Evidence for long-term Gamma-ray and X-ray variability from the unidentified TeV source HESS J0632+057

Evidence for long-term Gamma-ray and X-ray variability from the unidentified TeV source HESS J0632+057

Abstract

HESS J0632+057 is one of only two unidentified very-high-energy gamma-ray sources which appear to be point-like within experimental resolution. It is possibly associated with the massive Be star MWC 148 and has been suggested to resemble known TeV binary systems like LS I +61 303 or LS 5039. HESS J0632+057 was observed by VERITAS for 31 hours in 2006, 2008 and 2009. During these observations, no significant signal in gamma rays with energies above 1 TeV was detected from the direction of HESS J0632+057. A flux upper limit corresponding to 1.1% of the flux of the Crab Nebula has been derived from the VERITAS data. The non-detection by VERITAS excludes with a probability of 99.993% that HESS J0632+057 is a steady gamma-ray emitter. Contemporaneous X-ray observations with Swift XRT reveal a factor of higher flux in the 1-10 keV range than earlier X-ray observations of HESS J0632+057. The variability in the gamma-ray and X-ray fluxes supports interpretation of the object as a gamma-ray emitting binary.

acceleration of particles Ñ binaries: general Ñ gamma rays: observations Ñ stars: individual ( HESS J0632+057, MWC 148 )

1 Introduction

HESS J0632+057 is one of about 20 very-high-energy (VHE) gamma-ray sources with no known counterparts at other wavelengths (for a recent review see e.g. Weekes (2008)). Gamma-ray emission was discovered by the High Energy Stereoscopic System (H.E.S.S.) during observations of the Monoceros Loop Supernova Remnant in 2004 and 2005 (Aharonian et al, 2007). It appears to be point-like within experimental resolution; the limit on the size of the emission region was given as 2’ (95% confidence level). The reported flux of gamma rays with energies above 1 TeV from HESS J0632+057 corresponds to about 3% of the flux of the Crab Nebula, with a differential photon spectrum consistent with a power-law function with index of . Possible associations considered by Aharonian et al (2007) are the Monoceros Loop Supernova remnant, the weak X-ray source 1RXS J063258.3+054857, the B0pe-star MWC 148 (HD 259440) and the unidentified GeV gamma-ray source 3EG J0634+0521 (Hartman et al, 1999). Follow-up X-ray observations with XMM-Newton by Hinton et al (2009) revealed a variable X-ray source (XMMU J063259.3+054801) with a position compatible with HESS J0632+057 and MWC 148. It should be noted that 3EG J0634+0521 is absent in the EGR catalogue of EGRET gamma-ray sources, a reanalysis of the EGRET data with new Galactic interstellar emission models and interstellar radiation field data (Casandjian & Grenier, 2008). 3EG J0634+0521 is, as expected from the reported flux, not in the Fermi bright gamma-ray source list (Abdo et al, 2009).

Point-like gamma-ray sources stand out among the many galactic VHE objects with spatially extended gamma-ray emission. The latter are usually associated with either pulsar wind nebulae or supernova remnants. High-mass X-ray binaries constitute the only known class of galactic objects with variable point-like VHE emission; this class currently contains three members only: PSR B1959-63/SS 2883 (Aharonian et al, 2005b), LS 5039 (Aharonian et al, 2005c) and LS I +61 303 (Albert et al, 2006; Acciari et al, 2008). Additionally, marginal evidence at a level of 3.2 from the black-hole binary Cyg X-1 has been reported by Albert et al (2007). TeV binaries show variable emission of gamma rays, likely connected to changes in physical parameters associated with the orbital movement. VHE gamma-ray production in these objects is explained by the acceleration of charged particles in accretion-powered relativistic jets (Taylor & Gregory, 1984; Mirabel & Rodriguez, 1994) or in shocks created by the collision of the expanding pulsar wind with the wind from the stellar companion (Maraschi & Treves, 1981). Subsequent inverse-Compton scattering on low-energy stellar photons produces gamma rays at GeV and TeV energies. While there has been no compact companion discovered for MWC 148, the point-like nature of the VHE emission combined with the variable X-ray emission can easily be explained by a production scenario similar to those in TeV binaries. A second possible scenario is that MWC 148 is a representative of a new type of VHE emitter as proposed by Babel & Montmerle (1997) and Townsend et al (2007). In their picture strong magnetic fields around the massive star lead to magnetically channeled wind shocks in which second-order Fermi acceleration might occur. However it is not clear if the circumstellar environment of MWC 148 is strongly magnetized, or if this acceleration mechanism is able to produce particles of sufficiently high energy to produce a measurable TeV flux. An association of HESS J0632+057 with the Monoceros loop SNR is unlikely given the point-like nature of the gamma-ray emission and the non-correlation of possible target material with the position of the VHE source (Aharonian et al, 2007).

