Fermi lat observations of LS i +61°303: First detection of an orbital modulation in GeV gamma rays

A. A. Abdo; M. Ackermann; M. Ajello; W. B. Atwood; M. Axelsson; L. Baldini; J. Ballet; G. Barbiellini; D. Bastieri; B. M. Baughman; K. Bechtol; R. Bellazzini; B. Berenji; R. Blandford; E. D. Bloom; E. Bonamente; A. W. Borgland; J. Bregeon; A. Brez; M. Brigida; P. Bruel; T. H. Burnett; G. A. Caliandro; R. A. Cameron; P. A. Caraveo; J. M. Casandjian; E. Cavazzuti; C. Cecchi; Ö Çelik; E. Charles; S. Chaty; A. Chekhtman; C. C. Cheung; J. Chiang; S. Ciprini; R. Claus; J. Cohen-Tanugi; L. R. Cominsky; J. Conrad; S. Corbel; R. Corbet; S. Cutini; C. D. Dermer; A. De Angelis; A. De Luca; F. De Palma; S. W. Digel; M. Dormody; E. Do Couto E Silva; P. S. Drell; R. Dubois; G. Dubus; D. Dumora; C. Farnier; C. Favuzzi; S. J. Fegan; W. B. Focke; M. Frailis; Y. Fukazawa; S. Funk; P. Fusco; F. Gargano; D. Gasparrini; N. Gehrels; S. Germani; B. Giebels; N. Giglietto; F. Giordano; T. Glanzman; G. Godfrey; I. A. Grenier; M. H. Grondin; J. E. Grove; L. Guillemot; S. Guiriec; Y. Hanabata; A. K. Harding; M. Hayashida; E. Hays; A. B. Hill; R. E. Hughes; G. Jóhannesson; A. S. Johnson; R. P. Johnson; T. J. Johnson; W. N. Johnson; T. Kamae; H. Katagiri; J. Kataoka; N. Kawai; M. Kerr; J. Knödlseder; M. L. Kocian; F. Kuehn; M. Kuss; J. Lande; S. Larsson; L. Latronico; F. Longo; F. Loparco; B. Lott; M. N. Lovellette; P. Lubrano; G. M. Madejski; A. Makeev; M. Marelli; M. N. Mazziotta; J. E. McEnery; C. Meurer; P. F. Michelson; W. Mitthumsiri; T. Mizuno; C. Monte; M. E. Monzani; A. Morselli; I. V. Moskalenko; S. Murgia; P. L. Nolan; E. Nuss; T. Ohsugi; A. Okumura; N. Omodei; E. Orlando; J. F. Ormes; D. Paneque; J. H. Panetta; D. Parent; V. Pelassa; M. Pepe; M. Pesce-Rollins; F. Piron; T. A. Porter; S. Rainò; R. Rando; P. S. Ray; M. Razzano; N. Rea; A. Reimer; O. Reimer; T. Reposeur; S. Ritz; L. S. Rochester; A. Y. Rodriguez; R. W. Romani; F. Ryde; H. F.W. Sadrozinski; D. Sanchez; A. Sander; P. M. Saz Parkinson; J. D. Scargle; C. Sgrò; M. S. Shaw; A. Sierpowska-Bartosik; E. J. Siskind; D. A. Smith; P. D. Smith; G. Spandre; P. Spinelli; E. Striani; M. S. Strickman; D. J. Suson; H. Tajima; H. Takahashi; T. Takahashi; T. Tanaka; J. B. Thayer; J. G. Thayer; D. J. Thompson; L. Tibaldo; D. F. Torres; G. Tosti; A. Tramacere; Y. Uchiyama; T. L. Usher; V. Vasileiou; N. Vilchez; V. Vitale; A. P. Waite; P. Wang; B. L. Winer; K. S. Wood; T. Ylinen; M. Ziegler

doi:10.1088/0004-637X/701/2/L123

Fermi lat observations of LS i +61°303: First detection of an orbital modulation in GeV gamma rays

