VERITAS Detection of LS 5039 and HESS J1825-137
VERITAS Collaboration, A. U. Abeysekara, W. Benbow, R. Bird, R. Brose, J. L. Christiansen, A. J. Chromey, W. Cui, M. K. Daniel, A. Falcone, L. Fortson, D. Hanna, T. Hassan, O. Hervet, J. Holder, G. Hughes, T. B. Humensky, P. Kaaret, P. Kar, N. Kelley-Hoskins, M. Kertzman, D. Kieda, M. Krause, M. J. Lang, G. Maier, P. Moriarty, D. Nieto, M. Nievas-Rosillo, R. A. Ong, D. Pandel, M. Pohl, R. R. Prado, E. Pueschel, J. Quinn, K. Ragan, P. T. Reynolds, G. T. Richards, E. Roache, I. Sadeh, M. Santander, G. H. Sembroski, A. Weinstein, P. Wilcox, D. A. Williams, T. J Williamson
aa r X i v : . [ a s t r o - ph . H E ] J a n VERITAS Detection of LS 5039 and HESS J1825-137
A. U. Abeysekara a , W. Benbow b , R. Bird c , R. Brose d,e , J. L. Christiansen f , A. J. Chromey g , W. Cui h,i , M. K. Daniel b ,A. Falcone j , L. Fortson k , D. Hanna l , T. Hassan e , O. Hervet m , J. Holder n , G. Hughes b , T. B. Humensky o , P. Kaaret p ,P. Kar a , N. Kelley-Hoskins e , M. Kertzman q , D. Kieda a , M. Krause e , M. J. Lang r , G. Maier e , P. Moriarty r , D. Nieto s ,M. Nievas-Rosillo e , R. A. Ong c , D. Pandel t , M. Pohl d,e , R. R. Prado e , E. Pueschel e , J. Quinn u , K. Ragan l ,P. T. Reynolds v , G. T. Richards n , E. Roache b , I. Sadeh e , M. Santander w , G. H. Sembroski h , A. Weinstein g ,P. Wilcox p , D. A. Williams m , T. J Williamson n a Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA b Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA 02138, USA c Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA d Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany e DESY, Platanenallee 6, 15738 Zeuthen, Germany f Physics Department, California Polytechnic State University, San Luis Obispo, CA 94307, USA g Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA h Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA i Department of Physics and Center for Astrophysics, Tsinghua University, Beijing 100084, China. j Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA k School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA l Physics Department, McGill University, Montreal, QC H3A 2T8, Canada m Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA n Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA o Physics Department, Columbia University, New York, NY 10027, USA p Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA q Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135-0037, USA r School of Physics, National University of Ireland Galway, University Road, Galway, Ireland s Institute of Particle and Cosmos Physics, Universidad Complutense de Madrid, 28040 Madrid, Spain t Department of Physics, Grand Valley State University, Allendale, MI 49401, USA u School of Physics, University College Dublin, Belfield, Dublin 4, Ireland v Department of Physical Sciences, Cork Institute of Technology, Bishopstown, Cork, Ireland w Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA
Abstract
With 8 hours of observations, VERITAS confirms the detection of two very high energy gamma-ray sources. Thegamma-ray binary LS 5039 is detected with a statistical significance of 8 . σ . The measured flux above 1 TeV is(2 . ± . × − cm − s − near inferior conjunction and (7 . ± . × − cm − s − near superior conjunction. Thepulsar wind nebula HESS J1825-137 is detected with a statistical significance of 6 . σ and a measured flux above 1 TeVof (3 . ± . × − cm − s − . Keywords: binaries: close; gamma rays: general; ISM: individual objects: HESS J1825-137; stars: individual: LS 5039
1. Introduction
We report observations of two known, bright, very-high-energy (VHE; E >
50 GeV) gamma-ray emitting ob-jects: the gamma-ray binary LS 5039 and the very ex-tended pulsar wind nebula (PWN) HESS J1825-137. Thesetwo objects are separated by about 1 ◦ on the sky and canboth be observed in a single field of view with the Very En-ergetic Radiation Imaging Telescope Array System (VER-ITAS) .The observations are described in Section 2. Back-ground, spectra, timing, and morphological studies are de-scribed for each source in Section 3. http://veritas.sao.arizona.edu
2. Observations
VERITAS is a four-telescope array exploiting the imag-ing atmospheric Cherenkov technique to detect VHE gammarays and is most sensitive to energies between 0.085–30TeV. The array is located at the Fred Lawrence WhippleObservatory (FLWO) in southern Arizona (31 ◦ ′ N , ◦ ′ W , ◦ . Standard anal-ysis was performed [2] with background extracted fromregions near the sources excluding a 0 . ◦ radius regionaround LS 5039 and a 0 . ◦ radius region around HESSJ1825-137. With 7.9 hours of usable observations after Preprint submitted to Astroparticle Physics January 13, 2020 uality cuts, we confirm the HESS detections of thesesources [3, 4]. Figure 1 shows the VERITAS skymap madewith an integration region radius of θ = 0 . ◦ . The av-erage low-energy threshold for VERITAS observations is420 GeV due to the relatively large zenith angle of theobservations.