2 Observations

VERITAS is an array of four imaging atmospheric-Cherenkov telescopes located at the Fred Lawrence Whipple Observatory in southern Arizona. It combines a large effective area (up to m) over a wide energy range (100 GeV to 30 TeV) with good energy (15-20%) and angular () resolution. The field of view of the VERITAS telescopes is 3.5. The high sensitivity of VERITAS enables the detection of sources with a flux of 1% of the Crab Nebula in less than 50 hours of observations. For more details on the VERITAS instrument, see e.g. Acciari et al (2008).

VERITAS observed the sky around HESS J0632+057 during three periods in December 2006, December 2008 and January 2009; see Table 1 for details. For each period, data equivalent to 10 hours of observations passed quality selection criteria, which remove data taken during bad weather or with hardware-related problems. Data were taken on moonless nights in wobble mode, wherein the source was positioned 0.5 degrees from the camera center with the offsets in different positions for different runs. The first data set (Set I) consists of observations taken during the construction phase of VERITAS with only 3 telescopes. These observations were pointed towards the centre of the Monoceros region (at an angular distance of from HESS J0632+057), while observations in the second and third set were targeted around the reported position of HESS J0632+057.

The data analysis steps consist of image calibration and cleaning, second-moment parameterization of these images (Hillas, 1985), stereoscopic reconstruction of the event impact position and direction, gamma-hadron separation, spectral energy reconstruction (see e.g. Krawczynski et al. (2006)) as well as the generation of photon sky maps. The majority of the far more numerous background events are rejected by comparing the shape of the event images in each telescope with the expected shapes of gamma-ray showers modeled by Monte Carlo simulations. These mean-reduced-scaled width and mean-reduced-scaled length cuts (see definition in Krawczynski et al. (2006)), and an additional cut on the arrival direction of the incoming gamma ray () reject more than 99.9% of the cosmic-ray background while keeping 45% of the gamma rays. The cuts applied here are: integrated charge per image 1200 digital counts (225 photoelectrons), mean-reduced-scaled width/length , and deg ( deg for the 3-telescope data set). The background in the source region is estimated from the same field of view using the “reflected-region” model with 8-10 background regions (Aharonian et al, 2001) and the “ring-background” model with a ring size of 0.5 (mean radius) and a ring width of 0.175 (Aharonian et al, 2005a). This analysis has been chosen to provide best sensitivity at high energies, in order to compare more directly with the results reported in Aharonian et al (2007). The resulting energy threshold is 720 GeV.