A. A. Abdo, M. Ackermann, M. Ajello, W. B. Atwood, M. Axelsson, L. Baldini, J. Ballet, G. Barbiellini, D. Bastieri, B. M. Baughman, K. Bechtol, R. Bellazzini, B. Berenji, R. Blandford, E. D. Bloom, E. Bonamente, A. W. Borgland, J. Bregeon, A. Brez, M. BrigidaP. Bruel, T. H. Burnett, G. A. Caliandro, R. A. Cameron, P. A. Caraveo, J. M. Casandjian, E. Cavazzuti, C. Cecchi, Ö Çelik, E. Charles, S. Chaty, A. Chekhtman, C. C. Cheung, J. Chiang, S. Ciprini, R. Claus, J. Cohen-Tanugi, L. R. Cominsky, J. Conrad, S. Corbel, R. Corbet, S. Cutini, C. D. Dermer, A. De Angelis, A. De Luca, F. De Palma, S. W. Digel, M. Dormody, E. Do Couto E Silva, P. S. Drell, R. Dubois, G. Dubus, D. Dumora, C. Farnier, C. Favuzzi, S. J. Fegan, W. B. Focke, M. Frailis, Y. Fukazawa, S. Funk, P. Fusco, F. Gargano, D. Gasparrini, N. Gehrels, S. Germani, B. Giebels, N. Giglietto, F. Giordano, T. Glanzman, G. Godfrey, I. A. Grenier, M. H. Grondin, J. E. Grove, L. Guillemot, S. Guiriec, Y. Hanabata, A. K. Harding, M. Hayashida, E. Hays, A. B. Hill, R. E. Hughes, G. Jóhannesson, A. S. Johnson, R. P. Johnson, T. J. Johnson, W. N. Johnson, T. Kamae, H. Katagiri, J. Kataoka, N. Kawai, M. Kerr, J. Knödlseder, M. L. Kocian, F. Kuehn, M. Kuss, J. Lande, S. Larsson, L. Latronico, F. Longo, F. Loparco, B. Lott, M. N. Lovellette, P. Lubrano, G. M. Madejski, A. Makeev, M. Marelli, M. N. Mazziotta, J. E. McEnery, C. Meurer, P. F. Michelson, W. Mitthumsiri, T. Mizuno, C. Monte, M. E. Monzani, A. Morselli, I. V. Moskalenko, S. Murgia, P. L. Nolan, E. Nuss, T. Ohsugi, A. Okumura, N. Omodei, E. Orlando, J. F. Ormes, D. Paneque, J. H. Panetta, D. Parent, V. Pelassa, M. Pepe, M. Pesce-Rollins, F. Piron, T. A. Porter, S. Rainò, R. Rando, P. S. Ray, M. Razzano, N. Rea, A. Reimer, O. Reimer, T. Reposeur, S. Ritz, L. S. Rochester, A. Y. Rodriguez, R. W. Romani, F. Ryde, H. F.W. Sadrozinski, D. Sanchez, A. Sander, P. M. Saz Parkinson, J. D. Scargle, C. Sgrò, M. S. Shaw, A. Sierpowska-Bartosik, E. J. Siskind, D. A. Smith, P. D. Smith, G. Spandre, P. Spinelli, E. Striani, M. S. Strickman, D. J. Suson, H. Tajima, H. Takahashi, T. Takahashi, T. Tanaka, J. B. Thayer, J. G. Thayer, D. J. Thompson, L. Tibaldo, D. F. Torres, G. Tosti, A. Tramacere, Y. Uchiyama, T. L. Usher, V. Vasileiou, N. Vilchez, V. Vitale, A. P. Waite, P. Wang, B. L. Winer, K. S. Wood, T. Ylinen, M. Ziegler

School of Advanced Science and Engineering

Research output: Contribution to journal › Article › peer-review

157 Citations (Scopus)

Abstract

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 that the link between HE and VHE gamma rays is nontrivial.

