AB magnitude

The AB magnitude system is an astronomical magnitude system. Unlike many other magnitude systems, it is based on flux measurements that are calibrated in absolute units, namely spectral flux densities.

Definition

The monochromatic AB magnitude is defined as the logarithm of a spectral flux density with the usual scaling of astronomical magnitudes and a zero-point of 3631 Jansky,^[1] where 1 Jansky = 1 Jy = 10⁻²⁶ W Hz⁻¹ m⁻² = 10⁻²³ erg s⁻¹ Hz⁻¹ cm⁻². If the spectral flux density is denoted f_ν, the monochromatic AB magnitude is:

$m_\text{AB} = -\frac{5}{2} \log_{10} \left(\frac{f_{\nu}}{3631\text{ Jy}}\right),$

or

$m_\text{AB} = -\frac{5}{2} \log_{10} \left(\frac{f_{\nu}}{\text{Jy}}\right) + 8.90.$

In cgs units of erg s⁻¹ cm⁻² Hz⁻¹, it is:

$m_\text{AB} = -\frac{5}{2} \log_{10} f_{\nu} - 48.600.$

Actual measurements are always made across some continuous range of wavelengths. The bandpass AB magnitude is defined so that the zero point corresponds to a bandpass-averaged spectral flux density of 3631 Jansky:

$m_\text{AB} = -\frac{5}{2} \log_{10} \left(\frac{\int f_\nu (h\nu)^{-1} e(\nu) d\nu}{\int 3631\text{ Jy} (h\nu)^{-1} e(\nu) d\nu}\right),$

where e(ν) is the "equal-energy" filter response function and the (hν)⁻¹ term assumes that the detector is a photon-counting device such as a CCD or photomultiplier.^[2] (Filter responses are sometimes expressed as quantum efficiencies, that is, in terms of their response per photon, rather than per unit energy. In those cases the (hν)⁻¹ term has been folded into the definition of e(ν) and should not be included.)

The STMAG system is similarly defined, but for constant flux per unit wavelength interval instead.

AB stands for "absolute" in the sense that no relative reference object is used (unlike using Vega as a baseline object).^[3]

Expression in terms of f_λ

In some fields, spectral flux densities are expressed per unit wavelength, f_λ, rather than per unit frequency, f_ν. At any specific wavelength,

$f_\nu = \frac{\lambda^2}{c} f_\lambda,$

where f_ν is measured per frequency (e.g., Hertz), and f_λ is measured per wavelength (e.g., cm). If the wavelength unit is Ångstrom,

$\frac{f_\nu}{\text{Jy}} = 3.34 \times 10^4 \left(\frac{\lambda}{\AA}\right)^2 \frac{f_\lambda}{\text{erg} \text{ cm}^{-2} \text{ s}^{-1} \AA^{-1}}$.

This can then be plugged into the equations above.

The “pivot wavelength” of a given bandpass is the value of λ that makes the above conversion exact for observations made in that bandpass. For an equal-energy response function as defined above, it is ^[4]

$\lambda_\text{piv} = \sqrt{\frac{\int e(\lambda) d\lambda}{\int e(\lambda) \lambda^{-2} d\lambda}}$.

For a response function expressed in the quantum-efficiency convention, it is:

$\lambda_\text{piv} = \sqrt{\frac{\int e(\lambda) \lambda d\lambda}{\int e(\lambda) \lambda^{-1} d\lambda}}$.

Conversion from other magnitude systems

Magnitudes in the AB system can be converted to other systems. However, because all magnitude systems involve integration of some assumed source spectrum over some assumed passband, such conversions are not necessarily trivial to calculate, and precise conversions depend on the actual spectrum of the source in question. Various authors have computed conversions for standard situations.^[5]

External links

• Conversion from AB magnitudes to Johnson magnitudes

References

1. Oke, J. B. (1983). "Secondary standard stars for absolute spectrophotometry". The Astrophysical Journal 266: 713–717. Bibcode:1983ApJ...266..713O. doi:10.1086/160817.
2. Tonry, J. L. (2012). "The Pan-STARRS1 Photometric System". The Astrophysical Journal 750: 99. arXiv:1203.0297. Bibcode:2012ApJ...750...99T. doi:10.1088/0004-637X/750/2/99.
3. Oke, J. B. (1974). "Absolute spectral energy distributions for white dwarfs". Astrophysical Journal Supplement Series 236 (27): 21–25. Bibcode:1974ApJS...27...21O. doi:10.1086/190287.
4. Tokunaga, A. T.; Vacca (April 2005). "The Mauna Kea Observatories Near‐Infrared Filter Set. III. Isophotal Wavelengths and Absolute Calibration". Publications of the Astronomical Society of the Pacific 117 (830): 421–426. arXiv:astro-ph/0502120. Bibcode:2005PASP..117..421T. doi:10.1086/429382. Retrieved 25 February 2014.
5. Blanton, M. R. (2007). "K-Corrections and Filter Transformations in the Ultraviolet, Optical, and Near-Infrared". The Astronomical Journal 133 (2): 734–754. arXiv:astro-ph/0606170. Bibcode:2007AJ....133..734B. doi:10.1086/510127.