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Chandra Spectroscopy of MAXI J1305-704

ATel #4191; J. M. Miller (Michigan), J. Raymond (SAO), J. Kennea (Swift/PSU), A. Beardmore (Swift/Leicester), M. T. Reynolds (Michigan), D. Maitra (Michigan), N. Degenaar (Michigan), E. M. Cackett (Wayne State), A. C. Fabian (Cambridge), C. S. Reynolds (Maryland)
on 20 Jun 2012; 13:22 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

Subjects: X-ray, Binary, Black Hole, Neutron Star, Transient

We briefly report on a Chandra observation of the new X-ray transient MAXI J1305-704, obtained on 29 April 2012. The source was observed using the High Energy Transmission Grating Spectrometer, for 30 ksec. There are no prominent dips nor flares in the observation, and so we analyzed the time-averaged, combined, first-order MEG spectrum.

ATEL #4070 noted an absorption line in the Fe K band, likely due to He-like Fe XXV, in a Swift/XRT observation of MAXI J1305-704. Fits with an XSTAR photoionization grid (developed for IGR J17091-3624, see King et al. 2012) were able to account for the Fe XXV line and much of the spectral complexity in the low energy part of the Swift spectrum, especially the broad negative flux trough at 1 keV. This suggested that the spectral complexity could be due to a combination of Fe L absorption lines.

The Chandra/MEG spectrum reveals a number of strong Fe L shell absorption lines in the region around 1 keV (12.39852 Angstroms). Fits with a dedicated XSTAR grid assuming a blackbody input spectrum are able to model most of the absorption lines well. The density-sensitive Fe XXII line pair (11.77 and 11.92 Angstroms) implied an hydrogen number density of n = 1 E+14 cm^-3 in GRO J1655-40 (e.g. Miller et al. 2006, 2008), and was instrumental in requiring a magnetically-driven wind in that source. These lines also appear to be detected in MAXI J1305-704. XSTAR models with a density of n = 1 E+12 cm^-3 are unable to match the line ratio; a density of at least n = 1 E+15 cm^-3 matches the pair much better. Assuming this high density and a luminosity of 1 E+36 erg/s, an ionization parameter of log(xi) = 2.8 and a column density of N = 8 E+21 cm^-2 are found.

The distance to MAXI J1305-704 is currently unknown, but for reasonable values of the luminosity, density, and ionization parameter, the absorption originates close to the compact object. The fits also suggest a statistically significant red shift of approximately 500 km/s, which would be particularly interesting. However, the actual source position differed substantially from the Chandra pointing position (based on MAXI coordinates); it is possible that the apparent velocity shift derives from the offset pointing. The resolution of the dispersed spectra is also reduced owing to the offset. Line identification and relative flux levels are likely not affected, however. These issues will be addressed in a forthcoming journal paper.

We note that the low energy spectral complexity seen in MAXI J1305-704 is not unlike that seen in some ultra-compact X-ray binaries (e.g. 4U 1850-087; see Juett & Chakrabarty 2005). This observation of MAXI J1305-704 may have simply obtained a more sensitive spectrum, owing to a high flux level and low line of sight column density. It is possible that the low energy spectral complexity in such sources is not exclusively due to ISM edges or broad emission lines (e.g. Madej & Jonker 2011), but may be partially due to complex absorption in e.g. a dense disk wind or disk atmosphere.

A plot of the MEG spectrum fit with a the XSTAR model described above is available here.


References

Juett, A. M., & Chakrabarty, D., 2005, ApJ, 627, 926
King, A. L., et al., 2012, ApJ, 746, L20
Madej, O., & Jonker, P. G., 2011, MNRAS, 412, L11
Miller, J. M., et al., 2006, Nature, 441, 953
Miller, J. M., et al., 2008, ApJ, 680, 1359