[Paper Review] The XMM-Newton Serendipitous Survey. VI. The X-ray Luminosity Function

J. Ebrero, F. J. Carrera+7 more|ArXiv.org|Nov 10, 2008

Astrophysical Phenomena and Observations68 references67 citations

TL;DR

This study presents the X-ray luminosity function (XLF) of absorbed and unabsorbed AGN in the soft (0.5–2 keV), hard (2–10 keV), and ultrahard (4.5–7.5 keV) bands using the XMM-Newton serendipitous survey and complementary deep/shallow surveys. The Luminosity-Dependent Density Evolution (LDDE) model best describes the data, revealing that high-luminosity AGN formed earlier (peaking at z ~ 1.5), while lower-luminosity AGN continue forming to lower redshifts, with stronger evolution in the ultrahard band due to luminosity-dependent evolution effects.

ABSTRACT

We present the X-ray luminosity function of AGN in three energy bands (Soft: 0.5-2 keV, Hard: 2-10 keV and Ultrahard: 4.5-7.5 keV). We have used the XMS survey along with other highly complete flux-limited deeper and shallower surveys for a total of 1009, 435 and 119 sources in the Soft, Hard and Ultrahard bands, respectively. We have modeled the intrinsic absorption of the Hard and Ultrahard sources (NH function) and computed the intrinsic X-ray luminosity function in all bands using a Maximum Likelihood fit technique to an analytical model. We find that the X-ray luminosity function (XLF) is best described by a Luminosity-Dependent Density Evolution (LDDE) model. Our results show a good overall agreement with previous results in the Hard band, although with slightly weaker evolution. Our model in the Soft band present slight discrepancies with other works in this band, the shape of our present day XLF being significantly flatter. We find faster evolution in the AGN detected in the Ultrahard band than those in the Hard band. The fraction of absorbed AGN in the Hard and Ultrahard bands is dependent on the X-ray luminosity. We find evidence of evolution of this fraction with redshift in the Hard band but not in the Ultrahard band, possibly due to the low statistics. Our best-fit XLF shows that the high-luminosity AGN are fully formed earlier than the less luminous AGN. The latter sources account for the vast majority of the accretion rate and mass density of the Universe, according to an anti-hierarchical black hole growth scenario.

Motivation & Objective

• To accurately determine the intrinsic X-ray luminosity function (XLF) of AGN, accounting for intrinsic absorption and cosmological evolution.
• To investigate the dependence of the absorbed AGN fraction on X-ray luminosity and redshift across different energy bands.
• To test whether the Luminosity-Dependent Density Evolution (LDDE) model provides a better fit than Pure Luminosity or Pure Density Evolution models.
• To estimate the accretion rate density and total black hole mass density across cosmic time, accounting for obscured AGN populations.
• To reconcile discrepancies in XLF shapes and evolution rates across previous soft and hard X-ray surveys, especially in the soft band.

Proposed method

• The study uses a sample of 1,009, 435, and 119 sources in the soft, hard, and ultrahard bands, respectively, from the XMM-Newton serendipitous survey and complementary flux-limited surveys.
• It applies a modified $1/V_a$ technique and a Maximum Likelihood fitting method with an analytic model to compute the intrinsic XLF without binning.
• The intrinsic absorption ($N_H$) function is modeled for hard and ultrahard band sources to correct for obscuration effects.
• The LDDE model is tested against Pure Luminosity Evolution (PLE) and Pure Density Evolution (PDE) models using statistical goodness-of-fit criteria.
• The XLF is used to compute the comoving accretion rate density and total black hole mass density as a function of redshift.
• The results are compared with prior works (e.g., Miyaji et al. 2000, Hasinger et al. 2005, La Franca et al. 2005) to assess consistency and resolve discrepancies.

Experimental results

Research questions

• RQ1 How does the intrinsic X-ray luminosity function of AGN vary across soft, hard, and ultrahard energy bands, and what is the best-fitting evolutionary model?
• RQ2 What is the dependence of the fraction of absorbed AGN on X-ray luminosity and redshift, and does this fraction evolve with cosmic time?
• RQ3 Why does the ultrahard band XLF show stronger evolution than the hard band, and is this due to luminosity-dependent evolution or sample bias?
• RQ4 How do the accretion rate density and total black hole mass density evolve with redshift, and do they agree with local observations and Compton-thick AGN contributions?
• RQ5 To what extent do the results resolve discrepancies in the soft X-ray band XLF shape and evolution compared to previous studies?

Key findings

• The Luminosity-Dependent Density Evolution (LDDE) model provides the best fit to the XLF across all three energy bands, outperforming PLE and PDE models.
• The high-luminosity AGN (log LX > 44) peak in comoving density at z ~ 1.5, while lower-luminosity AGN (log LX < 44) peak at z ~ 0.7, indicating earlier formation for more luminous sources.
• The fraction of absorbed AGN (log NH > 22) decreases with increasing X-ray luminosity, confirming a known trend in AGN demographics.
• In the hard band, the fraction of absorbed AGN evolves with redshift, while in the ultrahard band, no significant evolution is detected, likely due to low statistics.
• The ultrahard band XLF shows stronger evolution below the cut-off redshift than the hard band, possibly due to a luminosity-dependent evolution parameter not accounted for in the model.
• The predicted local black hole mass density from the best-fit LDDE model agrees with observations, and the discrepancy with Marconi et al. (2004) is explained by missing Compton-thick AGN, which contribute a factor of ~1.5 to the mass function.