New XMM-Newton observation of the Phoenix cluster: properties of the cool core

¹ INAF, Osservatorio Astrofisico di Firenze, Largo Enrico Fermi 5, 50125 Firenze, Italy
e-mail: ptozzi@arcetri.astro.it
² INAF, IASF Milano, via E. Bassini 15, 20133 Milano, Italy
³ INAF, Osservatorio Astronomico di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁴ INFN, Sezione di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁵ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁶ INAF, Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
⁷ Università degli Studi di Ferrara, via Savonarola, 9 – 44121 Ferrara , Italy
⁸ European Space Astronomy Centre (ESAC), European Space Agency, Apartado de Correos 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁹ National Center for Supercomputing Applications, University of Illinois at Urbana-Champaign, 1205 W. Clark St., Urbana, IL 61801, USA
¹⁰ Department of Astronomy, University of Illinois at Urbana-Champaign, W. Green Street, Urbana, IL 61801, USA

Received: 26 January 2015
Accepted: 25 May 2015

Abstract

Aims. We present a spectral analysis of a deep (220 ks) XMM-Newton observation of the Phoenix cluster (SPT-CL J2344-4243). We also use Chandra archival ACIS-I data that are useful for modeling the properties of the central bright active galactic nucleus and global intracluster medium.

Methods. We extracted CCD and reflection grating spectrometer (RGS) X-ray spectra from the core region to search for the signature of cold gas and to finally constrain the mass deposition rate in the cooling flow that is thought to be responsible for the massive star formation episode observed in the brightest cluster galaxy (BCG).

Results. We find an average mass-deposition rate of Ṁ = 620 (−190 + 200)_stat (−50 + 150)_syst M_⊙ yr^-1 in the temperature range 0.3−3.0 keV from MOS data. A temperature-resolved analysis shows that a significant amount of gas is deposited at about 1.8 keV and above, while only upper limits on the order of hundreds of  M_⊙ yr^-1 can be placed in the 0.3−1.8 keV temperature range. From pn data we obtain Ṁ = 210 (−80 + 85)_stat (−35 + 60)_syst M_⊙ yr^-1 in the 0.3−3.0 keV temperature range, while the upper limits from the temperature-resolved analysis are typically a factor of 3 lower than MOS data. No line emission from ionization states below Fe XXIII is seen above 12 Å in the RGS spectrum, and the amount of gas cooling below ~3 keV has a formal best-fit value Ṁ = 122_-122⁺³⁴³ M_⊙ yr^-1. In addition, our analysis of the far-infrared spectral energy distribution of the BCG based on Herschel data provides a star formation rate (SFR) equal to 530  M_⊙ yr^-1 with an uncertainty of 10%, which is lower than previous estimates by a factor 1.5. Overall, current limits on the mass deposition rate from MOS data are consistent with the SFR observed in the BCG, while pn data prefer a lower value of Ṁ ~ SFR/ 3, which is inconsistent with the SFR at the 3σ confidence level.

Conclusions. Current data are able to firmly identify a substantial amount of cooling gas only above 1.8 keV in the core of the Phoenix cluster. At lower temperatures, the upper limits on Ṁ from MOS and pn data differ by a factor of 3. While the MOS data analysis is consistent with values as high as Ṁ ~ 1000 within 1σ, pn data provide Ṁ < 500 M_⊙ yr^-1 at 3σ confidence level at a temperature below 1.8 keV. At present, this discrepancy cannot be explained on the basis of known calibration uncertainties or other sources of statistical noise.