Observations of XMMU J063259.3+054801 (HESS J0632+057) were taken with the Swift satellite over four consecutive nights, from January 26 to January 29, 2009, and were contemporaneous with VERITAS observations (Set III). The analysis is restricted to X-ray data from the XRT instrument (Burrows et al, 2005) since the source is not detected with BAT and the bright star MWC 148 causes a very high level of photon-coincidence losses in the UVOT instrument (Poole et al, 2008).84 The presented Swift XRT observations were conducted in photon-counting (PC) mode with no signatures of pile-up. Data reduction is performed with the HEAsoft 6.5 package. Events with grades 0–12 in the energy range of 0.3–10 keV are calibrated and cleaned using the xrtpipeline tool by applying the standard filtering criteria and latest Swift calibration files. Source counts are extracted from a circular region with a 30-pixel radius (47.2 arcsec), and background events are extracted from a 40-pixel radius circle in a source-free region. Ancillary response files are generated with the xrtmkarf tool applying corrections for the PSF losses and CCD defects. The latest response matrix from the XRT calibration files are used in the analysis. To ensure valid minimization statistics during spectral fitting, the extracted XRT energy spectra are rebinned to contain a minimum of 25 counts in each bin.

3 Results

Results for each of the three VERITAS data sets (see Table 1), as well as for the total observation are listed in Table 2. Figure 1 shows a sky map of the significance at energies above 720 GeV observed in the region around HESS J0632+057. The distribution of significances in the sky map is consistent with the expected distribution from a field with no gamma-ray source present. The significance at the position of HESS J0632+057 is ( for an energy threshold of 1 TeV, see Table 2). There is therefore no significant evidence for gamma-ray like events from HESS J0632+057 during the 31 hours of observations with VERITAS. The flux upper limit (E1 TeV) for the complete data set at the 99% confidence level (Helene, 1983) assuming a power-law like source spectrum with a spectral index of is F(1 TeV) cms (about 1.1% of the flux of the Crab Nebula; see Table 2). This flux limit is times lower than the flux reported by H.E.S.S in Aharonian et al (2007); see Figure 2 for a light curve. The probability for a non-variable flux of high-energy gamma rays from HESS J0632+057 is derived from the VERITAS data and the average of the reported fluxes from H.E.S.S. using a -test. The test gives a of 15.8 with 1 degree of freedom, corresponding to a probability of 0.007% (about 4).

The non-detection of HESS J0632+057 by VERITAS initiated additional and very thorough quality checks of the data. Optical pointing monitors which are installed on each telescope show that the systematic error on the pointing was less than 90” during the data-taking period for Set II and III85. The sensitivity of VERITAS in the elevation range of 50-60 was confirmed by observations of the Crab Nebula and several other weak gamma-ray sources in early 2009 (e.g. Ong et al (2009)).

The Swift XRT observations provide further evidence for X-ray flux variability in the object. The total photon spectrum from the four Swift XRT observations in January 2009 is well-described by an absorbed power law. Due to the low counting statistics, a fixed column density of cm is applied in the spectral analysis, as measured for the XMM-Newton EPIC spectrum (Hinton et al, 2009). The best-fit absorbed power law model yields a , a photon index , and a 1 keV normalization of cm s keV. The deabsorbed 1-10 keV flux is erg cm s, corresponding to a factor of higher flux than the XMM-Newton EPIC data. Among the four Swift XRT observations no significant flux variability is measured. Figure 3 shows the deabsorbed X-ray spectra from the Swift and XMM-Newton observations. Over the limited energy range of 0.8–4 keV, the Swift photon spectra is softer () than the XMM-Newton EPIC spectrum (). The relatively hard () X-ray spectrum is similar to the spectral slopes measured from the TeV binaries LS I +61 303 (Leahy et al, 1997), LS 5039 (Hoffmann et al, 2009), and PSR B1259-63/SS 2883 (Chernyakova et al, 2006). No definite conclusion can be drawn from the X-ray spectral variability of HESS J0632+057. It appears, for example from observations of PSR B1259-63/SS 2883 (Chernyakova et al, 2006) that the photon index of the X-ray spectra in TeV binaries is not a strictly monotonic function of flux. Detailed results from the full Swift observing campaign of XMMU J063259.3+054801 between January 2009 and April 2009 are presented in Falcone et al (2009).