Original language	English
Pages (from-to)	L123-L128
Journal	Astrophysical Journal
Volume	701
Issue number	2 PART 2
DOIs	https://doi.org/10.1088/0004-637X/701/2/L123
Publication status	Published - 2009

Keywords

Binaries: close
Gamma rays: observations
Stars: variables: other
X-rays: binaries

ASJC Scopus subject areas

Astronomy and Astrophysics
Space and Planetary Science

Access to Document

10.1088/0004-637X/701/2/L123

Cite this

Abdo, A. A., Ackermann, M., Ajello, M., Atwood, W. B., Axelsson, M., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., Baughman, B. M., Bechtol, K., Bellazzini, R., Berenji, B., Blandford, R., Bloom, E. D., Bonamente, E., Borgland, A. W., Bregeon, J., Brez, A., ... Ziegler, M. (2009). Fermi lat observations of LS i +61°303: First detection of an orbital modulation in GeV gamma rays. Astrophysical Journal, 701(2 PART 2), L123-L128. https://doi.org/10.1088/0004-637X/701/2/L123

Abdo, AA, Ackermann, M, Ajello, M, Atwood, WB, Axelsson, M, Baldini, L, Ballet, J, Barbiellini, G, Bastieri, D, Baughman, BM, Bechtol, K, Bellazzini, R, Berenji, B, Blandford, R, Bloom, ED, Bonamente, E, Borgland, AW, Bregeon, J, Brez, A, Brigida, M, Bruel, P, Burnett, TH, Caliandro, GA, Cameron, RA, Caraveo, PA, Casandjian, JM, Cavazzuti, E, Cecchi, C, Çelik, Ö, Charles, E, Chaty, S, Chekhtman, A, Cheung, CC, Chiang, J, Ciprini, S, Claus, R, Cohen-Tanugi, J, Cominsky, LR, Conrad, J, Corbel, S, Corbet, R, Cutini, S, Dermer, CD, De Angelis, A, De Luca, A, De Palma, F, Digel, SW, Dormody, M, Do Couto E Silva, E, Drell, PS, Dubois, R, Dubus, G, Dumora, D, Farnier, C, Favuzzi, C, Fegan, SJ, Focke, WB, Frailis, M, Fukazawa, Y, Funk, S, Fusco, P, Gargano, F, Gasparrini, D, Gehrels, N, Germani, S, Giebels, B, Giglietto, N, Giordano, F, Glanzman, T, Godfrey, G, Grenier, IA, Grondin, MH, Grove, JE, Guillemot, L, Guiriec, S, Hanabata, Y, Harding, AK, Hayashida, M, Hays, E, Hill, AB, Hughes, RE, Jóhannesson, G, Johnson, AS, Johnson, RP, Johnson, TJ, Johnson, WN, Kamae, T, Katagiri, H, Kataoka, J, Kawai, N, Kerr, M, Knödlseder, J, Kocian, ML, Kuehn, F, Kuss, M, Lande, J, Larsson, S, Latronico, L, Longo, F, Loparco, F, Lott, B, Lovellette, MN, Lubrano, P, Madejski, GM, Makeev, A, Marelli, M, Mazziotta, MN, McEnery, JE, Meurer, C, Michelson, PF, Mitthumsiri, W, Mizuno, T, Monte, C, Monzani, ME, Morselli, A, Moskalenko, IV, Murgia, S, Nolan, PL, Nuss, E, Ohsugi, T, Okumura, A, Omodei, N, Orlando, E, Ormes, JF, Paneque, D, Panetta, JH, Parent, D, Pelassa, V, Pepe, M, Pesce-Rollins, M, Piron, F, Porter, TA, Rainò, S, Rando, R, Ray, PS, Razzano, M, Rea, N, Reimer, A, Reimer, O, Reposeur, T, Ritz, S, Rochester, LS, Rodriguez, AY, Romani, RW, Ryde, F, Sadrozinski, HFW, Sanchez, D, Sander, A, Saz Parkinson, PM, Scargle, JD, Sgrò, C, Shaw, MS, Sierpowska-Bartosik, A, Siskind, EJ, Smith, DA, Smith, PD, Spandre, G, Spinelli, P, Striani, E, Strickman, MS, Suson, DJ, Tajima, H, Takahashi, H, Takahashi, T, Tanaka, T, Thayer, JB, Thayer, JG, Thompson, DJ, Tibaldo, L, Torres, DF, Tosti, G, Tramacere, A, Uchiyama, Y, Usher, TL, Vasileiou, V, Vilchez, N, Vitale, V, Waite, AP, Wang, P, Winer, BL, Wood, KS, Ylinen, T & Ziegler, M 2009, 'Fermi lat observations of LS i +61°303: First detection of an orbital modulation in GeV gamma rays', Astrophysical Journal, vol. 701, no. 2 PART 2, pp. L123-L128. https://doi.org/10.1088/0004-637X/701/2/L123

