (2002) 7

8 What does the flux teach us? How and where are particles accelerated in the pulsar magnetosphere? beam geometry phase resolved spectra altitude of acceleration & radiation spectral cut-offs Efficiency : rate for converting its rotational energy loss into gamma-rays Emission comparison : Maximum of emission in the hard Xand γ-ray range Radio beam is a small fraction of emitted energy 8

9 Instrument response functions - IRF (1) Large Area Telescope Effective area < Geometric surface Gamma = no Gamma = yes Tracker LAT picture Calorimeter Point Spread Funtion theta Mutiple scattering Gamma-ray Burst Monitor (GBM) 9

10 Instrument response functions - IRF (2) Point spread function ~ 0.5, 68% containment radius at 1 GeV Surface effective ~ 8000 cm² on axis Energy resolution Basically, Nnumber of photons = ʃ Flux(E) x IRF(E) de 10

11 Instrument performance et al. Point source sensitivity : Integral sensitivity plot (5-sigma sensitivity for E>E0), assuming a 1/E2 spectrum source at high latitude. Comparison between the EGRET and LAT location for CTA1 11

12 How to compute the point source flux with Fermi? A set of tools were built to analyse the Fermi LAT data : gtselect, gtbin, gtlike, Glast Tools These tools handle FITS files, the standard astronomical data format. Data will be public on the 2009 August 11 Link to the page : 12

13 Vela spectrum Spectrum from the new paper : Fermi Lat Observations of the Vela Pulsar (Abdo et al. 2008) (ApJ submittted) This phase-averaged gammaray energy spectrum can be represented by a power law with a exponential cut-off at 3 GeV According to this result, polar cap model are excluded. Phase-averaged Vela spectral energy distribution 13

14 J spectrum from AGILE Phase-averaged spectrum for J (Halpern et al. 2008) The energy range is fitted with a power law of photon index g = 1.86+/-0.18, while a turndown is seen above 1.5 GeV. 14

15 Conclusions Our best gamma-ray pulsar candidates are young, energetic and noisy The instrument performance are still under investigations but are in agreeement with our current simulation of the LAT response. The spectral analysis is a link to understand where and how are particles accelerated in the pulsar magnetosphere : polar cap models are excluded for the Vela pulsar 15

16 16

17 What does the flux teach us? How and where are particles accelerated in the pulsar magnetosphere? beam geometry phase resolved spectra altitude of acceleration & radiation spectral cut-offs Efficiency : rate for converting its rotational energy loss into gamma-rays ref 17

18 Multi-wavelength spectra of EGRET pulsars Emission comparison : Maximum of emission in the hard X- and γ-ray range Radio beam is a small fraction of emitted energy Spectra are very different above 1 GeV High energy spectral cutoffs 18

19 Models of gamma-ray emission for the pulsars Where do γ-rays come from? Outer gap, polar cap or slot gap? Discrimination between models Different models predict different spectral cutoff. Measuring the spectral tail is possible to distinguish between models. 19