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Title: PROBING THE NATURE OF THE UNIDENTIFIED TeV GAMMA-RAY SOURCE HESS J0632+057 WITH SWIFT

Abstract

New generation TeV gamma-ray telescopes have discovered many new sources, including several enigmatic unidentified TeV objects. HESS J0632+057 is a particularly interesting unidentified TeV source since: it is a point source, it has a possible hard-spectrum X-ray counterpart and a positionally consistent Be star, it has evidence of long-term very high energy gamma-ray flux variability, and it is postulated to be a newly detected TeV/X-ray binary. We have obtained Swift X-ray telescope observations of this source from MJD 54857 to 54965, in an attempt to ascertain its nature and to investigate the hypothesis that it is a previously unknown X-ray/TeV binary. Variability and spectral properties similar to those of the other three known X-ray/TeV binaries have been observed, with measured flux increases by factors of {approx}3. X-ray variability is present on multiple timescales including days to months; however, no clear signature of periodicity is present on the timescales probed by these data. If binary modulation is present and dominating the measured variability, then the period of the orbit is likely to be {>=}54 days (half of this campaign), or it has a shorter period with a variable degree of flux modulation on successive high states. If the two high statesmore » measured to date are due to binary modulation, then the favored period is approximately 35-40 days. More observations are required to determine if this object is truly a binary system and to determine the extent that the measured variability is due to inter-orbit flaring effects or periodic binary modulation.« less

Authors:
;  [1];  [2]; ;  [3];  [4];  [5];  [6]
  1. Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802 (United States)
  2. School of Physics, University College Dublin, Belfield, Dublin 4 (Ireland)
  3. School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT (United Kingdom)
  4. Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716 (United States)
  5. Physics Department, McGill University, Montreal, QC H3A 2T8 (Canada)
  6. Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027 (United States), E-mail: afalcone@astro.psu.edu
Publication Date:
OSTI Identifier:
21301434
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 708; Journal Issue: 1; Other Information: DOI: 10.1088/2041-8205/708/1/L52; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; 99 GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE; COSMIC GAMMA SOURCES; GAMMA RADIATION; MODULATION; ORBITS; PERIODICITY; POINT SOURCES; STARS; TELESCOPES; TEV RANGE; X RADIATION