@article{951d643719354eaa9275e1a54ba9b60c,

title = "Fermi lat observations of LS i +61°303: First detection of an orbital modulation in GeV gamma rays",

abstract = "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 that the link between HE and VHE gamma rays is nontrivial.",

keywords = "Binaries: close, Gamma rays: observations, Stars: variables: other, X-rays: binaries",

author = "Abdo, {A. A.} and M. Ackermann and M. Ajello and Atwood, {W. B.} and M. Axelsson and L. Baldini and J. Ballet and G. Barbiellini and D. Bastieri and Baughman, {B. M.} and K. Bechtol and R. Bellazzini and B. Berenji and R. Blandford and Bloom, {E. D.} and E. Bonamente and Borgland, {A. W.} and J. Bregeon and A. Brez and M. Brigida and P. Bruel and Burnett, {T. H.} and Caliandro, {G. A.} and Cameron, {R. A.} and Caraveo, {P. A.} and Casandjian, {J. M.} and E. Cavazzuti and C. Cecchi and {\"O} {\c C}elik and E. Charles and S. Chaty and A. Chekhtman and Cheung, {C. C.} and J. Chiang and S. Ciprini and R. Claus and J. Cohen-Tanugi and Cominsky, {L. R.} and J. Conrad and S. Corbel and R. Corbet and S. Cutini and Dermer, {C. D.} and {De Angelis}, A. and {De Luca}, A. and {De Palma}, F. and Digel, {S. W.} and M. Dormody and {Do Couto E Silva}, E. and Drell, {P. S.} and R. Dubois and G. Dubus and D. Dumora and C. Farnier and C. Favuzzi and Fegan, {S. J.} and Focke, {W. B.} and M. Frailis and Y. Fukazawa and S. Funk and P. Fusco and F. Gargano and D. Gasparrini and N. Gehrels and S. Germani and B. Giebels and N. Giglietto and F. Giordano and T. Glanzman and G. Godfrey and Grenier, {I. A.} and Grondin, {M. H.} and Grove, {J. E.} and L. Guillemot and S. Guiriec and Y. Hanabata and Harding, {A. K.} and M. Hayashida and E. Hays and Hill, {A. B.} and Hughes, {R. E.} and G. J{\'o}hannesson and Johnson, {A. S.} and Johnson, {R. P.} and Johnson, {T. J.} and Johnson, {W. N.} and T. Kamae and H. Katagiri and J. Kataoka and N. Kawai and M. Kerr and J. Kn{\"o}dlseder and Kocian, {M. L.} and F. Kuehn and M. Kuss and J. Lande and S. Larsson and L. Latronico and F. Longo and F. Loparco and B. Lott and Lovellette, {M. N.} and P. Lubrano and Madejski, {G. M.} and A. Makeev and M. Marelli and Mazziotta, {M. N.} and McEnery, {J. E.} and C. Meurer and Michelson, {P. F.} and W. Mitthumsiri and T. Mizuno and C. Monte and Monzani, {M. E.} and A. Morselli and Moskalenko, {I. V.} and S. Murgia and Nolan, {P. L.} and E. Nuss and T. Ohsugi and A. Okumura and N. Omodei and E. Orlando and Ormes, {J. F.} and D. Paneque and Panetta, {J. H.} and D. Parent and V. Pelassa and M. Pepe and M. Pesce-Rollins and F. Piron and Porter, {T. A.} and S. Rain{\`o} and R. Rando and Ray, {P. S.} and M. Razzano and N. Rea and A. Reimer and O. Reimer and T. Reposeur and S. Ritz and Rochester, {L. S.} and Rodriguez, {A. Y.} and Romani, {R. W.} and F. Ryde and Sadrozinski, {H. F.W.} and D. Sanchez and A. Sander and {Saz Parkinson}, {P. M.} and Scargle, {J. D.} and C. Sgr{\`o} and Shaw, {M. S.} and A. Sierpowska-Bartosik and Siskind, {E. J.} and Smith, {D. A.} and Smith, {P. D.} and G. Spandre and P. Spinelli and E. Striani and Strickman, {M. S.} and Suson, {D. J.} and H. Tajima and H. Takahashi and T. Takahashi and T. Tanaka and Thayer, {J. B.} and Thayer, {J. G.} and Thompson, {D. J.} and L. Tibaldo and Torres, {D. F.} and G. Tosti and A. Tramacere and Y. Uchiyama and Usher, {T. L.} and V. Vasileiou and N. Vilchez and V. Vitale and Waite, {A. P.} and P. Wang and Winer, {B. L.} and Wood, {K. S.} and T. Ylinen and M. Ziegler",