3. Results
LS 5039 was identified as a high mass X-ray binary inthe ROSAT Galactic plane survey [5] and radio emissionwas detected using the VLA [6]. It was also detected asa gamma-ray binary with EGRET [7] and at VHE withHESS [3]. Periodic emission is also detected by
Fermi -LAT above 300 MeV [8]. The companion star is spectraltype O6.5V [9], while the mass and nature of the compactobject is undetermined [10]. However, some evidence ex-ists for a neutron star being present in the system. Orbitalmodulation in TeV gamma-rays peaks when the star andcompact object are aligned relative to the observer whichcan be explained by the outward flow of relativistic parti-cles from a PWN boosting emission from the star [11].We detect VER J1826-148, the VHE source associatedwith LS 5039, at a significance of 8.8 standard deviations( σ ) [12] with 101 excess counts within a region of radius θ = 0 . ◦ centered at the known position from HESS( α, δ ) = (276 . ◦ , − . ◦ ) (J2000) [13]. Each observa-tion was a single ∼ −
30 minute exposure, and consecu-tive exposures were separated by 1-2 nights. This providedapproximately daily coverage of the 3.9 day orbital period[13] for 8 days in 2013 and 12 days in 2014. Spectra sep-arated into two orbital phase intervals, see Fig. 2, agreewithin the uncertainties of those measured with HESS[13]. The flux above 1 TeV is (2 . ± . stat ± . sys ) × − cm − s − near inferior conjunction (orbital phase 0 . <φ ≤ .
9, 232 minutes exposure) and (7 . ± . stat ± . sys ) × − cm − s − near superior conjunction (0 . ≤ φ ≤ . . < φ ≤ .
0, 230 minutes). This difference influx demonstrates that the previously observed orbital fluxvariability [3] is detected by VERITAS as well. Fittinga power law to the spectrum for each orbital phase in-terval gives a photon index near inferior conjunction ofΓ = 2 . ± . stat ± . sys with χ / DoF = 0 . / . ± . stat ± . sys with χ / DoF = 0 . / χ / DoF = 1 . / χ / DoF = 0 . / The first indication of a PWN in this region was due toROSAT observations of an extended ( ∼ ′ ) X-ray nebulaassociated with the young (21 kyr) and energetic ( ˙ E =2 . × ergs s − ) pulsar PSR B1823-13 [14]. HESS J1825-137 has a radius at least 5 times larger in VHE thanX-ray, and the larger extent is consistent with the longerlifetime of TeV emitting electrons [4]. The large angularsize of HESS J1825-137 makes it ideal for resolved VHEstudy of PWN to better understand the particle transportwithin[15].Using the same observations and analysis described forLS 5039, VERITAS detected HESS J1825-137, at the cen-ter of the PWN reported by HESS ( α, δ ) = (276 . ◦ , − . ◦ ) (J2000) [15] with a significance of 5 . σ ; thepeak significance of 6 . σ within a 0 . ◦ radius region. Fit-ting a spectrum extracted from this region to a powerlaw model gave a normalization at 1 TeV of A = (5 . ± . stat ± . sys ) × − cm − s − TeV − , a photon indexΓ = 2 . ± . stat ± . sys , and χ / DoF = 2 . /
1. Wefound the PWN (named VER J1825-138) to be centered at( α, δ ) = (276 . ◦ ± . ◦ stat ± . ◦ sys , − . ◦ ± . ◦ stat ± . ◦ sys ) (J2000) using a weighted centroid. The regionof interest, HESS centroid and VERITAS centroid aremarked in Fig. 1. Summing in azimuth around the PWN,the radial profile is defined by a Gaussian with σ = 0 . ◦ (1 σ confidence interval of 0 . ◦ − . ◦ ).HESS reports a power-law fit from a 0 . ◦ region ofA = (6 . ± . stat ± . sys ) × − cm − s − TeV − witha photon index Γ = 2 . ± . stat ± . sys [15]. TheVERITAS flux normalization is consistent with that foundby HESS. Similarly, the position of the PWN measuredwith VERITAS is within 0 . ◦ of that detected by HESS.The radial extent of the nebula measured by VERITASis consistent with the original HESS observations of σ =0 . ◦ ± . ◦ with a 52hr exposure [16], but the exception-ally deep observations from HESS [15] yields a different,larger, energy dependent extent.
4. Conclusion
VERITAS has confirmed the TeV gamma-ray detec-tions of LS 5039 and HESS J1825-137 and analysis agreeswith previous findings about the energetics and morphol-ogy.
5. Acknowledgements
This research is supported by grants from the U.S. De-partment of Energy Office of Science, the U.S. NationalScience Foundation and the Smithsonian Institution, andby NSERC in Canada. This research used resources pro-vided by the Open Science Grid, which is supported by theNational Science Foundation and the U.S. Department ofEnergy’s Office of Science, and resources of the NationalEnergy Research Scientific Computing Center (NERSC),a U.S. Department of Energy Office of Science User Facil-ity operated under Contract No. DE-AC02-05CH11231.We acknowledge the excellent work of the technical sup-port staff at the Fred Lawrence Whipple Observatory andat the collaborating institutions in the construction andoperation of the instrument.2 D e c li na t i on ( J ) Figure 1: Excess counts skymap of the region containing LS 5039 andHESS J1825-137. LS 5039 is marked with a star and HESS J1825-137 is enclosed within a 0 . ◦ radius circle. A cross and x respectivelymark the VERITAS and HESS [15] centroids of the PWN. Energy(TeV)10 -13 -12 -11 E d N / d E ( T e V c m − s − ) INFC (HESS)SUPC (HESS)INFC (VERITAS)SUPC (VERITAS)
Figure 2: Spectra of LS 5039 separated by orbital phase. The filledcircles and dashed line are at inferior conjunction. The x’s and solidlines are at superior conjunction. Points are from this analysis andthe fit spectra with shaded errors are from [13].
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