The non-detection of HESS J0632+057 by VERITAS provides evidence for variability in the flux of gamma-rays with energies above 1 TeV. HESS J0632+057 is unlikely to have a blazar counterpart. Blazars are generally hard X-ray point sources and bright radio sources. Neither the XMM observation nor the Swift images show the presence of any bright X-ray source other than the Be star within the HESS error circle. A search in the NVSS catalog also finds no radio source in the area. The VHE emission and variability can be easily explained if MWC 148 would be part of a binary system and high-energy photons are produced in a similar way as in LS I +61 303 or LS 5039. The available data do not allow any conclusion on a possible periodicity of the gamma-ray signal. A detection of the compact companion or of the orbital motion of the Be star is required to confirm or refute the binary nature of this system. Particle acceleration and VHE emission from massive stars with strong magnetic fields has also been suggested. A confirmation that MWC 148//HESS J0632+057 is surrounded by sufficiently strong magnetic fields, along with further theoretical work to explain the variability in the gamma-ray emission, would be needed to establish this potentially new class of galactic gamma-ray sources. Future multiwavelength observations combined with results from ground-based and space-based gamma-ray observatories will provide a deeper understanding of the true nature of HESS J0632+057.

This research is supported by grants from the US Department of Energy, the US National Science Foundation, and the Smithsonian Institution, by NSERC in Canada, by Science Foundation Ireland, and by STFC in the UK. We acknowledge the excellent work of the technical support staff at the FLWO and the collaborating institutions in the construction and operation of the instrument. We acknowledge the efforts of the Swift team for providing the UVOT/XRT observations. Facilities: VERITAS, Swift
Figure 1: VERITAS significance map of the region around HESS J0632+057 for the whole data set and an energy threshold of 720 GeV. The background is estimated using the ring background method. The location of HESS J0632+057 is indicated by a black triangle. Also shown are the 95% and 99% confidence regions of the EGRET sources 3EGJ0634+0521 (white lines) and 3EGJ0631+0642 (GeV J0633+0645) (black lines). The Fermi source 0FGLJ0633.5+0634 is indicated with a ‘x’ sign; the blue circle denotes the 95% confidence region (Abdo et al, 2009). The circle at the bottom right indicates the angular resolution of the VERITAS observations.
Figure 2: The light curve above 1 TeV from HESS J0632+057 is shown assuming a spectral shape of with . The downwards pointing arrows show the 99% confidence limits derived here from the VERITAS data. The H.E.S.S. fluxes are taken from Aharonian et al (2007). The X-ray fluxes measured by XMM-Newton and Swift are indicated by open symbols.
Figure 3: X-ray spectrum of HESS J0632+057 from XMM-Newton EPIC data in September 2007 (Hinton et al, 2009) and Swift XRT observations in January 2009.
Name Date range N86 Elevation Angular distance Observation
range between source and time
pointing direction [min]
Set I Dec 16 2006 - Jan 25 2007 3 55-65 0.15-0.8 580
Set II Dec 30 2008 - Jan 03 2009 4 59-65 0.5 560
Set III Jan 26 2009 - Jan 30 2009 3/4 59-65 0.5 722
H.E.S.S. P1 Dec 2004 282
H.E.S.S. P2 Nov 2005 - Dec 2005 372
Table 1: Details of the VERITAS and H.E.S.S. (Aharonian et al, 2007) observations of HESS J0632+057.
on off excess significance Flux or upper flux limit
events events events [] [ cms]
Set I 84 594 0.16 -8.8 -0.9
Set II 131 713 0.16 15.7 1.3
Set III 120 669 0.16 11.4 1.0 3.6
Total (I-III) 335 1976 0.16 19.4 1.0 2.6
H.E.S.S. P1
H.E.S.S. P2
Table 2: Analysis results for HESS J0632+057 for TeV. Upper limits TeV are given at 99% confidence level (after Helene (1983)). The integral fluxes and 1 errors above 1 TeV reported by H.E.S.S. are listed for comparison (Aharonian et al, 2007).