year = "2009",

doi = "10.1088/0004-637X/701/2/L123",

language = "English",

volume = "701",

pages = "L123--L128",

journal = "Astrophysical Journal",

issn = "0004-637X",

publisher = "IOP Publishing Ltd.",

number = "2 PART 2",

}

TY - JOUR

T1 - Fermi lat observations of LS i +61°303

T2 - First detection of an orbital modulation in GeV gamma rays

AU - Abdo, A. A.

AU - Ackermann, M.

AU - Ajello, M.

AU - Atwood, W. B.

AU - Axelsson, M.

AU - Baldini, L.

AU - Ballet, J.

AU - Barbiellini, G.

AU - Bastieri, D.

AU - Baughman, B. M.

AU - Bechtol, K.

AU - Bellazzini, R.

AU - Berenji, B.

AU - Blandford, R.

AU - Bloom, E. D.

AU - Bonamente, E.

AU - Borgland, A. W.

AU - Bregeon, J.

AU - Brez, A.

AU - Brigida, M.

AU - Bruel, P.

AU - Burnett, T. H.

AU - Caliandro, G. A.

AU - Cameron, R. A.

AU - Caraveo, P. A.

AU - Casandjian, J. M.

AU - Cavazzuti, E.

AU - Cecchi, C.

AU - Çelik, Ö

AU - Charles, E.

AU - Chaty, S.

AU - Chekhtman, A.

AU - Cheung, C. C.

AU - Chiang, J.

AU - Ciprini, S.

AU - Claus, R.

AU - Cohen-Tanugi, J.

AU - Cominsky, L. R.

AU - Conrad, J.

AU - Corbel, S.

AU - Corbet, R.

AU - Cutini, S.

AU - Dermer, C. D.

AU - De Angelis, A.

AU - De Luca, A.

AU - De Palma, F.

AU - Digel, S. W.

AU - Dormody, M.

AU - Do Couto E Silva, E.

AU - Drell, P. S.

AU - Dubois, R.

AU - Dubus, G.

AU - Dumora, D.

AU - Farnier, C.

AU - Favuzzi, C.

AU - Fegan, S. J.

AU - Focke, W. B.

AU - Frailis, M.

AU - Fukazawa, Y.

AU - Funk, S.

AU - Fusco, P.

AU - Gargano, F.

AU - Gasparrini, D.

AU - Gehrels, N.

AU - Germani, S.

AU - Giebels, B.

AU - Giglietto, N.

AU - Giordano, F.

AU - Glanzman, T.

AU - Godfrey, G.

AU - Grenier, I. A.

AU - Grondin, M. H.

AU - Grove, J. E.

AU - Guillemot, L.

AU - Guiriec, S.

AU - Hanabata, Y.

AU - Harding, A. K.

AU - Hayashida, M.

