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CONSTRAINING LONG-TERM AND SPATIAL VARIABILITY OF THE PN LOW ENERGY RESPONSE WITH RXJ1856
(IUSS Pavia; INAF, IASF-Milano) In collaboration with: N. Sartore, S. Mereghetti, A. De Luca, R. Turolla, F. Haberl

Andrea Tiengo


RX J1856.5-3754 (RXJ1856)
Brightest of the "Magnificent Seven", or X-ray Dim Isolated Neutron Stars (XDINSs)
(e.g. Turolla 2009 f or a review)

The closest known neutron star: d ~120 pc
(W alter et al. 2010)

No (detected) radio emission
(Kondriatev et al. 2010)

Flux ~1.5 x 10

-11

erg s-1 cm-2 [0.2 ­ 10 keV]

BB spectrum, kT ~61 eV, no features
(e.g. Burwitz et al. 2003)

P ~7.055 s, low PF (~ 1.2%)
(Tiengo & Mereghetti 2007)

B ~ 1. 5 x 10
-1

13

G

Pdot ~10

-13

­ 10

-14

ss

(van Kerkwijk & Kaplan 2008)

Optical/UV counterpart (mV ~26): optical excess ~7x RJ tail of X-ray BB
(Kaplan et al. 2011)


Why studying RXJ1856 variability?
Astrophysical goal: · Constrain spectral evolution (as observed in RX J0720.4-3125)
RX J0720.4-3125

Possible cause: Precession
(e.g. Haberl et al. 2006)

Glitch episode
(e.g. Kaplan et al. 2007)

· If no spectral changes, cumulative spectrum (huge statistics to test NS EO S) Instrum ental goal: · Constrain PN response stability at low energies (<1.2 keV)


Preliminary results
(presented last year by Nicola Sartore)

Significant changes, possibly correlated to position on the detector (and time?)


New calibration observation
October 2011 observation divided into 4 pointings to sample different RAW X positions

3

Q1 Q4 Q3 Q2

Observations at larger distance have higher temperatures, but in the same range as in previous observations (kTBB~1 eV) The effect is present also at different detector positions, but it does not strongly increase with RAXW


Re-analysis of all data
(Sartore et al. 2012, A&A in press, arXiv:1202.2121)
Equally good fit with absorbed BB model by freeing: · g ai n s l o pe ( ~ 4% ) and offset (~15 eV) OR · BB temperature and normalization Target positions clustered in two regions: hard and soft


Observations in "soft region" (I)
Include ~50% of all RXJ1856 observations: kT ~ 0.5% and fX < ~ 3%

T im e evolution:

No significant variability in 2005-2011 period Obs. of April 2002 is "anomalous": · fit with a constant profile rejected at 4


Observations in "soft region" (II)
Merged spectrum : ~250 kiloseconds; ~2 x 106 counts

Single BB

Two BB

· No spectral features (EW < 6 eV)

· Extrapolation of 2BB model in optical band is 5x higher than single BB consistent with optical/UV flux


(Sartore et al. 2012,A&A in press, arXiv:1202.2121)

Conclusions

Calibration results - Small PN gain variations (slope ~4% and offset ~15 eV) related to target position - PN response in "soft region" is very stable (with possible exception of earliest observation in 2002), slightly more unstable (>2 times larger kT) in "hard region" Astrophysical results (if calibration issues fully understood)

- RXJ1856 very stable in time (kT < 0.5% and fX < 3% from March 2005 to present) Still a good standard candle - April 2002: possible hint of small scale heating episode - High-statistic cumulative spectrum: 2BB model provides much better fit and its extrapolation is compatible with optical/UV emission


Future prospects
Re-analysis of all RXJ1856 observations with SASv12.0 to check whether

position dependent gain is correctly accounted for.

Further observations might show instrumental problems or onset of heating episode in RXJ1856

Improvements in calibration of PN low energy response are needed to fully exploit the cumulative spectrum of RXJ1856 (the best ever to study NS thermal emission)