Footnotes

  1. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
  2. affiliation: Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
  3. affiliation: Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
  4. affiliation: Department of Physics, Washington University, St. Louis, MO 63130, USA
  5. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
  6. affiliation: Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
  7. affiliation: School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
  8. affiliation: Department of Physics, Washington University, St. Louis, MO 63130, USA
  9. affiliation: Department of Physics, Washington University, St. Louis, MO 63130, USA
  10. affiliation: Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA
  11. affiliation: School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
  12. affiliation: School of Physics, National University of Ireland, Galway, Ireland
  13. affiliation: School of Physics, National University of Ireland, Galway, Ireland
  14. affiliation: Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
  15. affiliation: Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
  16. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
  17. affiliation: Department of Physics, Washington University, St. Louis, MO 63130, USA
  18. affiliation: Department of Physics, Grinnell College, Grinnell, IA 50112-1690, USA
  19. affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
  20. affiliation: Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
  21. affiliation: Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
  22. affiliation: Department of Physics, Purdue University, West Lafayette, IN 47907, USA
  23. affiliation: Physics Department, University of Utah, Salt Lake City, UT 84112, USA
  24. affiliation: Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA
  25. affiliation: Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
  26. affiliation: Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
  27. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
  28. affiliation: School of Physics, National University of Ireland, Galway, Ireland
  29. affiliation: School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
  30. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
  31. affiliation: Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
  32. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
  33. affiliation: Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
  34. affiliation: Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
  35. affiliation: Physics Department, University of Utah, Salt Lake City, UT 84112, USA
  36. affiliation: Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
  37. affiliation: Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
  38. affiliation: Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA
  39. affiliation: Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
  40. affiliation: Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135-0037, USA
  41. affiliation: Physics Department, University of Utah, Salt Lake City, UT 84112, USA
  42. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
  43. affiliation: Department of Physics, Pittsburg State University, 1701 South Broadway, Pittsburg, KS 66762, USA
  44. affiliation: Department of Physics, Washington University, St. Louis, MO 63130, USA
  45. affiliation: Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
  46. affiliation: School of Physics, National University of Ireland, Galway, Ireland
  47. affiliation: Physics Department, University of Utah, Salt Lake City, UT 84112, USA
  48. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
  49. affiliation: Corresponding author; gernot.maier@mcgill.ca
  50. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
  51. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
  52. affiliation: Department of Physics, Anderson University, 1100 East 5th Street, Anderson, IN 46012
  53. affiliation: Department of Life and Physical Sciences, Galway-Mayo Institute of Technology, Dublin Road, Galway, Ireland
  54. affiliation: Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA
  55. affiliation: Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
  56. affiliation: Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
  57. affiliation: Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA
  58. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
  59. affiliation: European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
  60. affiliation: Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
  61. affiliation: School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
  62. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
  63. affiliation: Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
  64. affiliation: Department of Applied Physics and Instrumentation, Cork Institute of Technology, Bishopstown, Cork, Ireland
  65. affiliation: School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
  66. affiliation: Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
  67. affiliation: Department of Physics, Purdue University, West Lafayette, IN 47907, USA
  68. affiliation: Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA
  69. affiliation: Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
  70. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
  71. affiliation: School of Physics, National University of Ireland, Galway, Ireland
  72. affiliation: Department of Physics, Purdue University, West Lafayette, IN 47907, USA
  73. affiliation: Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
  74. affiliation: Physics Department, University of Utah, Salt Lake City, UT 84112, USA
  75. affiliation: Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA
  76. affiliation: Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
  77. affiliation: School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
  78. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
  79. affiliation: Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
  80. affiliation: Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
  81. affiliation: Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
  82. affiliation: Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
  83. affiliation: Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
  84. It should be noted that MWC 148 with a B magnitude of less than 9 is too faint to introduce systematic effects into the analysis of the VERITAS data.
  85. There were no pointing monitors installed in December 2006 (Set I).
  86. Number of available telescopes

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