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J/MNRAS/414/1329    M82 X-ray sources long-term variability  (Chiang+, 2011)
The long-term variability of the X-ray sources in M82.
    Chiang Y.-K., Kong A.K.H.
   <Mon. Not. R. Astron. Soc., 414, 1329-1338 (2011)>
   =2011MNRAS.414.1329C
ADC_Keywords: Galaxies, nearby ; X-ray sources

Keywords: galaxies: individual: M82 - X-rays: binaries - X-rays: galaxies


Abstract:
    We investigate the long-term variability exhibited by the X-ray point
    sources in the starburst galaxy M82. By combining nine Chandra
    observations taken between 1999 and 2007, we detect 58 X-ray point
    sources within the D25 isophote of M82 down to a luminosity of
    ∼1037erg/s. Of these 58 sources, we identify three supernova remnant
    candidates and one supersoft source. 26 sources in M82 exhibit
    long-term (i.e. days to years) flux variability and three show
    long-term spectral variability. Furthermore, we classify 26 sources as
    variables and 10 as persistent sources. Among the total 26 variables,
    17 varied by a flux ratio of >3 and six are transient candidates. By
    comparing with other nearby galaxies, M82 shows extremely strong
    long-term X-ray variability that 47 per cent of the X-ray sources are
    variables with a flux ratio of >3. The strong X-ray variability of M82
    suggests that the population is dominated by X-ray binaries.

Description:
    Since the X-ray sources in the central region of M82 are crowded, a
    high spatial resolution instrument like the Chandra X-ray Observatory
    is needed to resolve the images. M82 has been observed 13 times
    between 1999 and 2007 with Chandra; some of them were designed to
    observe M82 X-1 with special configurations. To study the long-term
    variability of the whole galaxy, we used nine Chandra data sets with
    relatively long exposure time and large field of view from the Chandra
    Data Archive.

File Summary:

       FileName      Lrecl  Records   Explanations

ReadMe            80        .   This file
table2.dat       104       59   Chandra source properties for M82


Byte-by-byte Description of file: table2.dat

   Bytes Format Units   Label     Explanations

   1-  2  I2    ---     Seq       Sequential number
   4- 18  A15   ---     CXOU      Source name, HHMMSS.s+DDMMSS (CXOU J)
  20- 28  F9.5  deg     RAdeg     Right ascension in decimal degrees (J2000)
  30- 38  F9.6  deg     DEdeg     Declination in decimal degrees (J2000)
  40- 46  F7.2  10+31W  LX        Maximum X-ray luminosity (1)
      48  A1    ---     l_Sflux   Limit flag on SFlux
  49- 54  F6.2  ---     Sflux     Significance parameter of long-term flux
                               variability (2)
      56  A1    ---     l_R       Limit flag on R
  57- 62  F6.2  ---     R         Fmax/Fmin ratio (3)
  64- 68  F5.2  ---     e_R       ? rms uncertainty in R
  70- 73  F4.2  ---     SHR1      ?=- Significance parameter of HR1 variability (4)
      74  A1    ---     n_SHR1    [gi] Note on SHR1 (5)
  76- 79  F4.2  ---     SHR2      ?=- Significance parameter of HR2 variability (6)
      80  A1    ---     n_SHR2    [hjk] Note on SHR2 (5)
  82-104  A23   ---     Notes     Notes (7)

Note (1): The highest luminosity in 0.3-7keV of a source among the nine
  observations we used, assuming an absorbed power-law model
  with a photon index of 1.7, NH=3x1022cm-2 and d=3.6Mpc.
Note (2): Sflux=maxi,j(|Fi-Fj|/sqrt((σHR1i)2+(σHR1j)2)

Note (3): R=Fmax/Fmin with a standard error if the lowest flux is detected or
  a 90 per cent lower bound if the lowest flux is an upper limit.
Note (4): SHR1=maxi,j(|HR1i-HR1j|/sqrt((σHR1i)2+(σHR1j)2)
  of the five Chandra ACIS observations.
Note (5): Individual notes as follows:
  g, i = The SHR1 and SHR2 with a different extraction of colours in observation
           5 using a pile-up spectral fitting. This source suffers from mild
           pile-up in observation 5 with a pile-up fraction of ∼26 per cent.
  h, j = The SHR1 and SHR2 excluding observation 5
     k = Considering the strong contamination from the diffuse emission, this
          significant level is overestimated
Note (6): SHR2=maxi,j(|HR2i-HR2j|/sqrt((σHR2i)2+(σHR2j)2)
  of the five Chandra ACIS observations
Note (7): Codes as follows:
    P = persistent source
    V = variable
   V3 = variable with flux ratio >3
    T = transient candidate
   SV = spectral variable
  SSS = supersoft source candidate
  RSN = source coincides with a radio-selected SNR in the list of
          Fenech et al. (2008, Cat. J/MNRAS/391/1384)
  IOV(Obs. No.) = intra-observation variability detected in the specified
          observation


History:
    From electronic version of the journal
(End)                                      Patricia Vannier [CDS]    02-Feb-2012