AU - Hays, E.

AU - Hill, A. B.

AU - Hughes, R. E.

AU - Jóhannesson, G.

AU - Johnson, A. S.

AU - Johnson, R. P.

AU - Johnson, T. J.

AU - Johnson, W. N.

AU - Kamae, T.

AU - Katagiri, H.

AU - Kataoka, J.

AU - Kawai, N.

AU - Kerr, M.

AU - Knödlseder, J.

AU - Kocian, M. L.

AU - Kuehn, F.

AU - Kuss, M.

AU - Lande, J.

AU - Larsson, S.

AU - Latronico, L.

AU - Longo, F.

AU - Loparco, F.

AU - Lott, B.

AU - Lovellette, M. N.

AU - Lubrano, P.

AU - Madejski, G. M.

AU - Makeev, A.

AU - Marelli, M.

AU - Mazziotta, M. N.

AU - McEnery, J. E.

AU - Meurer, C.

AU - Michelson, P. F.

AU - Mitthumsiri, W.

AU - Mizuno, T.

AU - Monte, C.

AU - Monzani, M. E.

AU - Morselli, A.

AU - Moskalenko, I. V.

AU - Murgia, S.

AU - Nolan, P. L.

AU - Nuss, E.

AU - Ohsugi, T.

AU - Okumura, A.

AU - Omodei, N.

AU - Orlando, E.

AU - Ormes, J. F.

AU - Paneque, D.

AU - Panetta, J. H.

AU - Parent, D.

AU - Pelassa, V.

AU - Pepe, M.

AU - Pesce-Rollins, M.

AU - Piron, F.

AU - Porter, T. A.

AU - Rainò, S.

AU - Rando, R.

AU - Ray, P. S.

AU - Razzano, M.

AU - Rea, N.

AU - Reimer, A.

AU - Reimer, O.

AU - Reposeur, T.

AU - Ritz, S.

AU - Rochester, L. S.

AU - Rodriguez, A. Y.

AU - Romani, R. W.

AU - Ryde, F.

AU - Sadrozinski, H. F.W.

AU - Sanchez, D.

AU - Sander, A.

AU - Saz Parkinson, P. M.

AU - Scargle, J. D.

AU - Sgrò, C.

AU - Shaw, M. S.

AU - Sierpowska-Bartosik, A.

AU - Siskind, E. J.

AU - Smith, D. A.

AU - Smith, P. D.

AU - Spandre, G.

AU - Spinelli, P.

AU - Striani, E.

AU - Strickman, M. S.

AU - Suson, D. J.

AU - Tajima, H.

AU - Takahashi, H.

AU - Takahashi, T.

AU - Tanaka, T.

AU - Thayer, J. B.

AU - Thayer, J. G.

AU - Thompson, D. J.

AU - Tibaldo, L.

AU - Torres, D. F.

AU - Tosti, G.

AU - Tramacere, A.

AU - Uchiyama, Y.

AU - Usher, T. L.

AU - Vasileiou, V.

AU - Vilchez, N.

AU - Vitale, V.

AU - Waite, A. P.

AU - Wang, P.

AU - Winer, B. L.

AU - Wood, K. S.

AU - Ylinen, T.

AU - Ziegler, M.

PY - 2009

Y1 - 2009

N2 - 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 that the link between HE and VHE gamma rays is nontrivial.

AB - 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 that the link between HE and VHE gamma rays is nontrivial.

KW - Binaries: close

KW - Gamma rays: observations

KW - Stars: variables: other

KW - X-rays: binaries

UR - http://www.scopus.com/inward/record.url?scp=72149120487&partnerID=8YFLogxK

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U2 - 10.1088/0004-637X/701/2/L123

DO - 10.1088/0004-637X/701/2/L123

M3 - Article

AN - SCOPUS:72149120487

SN - 0004-637X

VL - 701

SP - L123-L128

JO - Astrophysical Journal

JF - Astrophysical Journal

IS - 2 PART 2

ER -

Fermi lat observations of LS i +61°303: First detection of an orbital modulation in GeV gamma rays

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