Citation Formats

Falcone, A. D., Stroh, M., Grube, J., Hinton, J., Skilton, J., Holder, J., Maier, G., and Mukherjee, R. PROBING THE NATURE OF THE UNIDENTIFIED TeV GAMMA-RAY SOURCE HESS J0632+057 WITH SWIFT. United States: N. p., 2010. Web. doi:10.1088/2041-8205/708/1/L52; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA).
Falcone, A. D., Stroh, M., Grube, J., Hinton, J., Skilton, J., Holder, J., Maier, G., & Mukherjee, R. PROBING THE NATURE OF THE UNIDENTIFIED TeV GAMMA-RAY SOURCE HESS J0632+057 WITH SWIFT. United States. doi:10.1088/2041-8205/708/1/L52; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA).
Falcone, A. D., Stroh, M., Grube, J., Hinton, J., Skilton, J., Holder, J., Maier, G., and Mukherjee, R. Fri . "PROBING THE NATURE OF THE UNIDENTIFIED TeV GAMMA-RAY SOURCE HESS J0632+057 WITH SWIFT". United States. doi:10.1088/2041-8205/708/1/L52; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA).
@article{osti_21301434,
title = {PROBING THE NATURE OF THE UNIDENTIFIED TeV GAMMA-RAY SOURCE HESS J0632+057 WITH SWIFT},
author = {Falcone, A. D. and Stroh, M. and Grube, J. and Hinton, J. and Skilton, J. and Holder, J. and Maier, G. and Mukherjee, R.},
abstractNote = {New generation TeV gamma-ray telescopes have discovered many new sources, including several enigmatic unidentified TeV objects. HESS J0632+057 is a particularly interesting unidentified TeV source since: it is a point source, it has a possible hard-spectrum X-ray counterpart and a positionally consistent Be star, it has evidence of long-term very high energy gamma-ray flux variability, and it is postulated to be a newly detected TeV/X-ray binary. We have obtained Swift X-ray telescope observations of this source from MJD 54857 to 54965, in an attempt to ascertain its nature and to investigate the hypothesis that it is a previously unknown X-ray/TeV binary. Variability and spectral properties similar to those of the other three known X-ray/TeV binaries have been observed, with measured flux increases by factors of {approx}3. X-ray variability is present on multiple timescales including days to months; however, no clear signature of periodicity is present on the timescales probed by these data. If binary modulation is present and dominating the measured variability, then the period of the orbit is likely to be {>=}54 days (half of this campaign), or it has a shorter period with a variable degree of flux modulation on successive high states. If the two high states measured to date are due to binary modulation, then the favored period is approximately 35-40 days. More observations are required to determine if this object is truly a binary system and to determine the extent that the measured variability is due to inter-orbit flaring effects or periodic binary modulation.},
doi = {10.1088/2041-8205/708/1/L52; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)},
journal = {Astrophysical Journal Letters},
number = 1,
volume = 708,
place = {United States},
year = {Fri Jan 01 00:00:00 EST 2010},
month = {Fri Jan 01 00:00:00 EST 2010}
}
  • HESS J0632+057 is the only gamma-ray binary known so far whose position in the sky allows observations with ground-based observatories in both the northern and southern hemispheres. Here we report on long-term observations of HESS J0632+057 conducted with the Very Energetic Radiation Imaging Telescope Array System and High Energy Stereoscopic System Cherenkov telescopes and the X-ray satellite Swift, spanning a time range from 2004 to 2012 and covering most of the system's orbit. The very-high-energy (VHE) emission is found to be variable and is correlated with that at X-ray energies. An orbital period of 315{sub −4}{sup +6} days is derivedmore » from the X-ray data set, which is compatible with previous results, P = (321 ± 5) days. The VHE light curve shows a distinct maximum at orbital phases close to 0.3, or about 100 days after periastron passage, which coincides with the periodic enhancement of the X-ray emission. Furthermore, the analysis of the TeV data shows for the first time a statistically significant (>6.5σ) detection at orbital phases 0.6-0.9. The obtained gamma-ray and X-ray light curves and the correlation of the source emission at these two energy bands are discussed in the context of the recent ephemeris obtained for the system. Our results are compared to those reported for other gamma-ray binaries.« less
  • We report on a 40 ks Chandra observation of the TeV emitting high-mass X-ray binary HESS J0632+057 performed in 2011 February during a high state of X-ray and TeV activity. We have used the ACIS-S camera in continuous clocking mode to search for a possible X-ray pulsar in this system. Furthermore, we compare the emission of the source during this high state, with its X-ray properties during a low state of emission, caught by a 47 ks XMM-Newton observation on 2007 September. We did not find any periodic or quasi-periodic signal in any of the two observations. We derived anmore » average pulsed fraction 3{sigma} upper limit for the presence of a periodic signal of {approx}<35% and 25% during the low and high emission states, respectively (although this limit is strongly dependent on the frequency and the energy band). Using the best X-ray spectra derived to date for HESS J0632+057, we found evidence for a significant spectral change between the low and high X-ray emission states, with the absorption value and the photon index varying between N{sub H} {approx_equal} (2.1-4.3) x 10{sup 21} cm{sup -2} and {Gamma} {approx_equal} 1.18-1.61. In contrast to what has been observed in other TeV binaries, it seems that in this source the higher the flux, the softer the X-ray spectrum.« less
  • HD 259440 is a B0pe star that was proposed as the optical counterpart to the {gamma}-ray source HESS J0632+057. Here, we present optical spectra of HD 259440 acquired to investigate the stellar parameters, the properties of the Be star disk, and evidence of binarity in this system. Emission from the H{alpha} line shows evidence of a spiral density wave in the nearly edge-on disk. We find a best-fit stellar effective temperature of 27,500-30,000 K and a log surface gravity of 3.75-4.0, although our fits are somewhat ambiguous due to scattered light from the circumstellar disk. We derive a mass ofmore » 13.2-19.0 M{sub sun} and a radius of 6.0-9.6 R{sub sun}. By fitting the spectral energy distribution, we find a distance between 1.1 and 1.7 kpc. We do not detect any significant radial velocity shifts in our data, ruling out orbital periods shorter than one month. If HD 259440 is a binary, it is likely a long-period (>100 d) system.« less
  • The very high energy (VHE) {gamma}-ray source HESS J0632+057 has recently been confirmed to be a {gamma}-ray binary. The optical counterpart is the Be star MWC 148, and a compact object of unknown nature orbits it every {approx}321 days with a high eccentricity of {approx}0.8. We monitored HESS J0632+057 with the stereoscopic MAGIC telescopes from 2010 October to 2011 March and detected significant VHE {gamma}-ray emission during 2011 February, when the system exhibited an X-ray outburst. We find no {gamma}-ray signal in the other observation periods when the system did not show increased X-ray flux. Thus, HESS J0632+057 exhibits {gamma}-raymore » variability on timescales of the order of one to two months possibly linked to the X-ray outburst that takes place about 100 days after the periastron passage. Furthermore, our measurements provide for the first time the {gamma}-ray spectrum down to about 140 GeV and indicate no turnover of the spectrum at low energies. We compare the properties of HESS J0632+057 with the similar {gamma}-ray binary LS I +61 Degree-Sign 303 and discuss the possible origin of the multi-wavelength emission of the source.« less
  • HESS J0632+057 is a variable, point-like source of very high energy (>100 GeV) gamma rays located in the Galactic plane. It is positionally coincident with a Be star, it is a variable radio and X-ray source, has a hard X-ray spectrum, and has low radio flux. These properties suggest that the object may be a member of the rare class of TeV/X-ray binary systems. The definitive confirmation of this would be the detection of a periodic orbital modulation of the flux at any wavelength. We have obtained Swift X-Ray Telescope observations of the source from MJD 54857 to 55647 (2009more » January-2011 March) to test the hypothesis that HESS J0632+057 is an X-ray/TeV binary. We show that these data exhibit flux modulation with a period of 321 {+-} 5 days and we evaluate the significance of this period by calculating the null hypothesis probability, allowing for stochastic flaring. This periodicity establishes the binary nature of HESS J0632+057.« less